WEBVTT

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<v Steve>Welcome and thank you for joining</v>

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the National Geodetic Survey's Monthly Webinar Series.

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My name is Steve Vogel

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and I'll be the moderator for today's presentation.

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I'm a Communications Specialist at NGS

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in Silver Spring, Maryland.

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Today, Phillip McFarland from NGS

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will present Global Reference Frames,

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what they are and how and why

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NGS aligns to them.

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The U.S. National Spatial Reference System

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is aligned with

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the International Terrestrial Reference Frame.

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This presentation discusses

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what that statement means,

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why it is done and how it is achieved.

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We've given this presentation

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a technical rating of intermediate,

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meaning some prior knowledge

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of the topic is helpful.

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Phillip is a Geodesist

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and reference frame scientist at NGS.

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He received a Bachelor's and a Master's degree

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in geosciences from the University of Arizona.

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Phillip is currently the Project Manager

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for NGS' contribution

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to IGS repro three,

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which is a reprocessing of all GPS orbits

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from 1994 to the present

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and is the first step to defining

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the GNSS contribution

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to the upcoming ITRF2020

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global reference frame.

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Thank you, Phillip, and you may begin.

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<v Phillip>Awesome, well, thank you very much</v>

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for the introduction, and thank you, everyone,

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for joining the webinar today.

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Okay, so yeah, today I'm gonna talk about

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global reference frames, what they are

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and how and why NGS aligns our frames to them,

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and sort of unpack that and explain

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what all that means.

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Before we get into the nitty-gritty,

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I just wanna give you a brief outline

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of the talk today, what we're gonna go over.

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So first, I'm gonna tell you about,

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I'm gonna answer the question, or try to answer

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the question, just what is a reference frame in general?

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And I'm gonna give you a very sort of basic example

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of what that looks like

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and some of the use cases for that.

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I'm gonna go briefly over

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traditional geodetic datums

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and how those are slightly different

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from sort of modern global reference frames,

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and actually kinda talk about

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some of the drawbacks of using

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a traditional geodetic datum and why we use

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global reference frames now.

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I'm gonna give you some kinda technical definitions

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on global reference frames.

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Talk about the International Terrestrial Reference System,

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the ITRS, and the International Terrestrial Reference Frame,

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the ITRF.

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I'm gonna talk about how the ITRF is realized,

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kinda we throw that term around a lot in geodesy

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and surveying, realizing the frame,

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and sort of kinda flesh that out,

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what that realization means

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in terms of the ITRF.

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We'll discuss the International GNSS Service,

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the IGS, and their realization

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of that frame, the ITRF.

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And then at the end,

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I'll talk about how NGS aligns

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the U.S. National Spatial Reference System,

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the NSRS, how we align our frames

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with the ITRF by using

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the IGS realization of the ITRF.

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So let's get into it.

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Oh, before we get into the details,

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I just wanna give a quick disclaimer,

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actually kinda referring back to that poll question.

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So this presentation, I'm gonna cover

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geometric reference frames.

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So I'm not gonna discuss,

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I'm not going to explicitly discuss gravity

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at all in this presentation today.

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We'll talk about gravity implicitly

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because we'll be talking about satellites

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and orbits, and, of course, gravitation is involved.

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So it'll be sort of in this implicit way,

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but we're not gonna explicitly talk about

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physical geodetic reference frames.

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Okay, so what is a reference frame?

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Well, a reference frame gives us a means

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to assign self-consistent coordinates

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to physical locations and describe how

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those coordinates change over time.

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So that's sort of a definition

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that I just came up with on my own.

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Maybe the other definitions might differ slightly.

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But I wanna sort of give you

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a really basic example in the next few slides

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to sort of unpack that statement

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about assigning coordinates in a self-consistent way

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and tracking how those coordinates

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of physical locations might change over time.

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So as my career, as my colleague, ‪Jarir Saleh,

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told me last week when he and I

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were discussing this presentation, he said,

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"Really, when we're talking about a reference frame,

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"we're talking about a set of axes in space."

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So in three dimensions, we need three axes.

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So what I'm showing you here is a triad

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of axes in space,

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with the x-axis sort of coming out

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of the page at you, the y-axis

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going off to the right,

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and the z-axis, the vertical axis.

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And the origin of this little reference frame

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that I have here is at the intersection

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of these axes.

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And I wanna use these, this frame,

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to start assigning coordinates

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to physical points in space.

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But first, before I can do that,

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I have to come up with a unit of length

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that I'm gonna use to describe those coordinates.

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And once I have that unit of length in mind,

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I can start to assign coordinates

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to physical locations, physical points in space.

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So let's say, for example,

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I have this point P1,

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and I wanna describe its location in space

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using this self-consistent frame.

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Well, I can use that unit of length

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that I came up with earlier

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and measure along each axis

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to come up with coordinates for the point P1

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that are self-consistent within this frame.

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Okay, that's great.

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How can I use that?

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Well, let's say we have some other point,

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P2, that we're interested in.

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And let's say, for some reason or another,

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when we want to assign coordinates

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to this point P2, for some reason,

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we're unable to access

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the origin of the frame.

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That is, we have no way to sense

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where the origin is or no way to get at it.

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But maybe we know where the point P1 is.

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Maybe someone's published those coordinates already.

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Well, if that's the case,

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we can use the known coordinates of P1

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and position ourselves relative to P1.

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And by adding those two vectors together,

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we can come up with an absolute set

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of coordinates within the frame

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that's self-consistent, it's consistent with point P1

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and it's consistent with the origin of the frame.

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And I just wanna take a second here

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and point out that this is sort of the general mode

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of business that we use here at NGS

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and for a lot of the surveying community.

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So typically what happens

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is an official organization,

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someone like NGS or the IGS or the IERS,

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they will assign official coordinates

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to some set of points, points like P1.

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They'll do the dirty work of determining

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where the origin is, defining the frame

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and assigning coordinates to some of these points,

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and then they'll publish those coordinates.

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And then other folks will come along,

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people like the surveying community

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or the Earth science community,

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and they'll have some points, like P2,

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that they're interested in positioning to.

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And they won't be able to get at the origin,

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necessarily, but they'll be able to use

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the published coordinates of points like P1

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to come up with coordinates for their points

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of interest, points like P2,

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that are self-consistent with the frame

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that's been defined.

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And then there's another use case

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that I wanna talk about really quickly.

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Let's say we have some third point, point P3.

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And let's say, by some means or another,

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we've come up with coordinates

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for the point P3 in this reference frame,

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absolute coordinates in the frame.

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Well, it's really handy.

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The reason that we use a reference frame,

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the reason why we need it

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a lotta times is because it's really simple

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to come up with a relative position

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for the point P3 and the point P1

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by simply taking a subtraction

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of their coordinates in the frame.

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And furthermore, if those points happen to be moving.

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So now my point P1 is moving

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with a velocity V1.

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My point P3 is moving with a velocity V3.

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If I can describe the velocities

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and the initial points in the frame,

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then I can compute the differential vector

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between the two points at any time, t,

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if I have a description of how things

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are moving in the frame

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in a self-consistent manner.

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And this is sort of more like

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a navigation-type example,

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where things are moving around,

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but we have coordinates for things within the frame

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that are well-defined,

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we know the velocities described within the frame,

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and we wanna keep track of where things are

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with respect to one another.

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This is another very common use case

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for reference frames.

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Okay, so I've given you this really simple example,

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just these three axes

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and just a couple points floating around in space,

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and maybe they're moving, maybe they're not.

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But it doesn't take much of an imagination

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to sort of think about how this complexity

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might grow for these types of systems

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and things that we might wanna keep track of.

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So I'm showing this overhead view

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of this busy harbor here.

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We have some freight ships and some tugboats.

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There's an airliner flying overhead.

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And in today's world, we need to keep track

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of where all these things are.

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And in the case of the harbor,

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it's actually vitally important

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to our nation's economy to be able

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to position things accurately,

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to keep track how they're moving

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and to be able to do so in a self-consistent manner

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using a reference frame.

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And this idea has applications, as we said,

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in navigation, Earth science, engineering

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and, of course, in surveying.

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So I'm gonna keep coming back to this outline

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as I'm going through the talk

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just to kind of recap what we've already talked about

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and keep our eyes on the road

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and talk about where we're going

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as we move forward.

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So in the last slides, I just gave you

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a really basic kinda definition for what

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a reference frame is and gave you

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a really simple example of what they are

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and how they're used.

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And really, the key thing that I want you

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to take away from those slides is that

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a reference frame is just a way to keep track

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of coordinates, of physical points

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on or near the Earth's surface in space,

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and assign coordinates to points in space

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and to keep track of how those points

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are moving in time.

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And in the next section,

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I'm gonna talk about traditional geodetic datums.

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So I'm just gonna give you an example

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of how this is done, or rather,

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how this has been done historically

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for points on or near

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the surface of the Earth.

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So to define a traditional

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horizontal geodetic datum.

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So I'm gonna talk about horizontal geodetic datums

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in particular, and I'm gonna leave out

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vertical geodetic datums

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in this discussion, but a lot of the things

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that I'm gonna say, they're similar for the two.

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For a traditional horizontal geodetic datum,

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we have to define what we call

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a reference ellipsoid.

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And this reference ellipsoid

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is sort of a rough estimate

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for the shape of the surface of the Earth.

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It's not an exact replica

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of the shape of the surface of the Earth.

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It's an estimate.

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It's a rough estimate of the shape of the Earth.

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And when we use that reference ellipsoid,

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we reduce our measurements

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from the surface of the Earth

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to that reference ellipsoid

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to define where coordinates are in space.

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So what I'm showing in this figure here is, again,

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this sort of reference ellipsoid here,

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and then this little patch that I'm showing here

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is supposed to represent

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the actual topographic surface of the Earth,

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so the actual surface.

00:11:34.560 --> 00:11:36.280
And what we do when we define

00:11:36.280 --> 00:11:37.340
a horizontal geodetic datum

00:11:37.340 --> 00:11:39.900
is we just pick some point.

00:11:39.900 --> 00:11:41.490
It's not exactly arbitrary,

00:11:41.490 --> 00:11:43.450
but we just pick a point and say,

00:11:43.450 --> 00:11:45.580
okay, that's gonna be the origin in the datum

00:11:45.580 --> 00:11:47.290
and that's gonna be our zero point.

00:11:47.290 --> 00:11:49.140
And we're gonna define all of our coordinates

00:11:49.140 --> 00:11:51.770
for the rest of our points of interest

00:11:51.770 --> 00:11:54.673
within the datum with respect to that origin.

00:11:58.530 --> 00:11:59.363
So an example of this

00:11:59.363 --> 00:12:00.810
is the North American Datum

00:12:00.810 --> 00:12:04.220
of 1927, NAD27.

00:12:04.220 --> 00:12:08.060
So for that datum, Meade's Ranch was selected

00:12:08.060 --> 00:12:09.140
as the origin.

00:12:09.140 --> 00:12:11.330
And what I'm showing you here is the disc,

00:12:11.330 --> 00:12:13.410
and this little tiny point, right here in the center

00:12:13.410 --> 00:12:15.700
of the disc, that's actually the origin,

00:12:15.700 --> 00:12:18.863
the zero point, for NAD27.

00:12:20.269 --> 00:12:23.150
And so with Meade's Ranch as the origin,

00:12:23.150 --> 00:12:26.280
it was possible to assign coordinates.

00:12:26.280 --> 00:12:28.830
Again, this is clearly a simplification.

00:12:28.830 --> 00:12:30.350
I'm just showing you a little cartoon.

00:12:30.350 --> 00:12:32.260
I'm trying to explain how this works.

00:12:32.260 --> 00:12:35.250
So when we define Meade's Ranch as the origin,

00:12:35.250 --> 00:12:37.780
we're able to position other physical points

00:12:37.780 --> 00:12:38.840
on the surface of the Earth

00:12:38.840 --> 00:12:40.873
with respect to that origin.

00:12:41.870 --> 00:12:43.330
And when we do that,

00:12:43.330 --> 00:12:45.890
we are realizing our frame.

00:12:45.890 --> 00:12:48.550
We're realizing our datum by assigning coordinates

00:12:48.550 --> 00:12:50.480
to these other five locations

00:12:50.480 --> 00:12:52.123
with respect to Meade's Ranch.

00:12:53.400 --> 00:12:54.830
And then other folks can come along.

00:12:54.830 --> 00:12:56.380
Say someone up here in Iowa

00:12:56.380 --> 00:12:58.730
is interested in the location of this point.

00:12:58.730 --> 00:13:00.660
They can position themselves with respect

00:13:00.660 --> 00:13:04.100
to this position, to this point in Northern Missouri.

00:13:04.100 --> 00:13:05.810
And if they do so, they will be in

00:13:05.810 --> 00:13:07.790
a self-consistent manner that puts them

00:13:07.790 --> 00:13:10.020
into the datum of NAD27,

00:13:10.020 --> 00:13:12.846
so long as this point in Northern Missouri

00:13:12.846 --> 00:13:15.790
is measured in a self-consistent way

00:13:15.790 --> 00:13:17.020
with the datum.

00:13:17.020 --> 00:13:20.150
And similar with like this point over here in Illinois.

00:13:20.150 --> 00:13:22.220
This person, if they're interested in a point

00:13:22.220 --> 00:13:23.620
in Illinois, they don't have to measure

00:13:23.620 --> 00:13:25.710
all the way back to the point in Meade's Ranch.

00:13:25.710 --> 00:13:27.800
They can simply position themselves with respect

00:13:27.800 --> 00:13:30.470
to this point in Northern Missouri,

00:13:30.470 --> 00:13:32.300
they can add those two vectors together

00:13:32.300 --> 00:13:33.990
and they can come up with their position

00:13:33.990 --> 00:13:35.140
in a self-consistent way

00:13:35.140 --> 00:13:37.980
in Central Illinois in NAD27.

00:13:37.980 --> 00:13:40.740
And, of course, you can continue to build on that

00:13:40.740 --> 00:13:41.743
as you move outward.

00:13:43.810 --> 00:13:47.550
So that's sort of a very, very brief explanation

00:13:47.550 --> 00:13:50.160
of a traditional horizontal datum.

00:13:50.160 --> 00:13:51.760
And I quickly wanna talk about

00:13:51.760 --> 00:13:55.093
some of the drawbacks of using that sorta system.

00:13:55.940 --> 00:13:56.890
The first is really

00:13:57.800 --> 00:14:00.230
with the definition of that reference ellipsoid.

00:14:00.230 --> 00:14:01.810
So I'm showing you a little diagram here

00:14:01.810 --> 00:14:03.970
on the right that shows a couple

00:14:04.840 --> 00:14:06.380
reference ellipsoids, so one

00:14:06.380 --> 00:14:09.970
for the North American Datum 1927, NAD27,

00:14:09.970 --> 00:14:13.203
and one for the European Datum of 1950 in red.

00:14:14.070 --> 00:14:16.060
And as you can see, the North American Datum

00:14:16.060 --> 00:14:18.070
in this cartoon, in this diagram,

00:14:18.070 --> 00:14:19.830
you can see that the reference ellipsoid

00:14:19.830 --> 00:14:22.030
aligns really well with the surface of the Earth

00:14:22.030 --> 00:14:23.610
in some parts of the Earth,

00:14:23.610 --> 00:14:24.770
but in other parts of the Earth,

00:14:24.770 --> 00:14:26.780
there's quite a bit of disagreement,

00:14:26.780 --> 00:14:28.830
and similar for the European datum.

00:14:28.830 --> 00:14:31.120
In some places, it agrees really well

00:14:31.120 --> 00:14:33.530
with the shape of the surface of the Earth,

00:14:33.530 --> 00:14:36.363
and other places, it doesn't agree quite so well.

00:14:37.410 --> 00:14:39.460
And to further complicate things,

00:14:39.460 --> 00:14:42.030
the center of neither ellipsoid

00:14:42.030 --> 00:14:44.290
is aligned with the center of mass of the Earth

00:14:44.290 --> 00:14:46.600
and the center of both ellipsoids

00:14:46.600 --> 00:14:48.430
don't align with each other.

00:14:48.430 --> 00:14:51.080
So these misalignments can cause complications.

00:14:51.080 --> 00:14:53.110
So let's say, for example,

00:14:53.110 --> 00:14:55.070
I've measured the coordinates

00:14:55.070 --> 00:14:57.270
of a point using the North American Datum,

00:14:57.270 --> 00:14:59.260
and I have a colleague who's measured

00:14:59.260 --> 00:15:01.440
the same coordinate using the European Datum,

00:15:01.440 --> 00:15:03.040
and we wanna compare notes.

00:15:03.040 --> 00:15:04.960
Well, this misalignment can cause issues,

00:15:04.960 --> 00:15:08.420
and it can cause discrepancies in coordinates

00:15:08.420 --> 00:15:10.320
on the order of several,

00:15:10.320 --> 00:15:13.350
many, many centimeters, significant discrepancies.

00:15:13.350 --> 00:15:15.090
And so this is a major drawback of using

00:15:15.090 --> 00:15:16.270
these regional datums.

00:15:16.270 --> 00:15:18.400
Each datum is optimized.

00:15:18.400 --> 00:15:20.740
Rather, each reference ellipsoid is optimized

00:15:20.740 --> 00:15:22.410
to match the surface of the Earth

00:15:22.410 --> 00:15:24.410
in the region where people are interested in working.

00:15:24.410 --> 00:15:27.380
So, of course, the North American Datum in 1927

00:15:27.380 --> 00:15:29.740
is optimized to fit the surface of the Earth

00:15:29.740 --> 00:15:30.950
in North America.

00:15:30.950 --> 00:15:33.250
Similarly, the European Datum of 1950

00:15:33.250 --> 00:15:35.410
was optimized to match the surface

00:15:35.410 --> 00:15:36.700
of the Earth in Europe.

00:15:36.700 --> 00:15:38.990
But if you want to compare measurements

00:15:38.990 --> 00:15:41.390
between the two, you can run into some problems.

00:15:42.710 --> 00:15:45.330
Another major issue with this sort of system

00:15:45.330 --> 00:15:46.550
is that it's static.

00:15:46.550 --> 00:15:49.110
So when I talked about assigning that coordinate

00:15:49.110 --> 00:15:52.810
to Meade's Ranch in the center of the United States,

00:15:52.810 --> 00:15:55.123
in the center of CONUS,

00:15:56.250 --> 00:15:59.900
that was just literally a static origin that was set.

00:15:59.900 --> 00:16:01.840
And then those positions were measured

00:16:01.840 --> 00:16:04.630
with respect to that origin in a static way.

00:16:04.630 --> 00:16:06.510
But we know that the Earth is not static.

00:16:06.510 --> 00:16:07.970
We know it's a dynamic system.

00:16:07.970 --> 00:16:10.010
We have tectonic motion,

00:16:10.010 --> 00:16:12.110
glacial isostatic adjustment.

00:16:12.110 --> 00:16:14.390
We have lots of deformation on the West Coast

00:16:14.390 --> 00:16:17.477
of North America from the San Andreas system

00:16:17.477 --> 00:16:20.340
and from subduction of the Juan de Fuca Plate.

00:16:20.340 --> 00:16:22.330
So we know we have all these dynamic processes

00:16:22.330 --> 00:16:23.163
happening on the Earth.

00:16:23.163 --> 00:16:24.660
So these static datums,

00:16:24.660 --> 00:16:26.590
they can't account for that in the ways

00:16:26.590 --> 00:16:28.783
that we need them to any longer.

00:16:31.720 --> 00:16:33.840
So in the last couple slides, I talked about,

00:16:33.840 --> 00:16:35.730
talked really briefly about

00:16:35.730 --> 00:16:37.700
this idea of traditional geodetic datums,

00:16:37.700 --> 00:16:38.890
some of the shortcomings.

00:16:38.890 --> 00:16:40.970
And I wanna move forward now

00:16:40.970 --> 00:16:42.520
and talk about global reference frames.

00:16:42.520 --> 00:16:43.890
And I really wanna talk about

00:16:43.890 --> 00:16:46.350
some of their strengths and how they sort of make up

00:16:46.350 --> 00:16:47.410
for some of those drawbacks

00:16:47.410 --> 00:16:49.797
from the traditional style of geodetic datums.

00:16:54.290 --> 00:16:56.090
Before we get into

00:16:56.090 --> 00:16:57.560
the global reference frames, first,

00:16:57.560 --> 00:17:01.220
I just wanna clear up a little distinction in terminology.

00:17:01.220 --> 00:17:02.120
So the first thing

00:17:03.360 --> 00:17:06.170
I wanna discuss is a reference system.

00:17:06.170 --> 00:17:07.710
So I got these definitions

00:17:07.710 --> 00:17:08.697
from the "Springer Handbook

00:17:08.697 --> 00:17:11.040
"of Global Navigation Satellite Systems."

00:17:11.040 --> 00:17:13.350
I use 'em 'cause I think they're really great definitions.

00:17:13.350 --> 00:17:15.470
They're very technical, and we're gonna unpack them

00:17:15.470 --> 00:17:17.000
later in the slides, but I thought

00:17:17.000 --> 00:17:18.620
I should give these up front.

00:17:18.620 --> 00:17:20.750
So a reference system is a set of prescriptions

00:17:20.750 --> 00:17:23.440
and conventions together with the modeling required

00:17:23.440 --> 00:17:27.580
to define at any time a triad of coordinate axes.

00:17:27.580 --> 00:17:29.130
And a reference frame

00:17:29.130 --> 00:17:31.530
realizes the system by means of coordinates

00:17:31.530 --> 00:17:33.450
of definite points that are accessible

00:17:33.450 --> 00:17:36.420
directly by occupation or observation.

00:17:36.420 --> 00:17:38.810
So the reference system is like the rules,

00:17:38.810 --> 00:17:40.160
the models, things that,

00:17:40.160 --> 00:17:42.093
how are we gonna define this system.

00:17:43.410 --> 00:17:45.900
That's packed up in the reference system.

00:17:45.900 --> 00:17:47.430
And we get the reference frame

00:17:47.430 --> 00:17:49.980
when we start actually assigning coordinates

00:17:49.980 --> 00:17:52.090
to physical locations on or near

00:17:52.090 --> 00:17:53.510
the Earth's surface.

00:17:53.510 --> 00:17:56.130
And my advisor in graduate school, Rick Bennett,

00:17:56.130 --> 00:17:58.350
he used the analogy of the recipe.

00:17:58.350 --> 00:18:00.870
So the system is like the recipe, it's the ingredients,

00:18:00.870 --> 00:18:02.390
how we're gonna prepare them,

00:18:02.390 --> 00:18:04.130
what temperature to set the oven to,

00:18:04.130 --> 00:18:06.890
all the rules that we're gonna use to create this thing.

00:18:06.890 --> 00:18:08.530
And the frame's like the cake,

00:18:08.530 --> 00:18:09.910
it's the thing we sorta,

00:18:09.910 --> 00:18:11.340
it's the product at the end

00:18:11.340 --> 00:18:13.490
that we're actually gonna sit down and eat.

00:18:14.440 --> 00:18:16.090
I don't recommend trying to eat a reference frame.

00:18:16.090 --> 00:18:17.890
Not a lotta nutritional value there.

00:18:19.250 --> 00:18:21.150
And in particular, for reference systems

00:18:21.150 --> 00:18:24.460
and reference frames, I'm gonna talk about the ITRS.

00:18:24.460 --> 00:18:26.660
It's the International Terrestrial Reference System.

00:18:26.660 --> 00:18:28.900
So that's the reference system we're gonna talk about.

00:18:28.900 --> 00:18:30.390
And the reference frame we're gonna talk about

00:18:30.390 --> 00:18:32.360
in this presentation is the ITRF,

00:18:32.360 --> 00:18:34.660
the International Terrestrial Reference Frame.

00:18:36.650 --> 00:18:37.850
And before I dig in too far,

00:18:37.850 --> 00:18:39.250
I just want to say quickly

00:18:39.250 --> 00:18:41.667
that the International Terrestrial Reference System

00:18:41.667 --> 00:18:43.560
and the International Terrestrial Reference Frame

00:18:43.560 --> 00:18:45.870
wouldn't be possible without the IERS,

00:18:45.870 --> 00:18:47.120
the International Earth Rotation

00:18:47.120 --> 00:18:48.890
and Reference System Service.

00:18:48.890 --> 00:18:51.920
And I've included their URL here.

00:18:51.920 --> 00:18:53.150
They've got a really great website

00:18:53.150 --> 00:18:55.660
with lots of information and tons of resources.

00:18:55.660 --> 00:18:57.720
So if people are really interested in this topic,

00:18:57.720 --> 00:18:59.470
I suggest checking out their website

00:18:59.470 --> 00:19:00.510
and learning a little bit more.

00:19:00.510 --> 00:19:02.110
They've got a lotta stuff there.

00:19:04.580 --> 00:19:06.510
All right, so let's talk about the ITRS,

00:19:06.510 --> 00:19:09.290
the International Terrestrial Reference System.

00:19:09.290 --> 00:19:10.730
In order to define the system,

00:19:10.730 --> 00:19:12.730
we basically need three things.

00:19:12.730 --> 00:19:14.070
We need an origin.

00:19:14.070 --> 00:19:15.680
So where's the zero point

00:19:15.680 --> 00:19:17.860
in this reference system gonna be?

00:19:17.860 --> 00:19:18.930
We need a scale.

00:19:18.930 --> 00:19:21.240
Like, what is our measure of length going to be?

00:19:21.240 --> 00:19:23.500
And we need to define the orientation of the frame.

00:19:23.500 --> 00:19:24.600
How are the axes going

00:19:24.600 --> 00:19:26.393
to be oriented in space?

00:19:27.990 --> 00:19:30.990
Well, for the ITRS, we use Earth's geocenter.

00:19:30.990 --> 00:19:33.090
So we use the average center of mass

00:19:33.090 --> 00:19:35.620
of the Earth to define the origin.

00:19:35.620 --> 00:19:37.460
And it's the total mass that we're talking about here.

00:19:37.460 --> 00:19:39.220
So the Earth oceans, the atmospheres

00:19:39.220 --> 00:19:41.590
and the solid Earth, the total of all that mass,

00:19:41.590 --> 00:19:43.770
the average of it, that's what we're gonna use

00:19:43.770 --> 00:19:46.063
as our origin for the frame.

00:19:47.420 --> 00:19:48.670
For our scale, we're gonna use

00:19:48.670 --> 00:19:51.313
the SI unit of length, the meter.

00:19:53.030 --> 00:19:54.470
And for the orientation,

00:19:54.470 --> 00:19:56.570
we're gonna define it as follows.

00:19:56.570 --> 00:20:00.110
So we're gonna set the x-axis so that it extends

00:20:00.110 --> 00:20:01.940
from the origin to the point

00:20:01.940 --> 00:20:02.910
on Earth's surface

00:20:04.844 --> 00:20:06.020
where it pierces the surface

00:20:06.020 --> 00:20:09.313
where the equator and the prime meridian intersect.

00:20:10.400 --> 00:20:11.650
We're gonna define the z-axis

00:20:11.650 --> 00:20:15.520
so it coincides with Earth's average rotation pole.

00:20:15.520 --> 00:20:17.390
And we'll define the y-axis

00:20:17.390 --> 00:20:19.630
so that it's orthogonal to the other two axes

00:20:19.630 --> 00:20:21.800
in a right-handed sense.

00:20:21.800 --> 00:20:23.550
So that's how we'll define the orientation

00:20:23.550 --> 00:20:25.523
of our axes of our frame.

00:20:27.090 --> 00:20:28.460
And one thing I wanna quickly point out

00:20:28.460 --> 00:20:31.450
is that for those definitions that I just laid out,

00:20:31.450 --> 00:20:33.340
for us to hold those,

00:20:33.340 --> 00:20:35.730
then our frame actually needs to move

00:20:35.730 --> 00:20:38.040
with the Earth through space.

00:20:38.040 --> 00:20:40.380
So obviously, the Earth rotates on its axis.

00:20:40.380 --> 00:20:41.940
So our frame is gonna rotate

00:20:41.940 --> 00:20:44.200
with the Earth as the Earth does so.

00:20:44.200 --> 00:20:46.340
The Earth is gonna rotate about the sun.

00:20:46.340 --> 00:20:48.180
Rather, it's gonna orbit the sun.

00:20:48.180 --> 00:20:49.650
And our frame is gonna move

00:20:49.650 --> 00:20:52.830
with the Earth as it orbits the sun.

00:20:52.830 --> 00:20:54.480
So we call this an Earth-centered,

00:20:54.480 --> 00:20:55.800
Earth-fixed frame.

00:20:55.800 --> 00:20:57.230
It moves with the Earth

00:20:57.230 --> 00:20:58.473
as the Earth moves.

00:21:01.850 --> 00:21:03.000
All right, and next, I wanna talk about

00:21:03.000 --> 00:21:04.520
the International Terrestrial Reference Frame.

00:21:04.520 --> 00:21:05.840
So as I said earlier,

00:21:05.840 --> 00:21:07.600
this is the realization

00:21:07.600 --> 00:21:10.390
of the International Terrestrial Reference System.

00:21:10.390 --> 00:21:13.300
And like I said, we throw around this term realization,

00:21:13.300 --> 00:21:15.740
realize the frame a lot, and it's kinda just

00:21:15.740 --> 00:21:17.530
a fancy way of saying we're gonna assign

00:21:17.530 --> 00:21:20.420
some coordinates to some points on the Earth.

00:21:20.420 --> 00:21:22.830
So we realize the frame when we follow

00:21:22.830 --> 00:21:24.120
all those rules.

00:21:24.120 --> 00:21:26.210
We use that system that's laid out

00:21:26.210 --> 00:21:28.920
and we assign coordinates to points

00:21:28.920 --> 00:21:30.730
on or near the Earth's surface in this

00:21:30.730 --> 00:21:32.110
self-consistent manner.

00:21:32.110 --> 00:21:34.030
That's what it means to realize the frame.

00:21:34.030 --> 00:21:36.730
So the frame itself is really, in essence,

00:21:36.730 --> 00:21:38.710
it's actually those coordinates that we've assigned

00:21:38.710 --> 00:21:40.063
to those physical points.

00:21:42.980 --> 00:21:44.150
So in those last couple slides,

00:21:44.150 --> 00:21:46.050
I just sort of introduced some

00:21:46.050 --> 00:21:47.300
really technical definitions

00:21:47.300 --> 00:21:49.960
for the International Terrestrial Reference System

00:21:49.960 --> 00:21:53.380
and gave you a layout of how that's implemented.

00:21:53.380 --> 00:21:56.400
And then I talked about the ITRF, the realization

00:21:56.400 --> 00:21:58.833
of the International Terrestrial Reference System.

00:22:00.010 --> 00:22:01.220
And in the next few slides,

00:22:01.220 --> 00:22:02.430
I just wanna get into

00:22:02.430 --> 00:22:03.840
sort of a little bit more detail

00:22:03.840 --> 00:22:06.560
about how the ITRF is realized.

00:22:06.560 --> 00:22:08.030
And this isn't gonna be super heavy

00:22:08.030 --> 00:22:09.970
on the mathematics or anything like that

00:22:09.970 --> 00:22:12.940
because, obviously, books and papers and everything,

00:22:12.940 --> 00:22:14.710
tons of stuff has been published on this stuff.

00:22:14.710 --> 00:22:17.680
But I just wanna give sort of a 30,000-foot-view

00:22:18.844 --> 00:22:19.677
of what this looks like.

00:22:19.677 --> 00:22:22.663
So how is the ITRF actually going to be realized?

00:22:23.630 --> 00:22:25.940
So as I said before, the realization of the ITRF

00:22:25.940 --> 00:22:28.260
requires assigning self-consistent coordinates

00:22:28.260 --> 00:22:31.675
to physical points on or near Earth's surface.

00:22:31.675 --> 00:22:33.570
And the key part to this statement here

00:22:33.570 --> 00:22:35.440
is that can be occupied

00:22:35.440 --> 00:22:37.120
or observed directly.

00:22:37.120 --> 00:22:38.940
So we need to be able to observe

00:22:38.940 --> 00:22:41.400
or occupy these points.

00:22:41.400 --> 00:22:43.170
Well, it turns out

00:22:43.170 --> 00:22:45.850
that a really convenient place

00:22:45.850 --> 00:22:48.490
to start, or rather, convenient places

00:22:48.490 --> 00:22:50.650
to start assigning coordinates to physical locations

00:22:50.650 --> 00:22:53.530
on Earth are places where we have

00:22:53.530 --> 00:22:55.090
instrumentation set up.

00:22:55.090 --> 00:22:56.200
So in the next few slides,

00:22:56.200 --> 00:22:57.490
I'm gonna kinda go through

00:22:57.490 --> 00:22:59.500
the different techniques,

00:22:59.500 --> 00:23:01.240
the different instruments that are used

00:23:01.240 --> 00:23:02.880
to observe these points

00:23:02.880 --> 00:23:04.800
and to start assigning coordinates

00:23:04.800 --> 00:23:06.500
to these physical points on Earth.

00:23:07.560 --> 00:23:09.000
The first technique I wanna tell you about

00:23:09.000 --> 00:23:11.280
that's used in the realization of the ITRF

00:23:11.280 --> 00:23:15.010
is very-long-baseline interferometry, VLBI.

00:23:15.010 --> 00:23:17.790
So this technique uses radio telescopes

00:23:17.790 --> 00:23:20.610
to observe quasars, these stellar objects

00:23:20.610 --> 00:23:22.950
that are hundreds of millions of light years away.

00:23:22.950 --> 00:23:25.310
So forms these really long

00:23:25.310 --> 00:23:28.360
sort of observations with these distant objects.

00:23:28.360 --> 00:23:30.160
And the quasars are observed

00:23:30.160 --> 00:23:32.400
by radio telescopes

00:23:32.400 --> 00:23:33.700
in different parts of the world.

00:23:33.700 --> 00:23:35.610
So I've got one here in Madagascar.

00:23:35.610 --> 00:23:37.030
I don't think there's actually

00:23:38.261 --> 00:23:39.610
a radio telescope array in Madagascar,

00:23:39.610 --> 00:23:41.930
but this is just for illustration purposes only.

00:23:41.930 --> 00:23:42.820
But you can see we can form

00:23:42.820 --> 00:23:44.610
these really long baselines.

00:23:44.610 --> 00:23:46.530
We can observe the same quasar

00:23:46.530 --> 00:23:48.010
in very distant parts of the Earth.

00:23:48.010 --> 00:23:49.190
So we can get these long baselines,

00:23:49.190 --> 00:23:51.300
12,000-kilometer-long baselines

00:23:51.300 --> 00:23:53.290
observing this very distant object,

00:23:53.290 --> 00:23:54.890
which gives us resolution on the order

00:23:54.890 --> 00:23:57.720
of millimeters for these incredibly long baselines.

00:23:57.720 --> 00:23:59.410
And this is very, very helpful

00:23:59.410 --> 00:24:02.320
for defining the orientation of Earth,

00:24:02.320 --> 00:24:05.551
for defining the orientation of the ITRF.

00:24:05.551 --> 00:24:08.200
And VLBI also contributes to the realization

00:24:08.200 --> 00:24:10.163
of the scale for the ITRF as well.

00:24:12.890 --> 00:24:16.270
The second technique I wanna tell you about is DORIS.

00:24:16.270 --> 00:24:18.700
It's Doppler Orbitography by Radiopositioning

00:24:18.700 --> 00:24:20.460
Integrated on Satellite.

00:24:20.460 --> 00:24:23.160
And really, the way I think about DORIS

00:24:23.160 --> 00:24:25.190
is sort of like GPS in reverse.

00:24:25.190 --> 00:24:26.950
So for GPS, obviously, we have

00:24:26.950 --> 00:24:28.920
the satellites transmitting these radio signals

00:24:28.920 --> 00:24:31.500
that we receive at stations on the surface of the Earth,

00:24:31.500 --> 00:24:33.810
and we use that to estimate position.

00:24:33.810 --> 00:24:35.640
But for DORIS, we turn that around.

00:24:35.640 --> 00:24:37.580
So we have these transmitting beacons

00:24:37.580 --> 00:24:38.530
on the surface of the Earth.

00:24:38.530 --> 00:24:40.100
They're sending out radio waves

00:24:40.100 --> 00:24:42.860
that are being received by these satellites.

00:24:42.860 --> 00:24:45.110
And then we actually use the Doppler shift

00:24:46.150 --> 00:24:47.930
in that radio transmission to estimate

00:24:47.930 --> 00:24:50.280
both the position and the velocity of the satellites.

00:24:50.280 --> 00:24:51.440
And we can also use that to estimate

00:24:51.440 --> 00:24:54.510
the position of the transmitting beacon back on Earth.

00:24:54.510 --> 00:24:56.930
And DORIS is used for the realization

00:24:56.930 --> 00:24:59.680
of the orientation and the scale

00:24:59.680 --> 00:25:00.683
for the ITRF.

00:25:03.960 --> 00:25:05.600
And the third technique I wanna tell you about

00:25:05.600 --> 00:25:08.360
is satellite laser ranging, SLR.

00:25:08.360 --> 00:25:10.480
So this is kinda one of the techniques

00:25:10.480 --> 00:25:11.610
that I know the least about,

00:25:11.610 --> 00:25:13.380
but that I think is really cool.

00:25:13.380 --> 00:25:15.700
I have this image of this laser

00:25:15.700 --> 00:25:18.330
shooting off into space that looks like "Star Wars."

00:25:18.330 --> 00:25:20.600
This is like the fun science, it looks like.

00:25:20.600 --> 00:25:22.530
I think these guys probably have a lotta fun,

00:25:22.530 --> 00:25:23.740
guys and gals.

00:25:23.740 --> 00:25:25.100
But the way SLR works

00:25:25.100 --> 00:25:27.960
is you're literally shooting a laser

00:25:27.960 --> 00:25:29.600
from a station down on Earth

00:25:29.600 --> 00:25:32.010
up to a satellite that's reflecting

00:25:32.010 --> 00:25:33.710
that laser back down, and you're using

00:25:33.710 --> 00:25:36.170
the two-way travel time of that laser

00:25:36.170 --> 00:25:39.010
to estimate both the position of the satellite

00:25:39.010 --> 00:25:41.810
and the position of the ground station on Earth.

00:25:41.810 --> 00:25:43.770
And this technique is very,

00:25:43.770 --> 00:25:46.000
very important for the realization of the ITRF.

00:25:46.000 --> 00:25:47.770
It's actually the only technique

00:25:47.770 --> 00:25:49.810
that contributes to our estimate of where

00:25:49.810 --> 00:25:51.360
the center of mass of Earth is.

00:25:52.228 --> 00:25:54.640
And the reason SLR is able to do that

00:25:54.640 --> 00:25:58.470
is we know the orbits of these satellites

00:25:58.470 --> 00:26:00.130
very, very well.

00:26:00.130 --> 00:26:02.870
And this technique is very, very precise.

00:26:02.870 --> 00:26:04.669
And as these satellites orbit the Earth,

00:26:04.669 --> 00:26:05.510
they're actually orbiting

00:26:05.510 --> 00:26:07.420
the center of mass of the Earth.

00:26:07.420 --> 00:26:08.820
So they're sensing the center of mass

00:26:08.820 --> 00:26:09.880
of the Earth directly.

00:26:09.880 --> 00:26:11.970
And because of the precision of this technique,

00:26:11.970 --> 00:26:13.400
it allows us to get at

00:26:13.400 --> 00:26:15.600
where that center of mass actually is.

00:26:15.600 --> 00:26:17.510
And we're not able to do that using GPS

00:26:17.510 --> 00:26:18.570
or some of those other techniques

00:26:18.570 --> 00:26:20.460
for a variety of reasons that I'm not really

00:26:20.460 --> 00:26:23.140
gonna go into in this talk, but SLR is very,

00:26:23.140 --> 00:26:25.490
very important for the realization of the ITRF.

00:26:27.527 --> 00:26:29.370
And the last technique I wanna tell you about

00:26:29.370 --> 00:26:31.640
that's used in the realization of the ITRF

00:26:31.640 --> 00:26:36.180
is GNSS, Global Navigation Satellite Systems.

00:26:36.180 --> 00:26:39.310
So GNSS is an umbrella term for these systems.

00:26:39.310 --> 00:26:41.930
GPS was the first GNSS system,

00:26:41.930 --> 00:26:43.430
the Global Positioning System.

00:26:44.390 --> 00:26:46.330
And we used to be the only kids on the block

00:26:46.330 --> 00:26:48.600
who had this, but other countries

00:26:48.600 --> 00:26:50.870
and other folks have joined in the game.

00:26:50.870 --> 00:26:53.260
So the Europeans now have Galileo,

00:26:53.260 --> 00:26:54.560
the Russians have GLONASS,

00:26:55.920 --> 00:26:57.410
Japanese have QZSS.

00:26:57.410 --> 00:26:59.770
So there are many systems similar to GPS

00:26:59.770 --> 00:27:01.840
now orbiting the Earth,

00:27:01.840 --> 00:27:05.810
and the umbrella term for all of those is GNSS.

00:27:05.810 --> 00:27:07.200
And so for those who aren't familiar,

00:27:07.200 --> 00:27:10.760
just GPS, in two seconds,

00:27:10.760 --> 00:27:12.300
the way GPS works

00:27:12.300 --> 00:27:14.960
is we have these, well, in terms of geodesy,

00:27:14.960 --> 00:27:19.020
the way we use GPS here at NGS, or GNSS at NGS,

00:27:19.020 --> 00:27:21.780
is we set up a ground tracking station

00:27:21.780 --> 00:27:24.920
that's fixed to a physical point on Earth.

00:27:24.920 --> 00:27:26.700
And as these satellites orbit the Earth,

00:27:26.700 --> 00:27:28.990
they're broadcasting radio signals.

00:27:28.990 --> 00:27:31.790
And we have an antenna down here

00:27:31.790 --> 00:27:32.950
fixed to the surface of the Earth

00:27:32.950 --> 00:27:34.590
that's attached to a receiver.

00:27:34.590 --> 00:27:37.370
And that receiver uses those radio signals

00:27:37.370 --> 00:27:39.580
to solve a trilateration problem

00:27:39.580 --> 00:27:41.820
and estimate both the position of the antenna

00:27:41.820 --> 00:27:42.760
on the surface of the Earth,

00:27:42.760 --> 00:27:44.240
as well as the position

00:27:44.240 --> 00:27:47.360
of the satellites orbiting the Earth

00:27:47.360 --> 00:27:49.543
at 20,000 kilometer elevation.

00:27:52.110 --> 00:27:54.560
So GPS or GNSS.

00:27:54.560 --> 00:27:56.470
I frequently slip into GPS because

00:27:56.470 --> 00:27:58.980
that's kinda what I came up on, but GNSS

00:28:00.320 --> 00:28:02.310
contributes to the orientation

00:28:02.310 --> 00:28:04.260
of the realization of the ITRF.

00:28:04.260 --> 00:28:06.400
But in my opinion, in my view,

00:28:06.400 --> 00:28:09.740
GNSS' most important job is allowing access

00:28:09.740 --> 00:28:11.120
to the ITRF.

00:28:11.120 --> 00:28:13.710
So these GNSS ground stations are much,

00:28:13.710 --> 00:28:16.070
much cheaper than, say, a radio telescope,

00:28:16.070 --> 00:28:19.320
a radio telescope array or an SLR station.

00:28:19.320 --> 00:28:20.730
They're actually, I mean, they're not cheap,

00:28:20.730 --> 00:28:22.800
but they're much less expensive

00:28:22.800 --> 00:28:24.060
than those other techniques,

00:28:24.060 --> 00:28:26.380
and so they're much more ubiquitous.

00:28:26.380 --> 00:28:28.380
They're all over the place.

00:28:28.380 --> 00:28:32.320
And they allow access to the ITRF

00:28:32.320 --> 00:28:34.190
because they're so common

00:28:34.190 --> 00:28:36.190
and because they're so inexpensive,

00:28:36.190 --> 00:28:37.280
and also because

00:28:38.290 --> 00:28:41.110
the GPS satellites, the GNSS satellites,

00:28:41.110 --> 00:28:42.410
they just broadcast their signal,

00:28:42.410 --> 00:28:44.100
and anyone can access that signal.

00:28:44.100 --> 00:28:45.750
You don't have to have any special stuff,

00:28:45.750 --> 00:28:48.410
other than a GNSS receiver, to use it for positioning,

00:28:48.410 --> 00:28:50.727
so it really allows access to the ITRF.

00:28:50.727 --> 00:28:54.330
And I would argue that's GNSS' most important role

00:28:54.330 --> 00:28:55.760
for the realization of the ITRF

00:28:55.760 --> 00:28:57.203
is actual dissemination.

00:28:59.130 --> 00:29:00.630
And to sorta drive that point home,

00:29:00.630 --> 00:29:02.580
I'm showing this screen capture that I took

00:29:02.580 --> 00:29:04.310
from the IERS website

00:29:04.310 --> 00:29:05.950
showing their network that was used

00:29:05.950 --> 00:29:09.060
for the realization of the ITRF2014.

00:29:09.060 --> 00:29:11.300
And the green dots here are showing

00:29:11.300 --> 00:29:15.480
locations of SLR stations.

00:29:15.480 --> 00:29:18.020
The black dots are showing DORIS stations.

00:29:18.020 --> 00:29:20.070
The red are showing VLBI,

00:29:20.070 --> 00:29:22.190
and the blue are showing GNSS stations.

00:29:22.190 --> 00:29:23.510
And it's pretty easy to see

00:29:23.510 --> 00:29:25.710
there are way more blue dots

00:29:25.710 --> 00:29:27.630
than there are of any other color,

00:29:27.630 --> 00:29:29.770
so a lot more GNSS stations.

00:29:29.770 --> 00:29:31.360
So maybe they don't give us the center of mass,

00:29:31.360 --> 00:29:34.030
but they're all over the place and they allow us

00:29:34.030 --> 00:29:35.160
to access the frame.

00:29:35.160 --> 00:29:37.480
And I'm gonna explain what I mean by that

00:29:37.480 --> 00:29:39.160
in some of these later slides,

00:29:39.160 --> 00:29:40.220
but I just kinda wanna show

00:29:40.220 --> 00:29:43.230
that GNSS is, by far, the most common type

00:29:43.230 --> 00:29:45.023
of ground station that we have.

00:29:48.360 --> 00:29:52.020
And before I get into how we access the ITRF

00:29:52.020 --> 00:29:54.020
here at NGS, I just wanna give a quick shout-out

00:29:54.020 --> 00:29:55.333
to our field crews.

00:29:56.280 --> 00:29:58.790
You know, as I said in the earlier slides,

00:29:58.790 --> 00:29:59.990
there's a variety of techniques

00:29:59.990 --> 00:30:01.920
that go into the realization of the ITRF,

00:30:01.920 --> 00:30:04.680
VLBI, SLR, DORIS, GNSS.

00:30:04.680 --> 00:30:06.720
And these are all observing different things.

00:30:06.720 --> 00:30:09.270
They have different things that they're measuring.

00:30:09.270 --> 00:30:11.170
And so it's really critical

00:30:11.170 --> 00:30:13.270
that we have sites where these techniques

00:30:13.270 --> 00:30:16.140
are co-located, where we have VLBI and SLR

00:30:16.140 --> 00:30:18.540
in the same place, where we have VLBI and GNSS

00:30:18.540 --> 00:30:20.850
or DORIS in the same place.

00:30:20.850 --> 00:30:22.870
And our field crews here at NGS,

00:30:22.870 --> 00:30:24.680
they actually go out and measure

00:30:24.680 --> 00:30:26.930
the offsets between the physical points

00:30:28.124 --> 00:30:29.920
that these hold so that we can tie

00:30:29.920 --> 00:30:32.060
these observations together during the realization

00:30:32.060 --> 00:30:33.720
of the ITRF so that

00:30:35.060 --> 00:30:36.330
when the ITRF is realized,

00:30:36.330 --> 00:30:38.490
these observations can be tied together.

00:30:38.490 --> 00:30:40.580
And our NGS field crews are actually

00:30:40.580 --> 00:30:43.270
really internationally recognized

00:30:43.270 --> 00:30:45.610
at the best at doing these local ties.

00:30:45.610 --> 00:30:47.700
So I just wanna give them a quick shout.

00:30:47.700 --> 00:30:49.650
And I'm showing Steve Breidenbach,

00:30:49.650 --> 00:30:52.500
who was my boss for a short time,

00:30:52.500 --> 00:30:54.923
doing some work here in Kauai.

00:30:58.364 --> 00:30:59.410
Okay, so in those last slides,

00:30:59.410 --> 00:31:00.910
I talked about the realization

00:31:02.140 --> 00:31:04.320
of the ITRF, and obviously didn't go into

00:31:04.320 --> 00:31:05.770
too much detail about the mathematics

00:31:05.770 --> 00:31:07.090
or anything like that, but just sorta wanted

00:31:07.090 --> 00:31:09.190
to introduce you to some of the techniques

00:31:09.190 --> 00:31:11.963
that we use, that are used for the realization.

00:31:13.020 --> 00:31:14.260
And in the next slides,

00:31:14.260 --> 00:31:15.093
I wanna talk about

00:31:15.093 --> 00:31:17.750
the International GNSS Service, the IGS,

00:31:17.750 --> 00:31:19.860
they fall under the umbrella of the IAG,

00:31:19.860 --> 00:31:22.450
the International Association of Geodesy,

00:31:22.450 --> 00:31:25.950
and the work that they do to realize the ITRF

00:31:25.950 --> 00:31:28.763
using only GNSS observations.

00:31:29.730 --> 00:31:32.310
And how we use that, their realization of the ITRF,

00:31:32.310 --> 00:31:33.550
to access the ITRF.

00:31:36.450 --> 00:31:38.420
So the International GNSS Service,

00:31:38.420 --> 00:31:41.010
their realization of the frame of the ITRF

00:31:41.010 --> 00:31:42.630
is computed using only data

00:31:42.630 --> 00:31:45.270
from GNSS tracking stations in the IGS network.

00:31:45.270 --> 00:31:49.020
So they're not using VLBI or SLR or DORIS.

00:31:49.020 --> 00:31:51.050
They're using only GNSS.

00:31:51.050 --> 00:31:53.660
But because of the way the ITRF is realized,

00:31:53.660 --> 00:31:54.980
with those co-located sites

00:31:54.980 --> 00:31:57.230
and these observations being tied together,

00:31:57.230 --> 00:31:59.420
they are able to align their frame,

00:31:59.420 --> 00:32:01.133
their realization of the ITRF,

00:32:02.120 --> 00:32:04.680
to the ITRF so that the origin,

00:32:04.680 --> 00:32:07.900
orientation and scale are identical.

00:32:07.900 --> 00:32:09.680
So it truly is

00:32:11.740 --> 00:32:14.060
a GNSS-only realization of that ITRF,

00:32:14.060 --> 00:32:16.510
of the International Terrestrial Reference Frame.

00:32:17.810 --> 00:32:20.660
And so I kinda wanna give a explanation

00:32:20.660 --> 00:32:22.130
of what that looks like.

00:32:22.130 --> 00:32:24.080
So to do that, I'm gonna

00:32:24.080 --> 00:32:25.350
play make-believe with you here.

00:32:25.350 --> 00:32:27.490
I've made a little cartoon for you.

00:32:27.490 --> 00:32:29.170
So I'm just showing on the right

00:32:29.170 --> 00:32:31.200
a GNSS ground tracking station.

00:32:31.200 --> 00:32:32.710
Like we said earlier, it's fixed

00:32:32.710 --> 00:32:33.670
to the surface of the Earth.

00:32:33.670 --> 00:32:35.810
It's holding some physical point

00:32:35.810 --> 00:32:37.470
and it's observing those satellites

00:32:37.470 --> 00:32:40.090
up in space. and it's estimating,

00:32:40.090 --> 00:32:41.690
and we're estimating its position.

00:32:41.690 --> 00:32:44.023
We're estimating coordinates for it.

00:32:44.860 --> 00:32:46.770
And then down here, I'm showing a plot.

00:32:46.770 --> 00:32:48.910
And on the vertical axes,

00:32:48.910 --> 00:32:50.240
I'm showing position,

00:32:50.240 --> 00:32:52.560
so dX, dY, dZ.

00:32:52.560 --> 00:32:54.710
And on the horizontal axis, I'm showing time.

00:32:54.710 --> 00:32:57.110
So we're going from the year 1995

00:32:57.110 --> 00:32:58.680
to the year 2020.

00:32:58.680 --> 00:33:00.250
And all these little dots in the plot

00:33:00.250 --> 00:33:02.800
represent an estimate of the position

00:33:02.800 --> 00:33:05.660
of the station for each day.

00:33:05.660 --> 00:33:08.200
And I wanna say here that this is actually,

00:33:08.200 --> 00:33:09.380
we call this a time series.

00:33:09.380 --> 00:33:11.710
It's a GNSS position time series,

00:33:11.710 --> 00:33:13.610
and this is actually a synthetic time series.

00:33:13.610 --> 00:33:15.810
It doesn't represent the position of any

00:33:15.810 --> 00:33:17.200
actual station on Earth.

00:33:17.200 --> 00:33:20.900
I just made it using sort of fake information

00:33:20.900 --> 00:33:23.250
for the purposes of illustration for this here.

00:33:24.380 --> 00:33:26.420
So let's focus on this top plot here,

00:33:26.420 --> 00:33:29.000
the x position of the station.

00:33:29.000 --> 00:33:31.440
So what pops out to me immediately

00:33:31.440 --> 00:33:35.010
is that this thing is not standing still, right?

00:33:35.010 --> 00:33:36.700
As we march forward in time,

00:33:36.700 --> 00:33:39.050
the x position is increasing.

00:33:39.050 --> 00:33:40.855
The position of the station,

00:33:40.855 --> 00:33:42.380
the position that this station is holding,

00:33:42.380 --> 00:33:43.940
is changing in time.

00:33:43.940 --> 00:33:45.860
So if we're trying to define a reference frame

00:33:45.860 --> 00:33:48.740
using this station, we have a decision to make, right?

00:33:48.740 --> 00:33:50.300
We have to decide, well,

00:33:50.300 --> 00:33:52.150
at what time are we gonna assign

00:33:52.150 --> 00:33:53.697
the coordinate to this station?

00:33:53.697 --> 00:33:55.820
And so we could call this our reference epoch

00:33:55.820 --> 00:33:57.430
or our t-naught.

00:33:57.430 --> 00:33:58.983
And we could say, okay, 2010,

00:34:00.260 --> 00:34:02.000
we like that, it was a great year.

00:34:02.000 --> 00:34:03.740
We can select that year

00:34:03.740 --> 00:34:04.847
as our reference epoch.

00:34:04.847 --> 00:34:06.970
And we can say, okay, great,

00:34:06.970 --> 00:34:08.880
that's our position for this station.

00:34:08.880 --> 00:34:10.640
We have an x-naught, y-naught, z-naught

00:34:10.640 --> 00:34:12.120
at the reference epoch,

00:34:12.120 --> 00:34:14.210
and those are the coordinates of the station,

00:34:14.210 --> 00:34:16.271
and just leave it at that.

00:34:16.271 --> 00:34:17.430
But it's pretty easy to see

00:34:17.430 --> 00:34:19.930
that as we get away from the reference epoch,

00:34:19.930 --> 00:34:21.940
as we keep marching forward in time,

00:34:21.940 --> 00:34:24.860
our position is gonna diverge from that x-naught, right?

00:34:24.860 --> 00:34:25.910
If we just drew that x-naught forward

00:34:25.910 --> 00:34:27.960
and it's a flat line going forward,

00:34:27.960 --> 00:34:29.290
we would slowly diverge

00:34:29.290 --> 00:34:31.240
from our actual estimates of where

00:34:31.240 --> 00:34:32.960
that station is.

00:34:32.960 --> 00:34:35.740
So we can see that this sort of assigning coordinates

00:34:35.740 --> 00:34:37.560
with the reference epoch and calling that good,

00:34:37.560 --> 00:34:39.810
that's not really gonna work for us anymore.

00:34:39.810 --> 00:34:42.273
We need to introduce the concept of velocity.

00:34:43.300 --> 00:34:44.790
So now what we're gonna do

00:34:44.790 --> 00:34:46.240
is we're gonna say we're gonna estimate

00:34:46.240 --> 00:34:50.220
the line that goes through these dots the best.

00:34:50.220 --> 00:34:51.750
We're still gonna use a reference epoch.

00:34:51.750 --> 00:34:54.680
We're gonna estimate the coordinates

00:34:54.680 --> 00:34:57.100
of the station at the reference epoch t-naught,

00:34:57.100 --> 00:34:58.900
but then we're also gonna add in this velocity.

00:34:58.900 --> 00:35:01.470
We're gonna add the ability to track

00:35:01.470 --> 00:35:03.230
the position of that station

00:35:03.230 --> 00:35:04.460
as we move through time.

00:35:04.460 --> 00:35:06.130
And I just wanna give a shout-out

00:35:06.130 --> 00:35:08.920
to my colleague Dr. Dr Smith here.

00:35:08.920 --> 00:35:10.950
I believe he coined the phrase

00:35:10.950 --> 00:35:14.300
coordinate function, and that's exactly what this is.

00:35:14.300 --> 00:35:15.980
It's a function of time

00:35:15.980 --> 00:35:18.890
that describe the coordinates of this station

00:35:18.890 --> 00:35:21.173
as we go forward, as we go through time.

00:35:22.320 --> 00:35:25.560
And this is literally how the IGS now defines

00:35:25.560 --> 00:35:26.920
their reference frame.

00:35:26.920 --> 00:35:28.260
They give us coordinates

00:35:28.260 --> 00:35:29.690
at the reference epoch,

00:35:29.690 --> 00:35:32.510
along with a velocity that's the best fit

00:35:32.510 --> 00:35:34.570
to all these dots as we go back

00:35:34.570 --> 00:35:35.500
and forth through time.

00:35:35.500 --> 00:35:37.930
So the best red line that fits through here

00:35:37.930 --> 00:35:39.510
describes the coordinates of the station.

00:35:39.510 --> 00:35:42.590
Now, no longer are we talking about just a static,

00:35:42.590 --> 00:35:44.060
fixed set of coordinates.

00:35:44.060 --> 00:35:45.080
We have to talk about

00:35:45.080 --> 00:35:46.643
coordinates with velocities.

00:35:49.310 --> 00:35:51.070
So I just wanted to show this figure really quick.

00:35:51.070 --> 00:35:54.520
This is a figure from Dr. Altamimi's paper,

00:35:54.520 --> 00:35:58.020
his 2016 paper with colleagues that was in JGR.

00:35:58.020 --> 00:36:00.110
And this shows the velocity field

00:36:00.110 --> 00:36:02.550
for all of the ITRF stations

00:36:02.550 --> 00:36:05.200
from the realization, the ITRF2014.

00:36:05.200 --> 00:36:08.560
And, I mean, you just look at it, you see these arrows.

00:36:08.560 --> 00:36:09.600
So the blue dots

00:36:11.660 --> 00:36:13.299
are the reference epoch coordinates,

00:36:13.299 --> 00:36:15.890
and the red arrows are the associated velocities.

00:36:15.890 --> 00:36:17.640
And you just see that, I mean,

00:36:17.640 --> 00:36:20.020
the static concept of a reference frame

00:36:20.020 --> 00:36:21.320
just isn't gonna cut it anymore.

00:36:21.320 --> 00:36:23.090
There's so much motion.

00:36:23.090 --> 00:36:25.800
We have deformation on the West Coast of North America.

00:36:25.800 --> 00:36:27.320
We have wholesale plate rotation

00:36:27.320 --> 00:36:28.930
of the North American Plate.

00:36:28.930 --> 00:36:31.690
Things are really moving in this frame,

00:36:31.690 --> 00:36:35.070
and so we really need to capture that with,

00:36:35.070 --> 00:36:36.930
again, reference epoch coordinates and velocities

00:36:36.930 --> 00:36:38.693
using those coordinate functions.

00:36:42.260 --> 00:36:43.560
Okay, so in the last slide,

00:36:43.560 --> 00:36:46.460
I talked about how the IGS,

00:36:46.460 --> 00:36:48.173
the International GNSS Service,

00:36:49.130 --> 00:36:52.170
realizes the ITRF

00:36:52.170 --> 00:36:54.180
by estimating positions

00:36:54.180 --> 00:36:56.447
that are consistent with the ITRF frame,

00:36:56.447 --> 00:36:58.480
and then fitting coordinate functions

00:36:59.820 --> 00:37:02.880
to those estimates as a function of time,

00:37:02.880 --> 00:37:05.150
and then giving us, disseminating,

00:37:05.150 --> 00:37:08.430
those reference epoch coordinates and their velocities.

00:37:08.430 --> 00:37:11.070
And that is, in a very literal sense,

00:37:11.070 --> 00:37:14.120
the actual realization of the frame.

00:37:14.120 --> 00:37:16.970
The coordinates and velocities for those stations

00:37:17.970 --> 00:37:19.123
realize the frame.

00:37:20.290 --> 00:37:22.330
And then I wanna talk about, in the next few slides,

00:37:22.330 --> 00:37:25.330
how we at NGS use those coordinates

00:37:25.330 --> 00:37:28.010
and velocities to align the United States

00:37:28.010 --> 00:37:31.470
National Spatial Reference System to the ITRF.

00:37:31.470 --> 00:37:33.020
And we actually use those coordinates

00:37:33.020 --> 00:37:35.220
and velocities from the IGS to do so.

00:37:35.220 --> 00:37:37.463
So I wanna talk about how we do that.

00:37:39.670 --> 00:37:42.920
So quickly, when we say the NSRS,

00:37:42.920 --> 00:37:44.230
National Spatial Reference System,

00:37:44.230 --> 00:37:47.270
is aligned with the ITRF and IGS frames,

00:37:47.270 --> 00:37:49.080
what we mean is that stations

00:37:49.080 --> 00:37:51.950
in the NOAA CORS Network, the NCN,

00:37:51.950 --> 00:37:54.720
are aligned with those frames in the computation

00:37:54.720 --> 00:37:56.980
of our Multi-Year CORS Solutions,

00:37:56.980 --> 00:37:59.023
so-called MYCSs.

00:38:02.250 --> 00:38:03.700
So I just wanted to quickly show you

00:38:03.700 --> 00:38:06.690
this screen grab I have from our website

00:38:06.690 --> 00:38:09.090
at NGS showing a map

00:38:09.090 --> 00:38:10.900
of the NOAA CORS Network.

00:38:10.900 --> 00:38:13.870
Again, it's these GNSS ground tracking stations

00:38:13.870 --> 00:38:15.110
that are observing satellites.

00:38:15.110 --> 00:38:16.170
As you can see,

00:38:16.170 --> 00:38:17.780
there is just a whole bunch of 'em,

00:38:17.780 --> 00:38:20.393
on order of 2,000, 3,000 at this point, I think.

00:38:21.350 --> 00:38:23.610
And they're all tracking those satellites.

00:38:23.610 --> 00:38:25.610
And we here at NGS

00:38:25.610 --> 00:38:27.910
are tracking their position as a function of time,

00:38:27.910 --> 00:38:30.433
just like I showed you in those previous slides.

00:38:32.700 --> 00:38:34.120
And we use these stations

00:38:34.120 --> 00:38:35.530
to align the United States

00:38:35.530 --> 00:38:38.140
National Spatial Reference System

00:38:38.140 --> 00:38:39.590
to the ITRF.

00:38:39.590 --> 00:38:40.850
And in the next few slides,

00:38:40.850 --> 00:38:42.610
I'm gonna explain how we do that

00:38:42.610 --> 00:38:44.210
using a really simple, well,

00:38:44.210 --> 00:38:46.933
hopefully simple cartoon that I came up with.

00:38:49.130 --> 00:38:50.370
Okay, so what I'm showing you here

00:38:50.370 --> 00:38:52.220
are three axes,

00:38:52.220 --> 00:38:53.330
two spatial axes.

00:38:53.330 --> 00:38:54.830
So I've got the x-axis coming out

00:38:54.830 --> 00:38:56.880
of the page at you, and the y-axis,

00:38:56.880 --> 00:38:58.130
this vertical axis here,

00:38:58.130 --> 00:39:00.830
and the horizontal axis is meant to represent time.

00:39:00.830 --> 00:39:02.230
So we're moving forward in time

00:39:02.230 --> 00:39:04.330
as we go to the right.

00:39:04.330 --> 00:39:06.470
And I'm gonna show you some snapshots in time

00:39:06.470 --> 00:39:08.750
of this two-dimensional space.

00:39:08.750 --> 00:39:11.060
So here I've got, again, our reference epoch,

00:39:11.060 --> 00:39:13.340
t-naught, and I'm showing you a snapshot

00:39:13.340 --> 00:39:16.440
of what this two-dimensional space looks like.

00:39:16.440 --> 00:39:17.790
And in this two-dimensional space,

00:39:17.790 --> 00:39:20.360
I'm plotting the positions

00:39:20.360 --> 00:39:23.820
of IGS GNSS tracking stations.

00:39:23.820 --> 00:39:25.270
So again, this is just a cartoon.

00:39:25.270 --> 00:39:27.410
We've got a little five-station network.

00:39:27.410 --> 00:39:29.820
And these are meant to show

00:39:29.820 --> 00:39:31.880
the reference epoch coordinates

00:39:31.880 --> 00:39:34.030
of these IGS stations, as defined

00:39:34.030 --> 00:39:37.120
by the IGS in their realization of the frame.

00:39:37.120 --> 00:39:39.680
But remember I said that the IGS

00:39:39.680 --> 00:39:42.350
doesn't use static coordinates anymore.

00:39:42.350 --> 00:39:44.500
They're giving us velocities now as well.

00:39:44.500 --> 00:39:46.380
So we have to account for the trajectory

00:39:46.380 --> 00:39:48.300
of these stations as we go through time.

00:39:48.300 --> 00:39:50.290
So these dashed lines are meant to show

00:39:50.290 --> 00:39:51.920
the trajectory of these stations

00:39:51.920 --> 00:39:54.144
as we march ahead in time.

00:39:54.144 --> 00:39:55.810
And I'm gonna show you snapshots

00:39:55.810 --> 00:39:57.870
at t1, t2, t3.

00:39:57.870 --> 00:39:59.830
I'm gonna represent this two-dimensional space

00:39:59.830 --> 00:40:01.930
at these three different times.

00:40:01.930 --> 00:40:04.440
And where these dashed lines pierce

00:40:04.440 --> 00:40:07.280
these two-dimensional spaces, at these snapshots,

00:40:07.280 --> 00:40:09.710
that shows where the IGS says

00:40:09.710 --> 00:40:12.063
the station should be at these times.

00:40:14.180 --> 00:40:16.000
So we have, we've used

00:40:16.000 --> 00:40:17.784
the reference epoch coordinates,

00:40:17.784 --> 00:40:20.130
and the velocity is given to us by the IGS

00:40:20.130 --> 00:40:22.480
to predict where these stations

00:40:22.480 --> 00:40:25.200
should be at times t1,

00:40:25.200 --> 00:40:26.033
t2 and t3.

00:40:27.085 --> 00:40:29.463
And that's what those little black dots represent.

00:40:30.420 --> 00:40:33.610
So what we do at NGS is we actually go ahead

00:40:33.610 --> 00:40:34.990
and we say, okay, well,

00:40:34.990 --> 00:40:36.600
we're gonna also estimate

00:40:36.600 --> 00:40:38.960
the positions of those IGS stations on our own,

00:40:38.960 --> 00:40:40.730
using our own computers, our own software,

00:40:40.730 --> 00:40:42.760
our own techniques, and we're gonna estimate

00:40:42.760 --> 00:40:44.460
the position for those stations.

00:40:44.460 --> 00:40:46.250
And what we find is our estimates

00:40:46.250 --> 00:40:49.140
don't quite agree with where the IGS says

00:40:49.140 --> 00:40:51.660
those stations should be at these snapshots in time.

00:40:51.660 --> 00:40:52.870
You see, there's some misalignment

00:40:52.870 --> 00:40:55.403
between our estimate and the IGS estimate.

00:40:56.420 --> 00:40:57.450
Well, that's okay.

00:40:57.450 --> 00:40:58.940
What we do is we compute

00:40:58.940 --> 00:41:01.180
some transformation parameters,

00:41:01.180 --> 00:41:02.760
and we're actually able to use those

00:41:02.760 --> 00:41:06.000
to transform our sort of realization

00:41:06.000 --> 00:41:09.110
of this little network so that it aligns

00:41:09.110 --> 00:41:10.700
with where the IGS says

00:41:10.700 --> 00:41:12.950
these stations should be in time.

00:41:12.950 --> 00:41:14.300
So we've gone ahead and we've estimated

00:41:14.300 --> 00:41:15.970
these positions, and we've used

00:41:16.836 --> 00:41:18.340
the transformation parameters to transform

00:41:18.340 --> 00:41:20.890
our realization and align this little mini network,

00:41:20.890 --> 00:41:22.230
these little five stations,

00:41:22.230 --> 00:41:24.900
with where the IGS says that they should be.

00:41:24.900 --> 00:41:27.030
Okay, that's a nice little academic exercise,

00:41:27.030 --> 00:41:28.880
but where's the actual utility in that?

00:41:28.880 --> 00:41:30.490
How is that useful?

00:41:30.490 --> 00:41:32.920
Well, that becomes useful when we hang

00:41:32.920 --> 00:41:35.730
the NOAA CORS Network on top of our estimates

00:41:35.730 --> 00:41:37.960
for where these IGS stations should be.

00:41:37.960 --> 00:41:40.200
So these green dots are meant to represent

00:41:40.200 --> 00:41:42.930
the NOAA CORS Network, those CORS stations.

00:41:42.930 --> 00:41:45.000
I showed you that map just a few slides ago.

00:41:45.000 --> 00:41:47.270
So our network, so we go ahead and estimate.

00:41:47.270 --> 00:41:49.677
At all these times, we estimate the position

00:41:49.677 --> 00:41:52.210
of the IGS stations, these yellow dots.

00:41:52.210 --> 00:41:54.080
And at the same time, we estimate positions

00:41:54.080 --> 00:41:56.620
for stations in the NOAA CORS Network.

00:41:56.620 --> 00:41:58.070
And then we go through the same procedure,

00:41:58.070 --> 00:42:00.400
where we compute transformation parameters.

00:42:00.400 --> 00:42:01.930
We align the IGS network,

00:42:01.930 --> 00:42:04.820
the IGS stations with where the IGS says

00:42:04.820 --> 00:42:07.970
they should be at each of these snapshots in time,

00:42:07.970 --> 00:42:10.010
and we bring the CORS stations along with it.

00:42:10.010 --> 00:42:12.083
You see as I align the two,

00:42:13.868 --> 00:42:15.880
the CORS stations come along for the ride.

00:42:15.880 --> 00:42:18.430
And then that way, at these snapshots in time,

00:42:18.430 --> 00:42:20.750
we actually align our estimates

00:42:20.750 --> 00:42:22.500
for the positions of the NOAA CORS Network

00:42:22.500 --> 00:42:25.360
to be consistent with the IGS frame.

00:42:25.360 --> 00:42:28.030
So in this way, we are able to realize the ITRF

00:42:28.030 --> 00:42:30.470
by using the IGS realization

00:42:30.470 --> 00:42:31.880
and bring the NOAA CORS Network

00:42:31.880 --> 00:42:33.700
along for the ride to snap everything

00:42:33.700 --> 00:42:36.283
into a self-consistent frame.

00:42:37.280 --> 00:42:39.690
And then we take that a little, one step further

00:42:39.690 --> 00:42:41.950
when we do the Multi-Year CORS Solutions.

00:42:41.950 --> 00:42:43.960
We say, okay, we now have

00:42:43.960 --> 00:42:46.440
this self-consistent realization where everything

00:42:46.440 --> 00:42:49.006
is in this IGS frame at these times,

00:42:49.006 --> 00:42:50.680
t1, t2 and t3.

00:42:50.680 --> 00:42:52.560
Now we're gonna estimate velocities

00:42:52.560 --> 00:42:54.970
for those stations, just like the IGS did.

00:42:54.970 --> 00:42:56.820
We recognize that these,

00:42:56.820 --> 00:42:58.810
the NOAA CORS Network stations,

00:42:58.810 --> 00:43:01.047
their positions are changing as a function of time,

00:43:01.047 --> 00:43:02.940
and we wanna represent the velocity,

00:43:02.940 --> 00:43:04.650
the trajectory, that best fits

00:43:06.076 --> 00:43:07.140
those changing positions.

00:43:07.140 --> 00:43:09.590
So we do that, and we can back project

00:43:09.590 --> 00:43:11.850
to the exact same reference epoch.

00:43:11.850 --> 00:43:15.670
For ITR2014, that would be 2010.

00:43:15.670 --> 00:43:18.550
And then we come up with reference epoch coordinates

00:43:18.550 --> 00:43:21.630
and velocities for the NOAA CORS Network

00:43:21.630 --> 00:43:23.470
in a way that is aligned and consistent

00:43:23.470 --> 00:43:25.817
with the ITRF and, in particular,

00:43:25.817 --> 00:43:29.107
the IGS realization of the ITRF.

00:43:29.107 --> 00:43:31.150
And this is actually what we provide to our users.

00:43:31.150 --> 00:43:33.090
We provide reference epoch coordinates

00:43:33.090 --> 00:43:35.120
and velocities to our users.

00:43:35.120 --> 00:43:36.920
And then our users can do a very,

00:43:36.920 --> 00:43:39.640
very similar procedure to what I just laid out.

00:43:39.640 --> 00:43:42.290
Exactly what I just showed, how we align to the IGS,

00:43:42.290 --> 00:43:44.360
our users are able to use these

00:43:44.360 --> 00:43:46.770
reference epoch coordinates and velocities

00:43:46.770 --> 00:43:48.360
to align their measurements,

00:43:48.360 --> 00:43:49.740
their points of interest,

00:43:49.740 --> 00:43:51.223
to the NSRS.

00:43:52.468 --> 00:43:54.530
And something very similar to this happens

00:43:54.530 --> 00:43:57.480
when you submit your GNSS data to OPUS,

00:43:57.480 --> 00:43:59.480
the Online Positioning User Service.

00:43:59.480 --> 00:44:01.720
A very similar-type procedure happens at your epoch

00:44:01.720 --> 00:44:03.420
and points of interest, where things are aligned

00:44:03.420 --> 00:44:06.110
in this way to be in the NSRS,

00:44:06.110 --> 00:44:08.203
to be in the ITRF.

00:44:10.360 --> 00:44:11.430
And I just quickly wanna show,

00:44:11.430 --> 00:44:13.580
for our super users out there,

00:44:13.580 --> 00:44:16.100
for people who are very familiar with this stuff,

00:44:16.100 --> 00:44:18.100
when you actually pull down a position

00:44:18.100 --> 00:44:20.170
and velocity file for a particular CORS.

00:44:20.170 --> 00:44:23.153
So I'm showing a CORS that's in Illinois, ILSA.

00:44:24.550 --> 00:44:26.500
When you download one of these files,

00:44:26.500 --> 00:44:29.430
we actually, that's exactly what we give you here.

00:44:29.430 --> 00:44:31.170
We give you X, Y, and Z

00:44:31.170 --> 00:44:32.740
reference epoch coordinates,

00:44:32.740 --> 00:44:34.297
like these green dots here,

00:44:34.297 --> 00:44:35.910
and the associated velocities,

00:44:35.910 --> 00:44:37.820
VX, VY, VZ, these dashed lines.

00:44:37.820 --> 00:44:40.390
And that's what you're actually pulling down,

00:44:40.390 --> 00:44:41.784
these coordinates that have been aligned

00:44:41.784 --> 00:44:42.784
to the ITRF.

00:44:45.760 --> 00:44:46.947
Okay, so this is my last slide.

00:44:46.947 --> 00:44:48.220
And I just wanna say

00:44:49.100 --> 00:44:51.140
that this is ongoing work.

00:44:51.140 --> 00:44:52.180
You know, as we talked about,

00:44:52.180 --> 00:44:54.940
the Earth is a dynamic system, things are moving.

00:44:54.940 --> 00:44:58.180
And we recently had the Multi-Year CORS Solution 2

00:44:58.180 --> 00:45:00.300
come out, MYCS2, and that was aligned

00:45:00.300 --> 00:45:03.243
with ITRF2014, IGS14.

00:45:04.190 --> 00:45:05.950
We aligned the National Spatial Reference System,

00:45:05.950 --> 00:45:09.342
the NOAA CORS Network, with those frames,

00:45:09.342 --> 00:45:10.175
and that was a huge effort.

00:45:10.175 --> 00:45:11.757
And again, another shout-out to my colleague

00:45:11.757 --> 00:45:13.860
Jarir Saleh, who really did the heavy lifting

00:45:13.860 --> 00:45:16.600
on that project and got us

00:45:16.600 --> 00:45:18.690
coordinates for the NOAA CORS Network

00:45:18.690 --> 00:45:20.600
for the National Spatial Reference System

00:45:20.600 --> 00:45:24.240
that are consistent and aligned with the ITRF2014.

00:45:24.240 --> 00:45:26.070
But like we said, the Earth's not static.

00:45:26.070 --> 00:45:27.810
It's a dynamic system, things change.

00:45:27.810 --> 00:45:31.630
So since the ITRF2014 was first released,

00:45:31.630 --> 00:45:33.363
we've had massive earthquakes.

00:45:34.960 --> 00:45:37.010
Antennas have changed and various things have happened

00:45:37.010 --> 00:45:39.530
to disrupt the frame so that those velocities

00:45:39.530 --> 00:45:40.960
that I showed you, those nice, neat,

00:45:40.960 --> 00:45:42.680
straight lines that I showed you

00:45:42.680 --> 00:45:44.720
in those earlier pictures,

00:45:44.720 --> 00:45:47.280
they become less straight and more jagged

00:45:47.280 --> 00:45:48.730
or they have funky shapes.

00:45:48.730 --> 00:45:52.640
So at different times,

00:45:52.640 --> 00:45:54.710
they actually have to release updates

00:45:54.710 --> 00:45:55.710
to the ITRF.

00:45:55.710 --> 00:45:57.880
And so recently, one was introduced

00:45:57.880 --> 00:46:00.270
just a few months ago, IGb14,

00:46:00.270 --> 00:46:01.730
which accounted for some large earthquakes

00:46:01.730 --> 00:46:03.337
that had happened, some various other things

00:46:03.337 --> 00:46:04.170
that had happened.

00:46:04.170 --> 00:46:06.100
And so we had to go in and make

00:46:06.100 --> 00:46:09.240
some small corrections to some of our station coordinates

00:46:09.240 --> 00:46:12.630
so that we could be consistent with IGb14,

00:46:12.630 --> 00:46:16.260
which is another realization of the ITRF2014.

00:46:16.260 --> 00:46:17.840
And this work is never gonna stop.

00:46:17.840 --> 00:46:18.930
It's gonna keep going.

00:46:18.930 --> 00:46:21.490
The ITRF2020 is forthcoming.

00:46:21.490 --> 00:46:23.870
It'll be released sometime in probably late 2021,

00:46:23.870 --> 00:46:25.610
maybe early 2022.

00:46:25.610 --> 00:46:26.610
And we're gonna have to go through

00:46:26.610 --> 00:46:27.950
this process again at NGS.

00:46:27.950 --> 00:46:28.783
We're gonna have to come out

00:46:28.783 --> 00:46:30.620
with Multi-Year CORS Solution 3.

00:46:30.620 --> 00:46:33.200
We're gonna have to realign to the new ITRF

00:46:33.200 --> 00:46:35.255
and we're gonna have to keep doing so

00:46:35.255 --> 00:46:37.610
every so many years to keep ourselves coordinate,

00:46:37.610 --> 00:46:40.400
to provide that level of geodetic control

00:46:40.400 --> 00:46:42.710
to our users that they demand

00:46:44.278 --> 00:46:46.810
so we can keep delivering on that promise.

00:46:46.810 --> 00:46:47.860
If you're interested in learning more

00:46:47.860 --> 00:46:50.220
about Multi-Year CORS Solution 2,

00:46:50.220 --> 00:46:52.550
I have the URL for the website

00:46:53.470 --> 00:46:55.245
that describes that.

00:46:55.245 --> 00:46:56.140
That was a huge lift for us

00:46:56.140 --> 00:46:57.290
and a really awesome project.

00:46:57.290 --> 00:46:58.620
And big shout-out to my colleagues

00:46:58.620 --> 00:46:59.720
who did all that work.

00:47:00.670 --> 00:47:02.380
And I'm just gonna leave you with a recap

00:47:02.380 --> 00:47:05.680
of what I tried to explain today up here,

00:47:05.680 --> 00:47:07.710
and turn it back over to Steve

00:47:07.710 --> 00:47:09.673
and take any questions, if there are any.

00:47:15.660 --> 00:47:18.070
<v Steve>Thank you very much, Phillip.</v>

00:47:18.070 --> 00:47:20.500
We have a few questions

00:47:20.500 --> 00:47:22.620
that have come in via

00:47:22.620 --> 00:47:24.930
the question box.

00:47:24.930 --> 00:47:27.010
<v Phillip>Hopefully not from the experts.</v>

00:47:27.010 --> 00:47:31.090
Oh, sorry. (Steve and Phillip laugh)

00:47:31.090 --> 00:47:33.570
<v Steve>The first question is why is it important</v>

00:47:33.570 --> 00:47:36.220
to have the center of the ellipsoid

00:47:36.220 --> 00:47:38.683
coincide with the Earth's center of mass?

00:47:44.840 --> 00:47:47.200
<v Phillip>Well, in the context</v>

00:47:47.200 --> 00:47:49.600
of a global reference frame,

00:47:49.600 --> 00:47:51.170
my understanding is that

00:47:52.760 --> 00:47:54.150
since we are using

00:47:56.160 --> 00:47:59.093
satellites to sort of define this frame,

00:48:00.987 --> 00:48:02.990
and we know that those satellites orbit

00:48:02.990 --> 00:48:04.340
the Earth's center of mass,

00:48:05.260 --> 00:48:07.900
since we wanna use those data

00:48:07.900 --> 00:48:10.483
to help realize the frame,

00:48:11.570 --> 00:48:15.920
it's very helpful if the center

00:48:15.920 --> 00:48:18.670
of our frame is coincident,

00:48:18.670 --> 00:48:21.190
co-located with the center of mass of the Earth.

00:48:21.190 --> 00:48:22.763
It simplifies things.

00:48:25.479 --> 00:48:26.420
In terms of like that example

00:48:26.420 --> 00:48:28.253
with the traditional geodetic datums

00:48:28.253 --> 00:48:29.393
that I showed,

00:48:32.466 --> 00:48:33.893
it's not necessarily, I guess, important

00:48:33.893 --> 00:48:36.010
that the center of the ellipsoid coincide

00:48:36.010 --> 00:48:39.070
or be co-located with the center mass of the Earth.

00:48:39.070 --> 00:48:41.250
What's really important, in a global context,

00:48:41.250 --> 00:48:42.610
is consistency, right?

00:48:42.610 --> 00:48:45.030
That's the whole point of these frames

00:48:45.030 --> 00:48:46.860
is so that we can assign coordinates

00:48:46.860 --> 00:48:48.160
in a self-consistent manner,

00:48:48.160 --> 00:48:50.160
and we don't need to sort of jump around

00:48:50.160 --> 00:48:52.050
from datum to datum to do so.

00:48:52.050 --> 00:48:53.637
So I guess it's not so much important

00:48:53.637 --> 00:48:54.710
that they're aligned with the center mass

00:48:54.710 --> 00:48:55.670
of the Earth in that context,

00:48:55.670 --> 00:48:58.880
but it is important that those reference ellipsoids

00:48:58.880 --> 00:49:01.290
are aligned with each other

00:49:01.290 --> 00:49:03.440
so that we can communicate coordinates

00:49:05.011 --> 00:49:06.830
to our colleagues in other nations

00:49:08.090 --> 00:49:10.340
in a self-consistent way.

00:49:10.340 --> 00:49:11.990
I hope that answers the question.

00:49:13.350 --> 00:49:15.750
<v Steve>Okay, thank you, Phillip.</v>

00:49:15.750 --> 00:49:17.760
And another question we received

00:49:17.760 --> 00:49:21.300
is what are the easiest free tools

00:49:21.300 --> 00:49:24.493
to convert between coordinate systems?

00:49:30.420 --> 00:49:33.480
<v Phillip>I apologize, I do not know.</v>

00:49:33.480 --> 00:49:36.943
I'll get back to that questioner.

00:49:37.790 --> 00:49:39.533
I don't know off the top of my head.

00:49:40.580 --> 00:49:41.810
<v Steve>All right, well, this may</v>

00:49:41.810 --> 00:49:44.000
be a good time to mention

00:49:44.000 --> 00:49:46.600
that any questions we receive

00:49:47.640 --> 00:49:49.890
after the webinar ends,

00:49:49.890 --> 00:49:52.730
we will send a response by email

00:49:53.920 --> 00:49:55.810
to those participants

00:49:55.810 --> 00:49:58.800
and to all of our webinar participants.

00:49:58.800 --> 00:50:01.190
Let me see if we have

00:50:01.190 --> 00:50:02.840
any other questions we can answer

00:50:02.840 --> 00:50:04.800
during the presentation.

00:50:04.800 --> 00:50:05.737
<v Phillip>Sorry, Steve, sorry.</v>

00:50:05.737 --> 00:50:07.605
Just back to that last question.

00:50:07.605 --> 00:50:09.350
I just wanna say quickly,

00:50:09.350 --> 00:50:11.640
we have our own internal set of tools

00:50:11.640 --> 00:50:12.730
that we use for this work.

00:50:12.730 --> 00:50:15.280
And I just have to apologize,

00:50:15.280 --> 00:50:18.210
I pretty much rely on those exclusively,

00:50:18.210 --> 00:50:19.600
so I'm not a good source

00:50:20.737 --> 00:50:23.260
for looking outside of our routine work

00:50:23.260 --> 00:50:26.870
to look at free programs

00:50:26.870 --> 00:50:28.480
that do those types of conversions.

00:50:28.480 --> 00:50:30.980
We have our own software that does all that stuff.

00:50:32.200 --> 00:50:34.890
<v Steve>Fair enough, and NGS does have</v>

00:50:34.890 --> 00:50:37.093
a free tool called NCAT.

00:50:38.060 --> 00:50:39.340
That's our nickname for it.

00:50:39.340 --> 00:50:42.640
It stands for NGS Coordinate Transformation

00:50:42.640 --> 00:50:45.410
and Conversion Tool.

00:50:45.410 --> 00:50:47.880
And that is a very robust tool

00:50:47.880 --> 00:50:50.993
that is free to use on our website.

00:50:52.090 --> 00:50:53.090
<v Phillip>Awesome.</v>

00:50:56.700 --> 00:50:58.970
<v Steve>Let me see if we have any other questions</v>

00:50:58.970 --> 00:51:00.673
we can queue up now.

00:51:09.220 --> 00:51:10.780
Here's another similar question.

00:51:10.780 --> 00:51:12.420
Is there a list of best practices

00:51:12.420 --> 00:51:14.600
for transformations

00:51:14.600 --> 00:51:16.490
between different software and hardware

00:51:16.490 --> 00:51:18.720
throughout a workflow

00:51:18.720 --> 00:51:22.450
such as Esri, MicroStation, Trimble?

00:51:22.450 --> 00:51:25.543
Is that something that you can address, Phillip?

00:51:26.800 --> 00:51:29.220
<v Phillip>Again, I feel like I'm falling down</v>

00:51:29.220 --> 00:51:30.320
on the question-and-answer portion

00:51:30.320 --> 00:51:32.170
of the webinar here.

00:51:32.170 --> 00:51:34.900
But again, we have a lot of internal tools,

00:51:34.900 --> 00:51:36.320
and I'm really unfamiliar with a lot

00:51:36.320 --> 00:51:39.750
of those proprietary pieces of software,

00:51:39.750 --> 00:51:41.580
like the Esri products

00:51:41.580 --> 00:51:44.810
and Trimble products and stuff.

00:51:44.810 --> 00:51:46.420
I didn't come up through a traditional

00:51:46.420 --> 00:51:49.480
sort of surveying background.

00:51:49.480 --> 00:51:51.750
I'm more from the Earth sciences side of things

00:51:51.750 --> 00:51:55.150
and I've used a lot of Linux tools

00:51:55.150 --> 00:51:58.260
and local, like in-house software

00:51:58.260 --> 00:51:59.093
for a lot of this work.

00:51:59.093 --> 00:52:03.150
So, again, I can't really speak to that too well.

00:52:03.150 --> 00:52:04.053
I apologize.

00:52:05.460 --> 00:52:06.720
<v Steve>Fair enough.</v>

00:52:06.720 --> 00:52:09.250
We are at the top of our hour,

00:52:09.250 --> 00:52:12.990
so it'll be time to wrap up.

00:52:12.990 --> 00:52:14.270
Thank you again, Phillip,

00:52:14.270 --> 00:52:17.410
for the excellent presentation.

00:52:17.410 --> 00:52:18.250
And

00:52:20.090 --> 00:52:21.130
before I conclude,

00:52:21.130 --> 00:52:23.460
I'd like to invite you

00:52:23.460 --> 00:52:25.360
to our next monthly webinar

00:52:25.360 --> 00:52:27.470
on November 12th,

00:52:27.470 --> 00:52:30.160
when my guest, Laksan, from NGS will present

00:52:30.160 --> 00:52:33.850
an update on the NGS coastal mapping program.

00:52:33.850 --> 00:52:35.827
And again, I'll mention that

00:52:35.827 --> 00:52:39.860
the presentation slides and recorded webinar

00:52:39.860 --> 00:52:44.490
will be available on our recorded webinars page,

00:52:44.490 --> 00:52:46.693
web page, in the next week or so.

00:52:47.530 --> 00:52:50.370
And we will send a follow-up email

00:52:50.370 --> 00:52:52.640
with any relevant links

00:52:52.640 --> 00:52:55.293
and answers to your questions.

00:52:56.500 --> 00:52:57.400
And

00:52:59.110 --> 00:53:00.840
please take a minute to complete

00:53:00.840 --> 00:53:03.190
the very brief evaluation that'll appear

00:53:03.190 --> 00:53:05.893
on your computer screen when the webinar ends.

00:53:07.550 --> 00:53:09.540
We appreciate you taking the time,

00:53:09.540 --> 00:53:11.740
and we use your comments

00:53:11.740 --> 00:53:14.880
to improve this webinar series.

00:53:14.880 --> 00:53:17.180
Thank you, again, to everyone in our audience

00:53:17.180 --> 00:53:19.430
for joining our webinar today.

00:53:19.430 --> 00:53:21.343
We hope you'll join us again soon.

00:53:22.360 --> 00:53:23.533
<v Phillip>Thanks, Steve.</v>