Acronyms and Glossary
- A Priori Coordinates
The initial coordinates assigned to survey marks by OP. This effectively gives a positional starting point for the subsequent least squares adjustment. In the absence of any other information, a priori coordinates come directly from the OPUS solutions for individual user marks. If all observations for a given user mark are loaded prior to accessing the project, OP will sort the solutions according to the dates and times of observations, and will use the first solution as the a priori. If only one observation is loaded prior to accessing the project, OP will use that solution as the a priori, regardless of when the observation was taken. Once the project has been accessed/opened, uploading subsequent observation files and obtaining additional OPUS solutions will not change the a priori coordinate. The user can always verify which RINEX file was used for the ap priori coordinates in the Marks page Processing Results drop-down menu.If the user selects “Use Coordinates” on the mark’s datasheet, the published coordinates will automatically replace the a priori coordinates from OPUS. This means that the coordinates from the datasheet are believed to be better than the ones created from your GNSS project. This should occur rarely.
- A Priori Coordinate Shifts
These numbers (given either in centimeters or meters) show by how much the a priori coordinates changed as a result of the least squares adjustment during session processing or network solutions. Note that even positions held fixed (constraints) will still show small coordinate shifts, especially if the “normal” or “loose” constraint preferences are selected. These preferences allow for small shifts to minimize overall least squares error in the adjustment.
- ADJUST
An NGS computer program for the least squares adjustment of horizontal, vertical angle, and GNSS survey networks. For GNSS networks, the input (reduced) observations are Earth-Centered, Earth-Fixed (ECEF) Cartesian vectors and their uncertainties, obtained from a GNSS baseline processing engine (such as PAGES). ADJUST is used in OP for horizontal and vertical network adjustments, after initial adjustments using the NGS program GPSCOM. ADJUST is also available as a separate PC executable.
- Ambiguities
See Integer Ambiguities
- Antenna Reference Point
The antenna reference point (ARP) is the physical point on the antenna to which antenna calibration values are referenced. ARP is preferably an easily accessible point on the lowest non-removable horizontal surface of the antenna. Typically, the ARP coincides with the axis of attachment of the antenna to a monument or surveying instrument. To identify the ARP for a given antenna, visit the NGS Antenna Calibrations page. (https://geodesy.noaa.gov/ANTCAL/)
- ARP
See “Antenna Reference Point”
- B-file
An ASCII text file generated as an output of network adjustment that contains mark coordinates, network and local accuracies, and mark occupation information, in standard “Bluebook” format (https://geodesy.noaa.gov/FGCS/BlueBook/pdf/Chapter%204.pdf).
- Baseline
An idealized line between 2 marks. There can only be one baseline between two marks. There can be multiple measurements (vectors) of that baseline.
- Baseline Processing
In GNSS-based positioning, baseline processing is the computation of vectors (distance and direction in ΔX, ΔY, ΔZ) between points observed at the same time with GNSS. A vector can be considered as a measurement of a baseline.
- Bluebook or Bluebooking
This term refers to a collection of defined procedures by which a user could submit observations to NGS for possible inclusion in the authoritative NGS database of survey control (the Integrated Database or “IDB”). Originally, the procedures were published in blue 3 ring binders, and came to be called the bluebooks, which explains the origination of the action term “bluebooking.” See “Bluebook” format at: (https://geodesy.noaa.gov/FGCS/BlueBook/pdf/Chapter%204.pdf).
- Constraints
OP allows the user to select which marks or CORS have their coordinates held fixed (“constrained”) in a least-squares adjustment of all of the GNSS data. Three levels of constraints are available. The NORMAL constraint weights instruct the program to limit the adjustment to less than about 1 cm, but if it “wants” to be greater than 1 cm, it will, although the constraint will increasingly hinder taking on larger and larger values. Think of it as a rubber band. For adjustments less than a couple cm, the rubber band is not stretched tightly and the adjustment can “easily” reach (take on) any of those values if the solution demands. Beyond that, the rubber band is stretched tighter and tighter. The adjustment can reach (take on) larger values, but it must work harder to do so. Be aware that the rubber band is never slack. It is always “pulling” towards the specified constraint value to some extent. The LOOSE constraint weights instructs the program to limit the adjustment to less than about one meter of float for the constrained points. The TIGHT constraint weights forces the adjustment to less than one-tenth of a millimeter of float for the constrained points in the adjustment (effectively fixing and not allowing the constrained control points to move).
- CORS
Continuously Operating Reference Station. In this document, CORS refers to active GNSS control from the NOAA CORS Network and IGS CORS stations that are in the NGS IDB.
- Datasheet
See NGS Datasheet a report containing the published NSRS time dependent coordinates on a survey mark, as well as subsidiary information and metadata such as superseded coordinates, descriptions and recovery history of the survey mark.
- Double-differencing
The mathematical process of solving for unknown positional errors inherent in GNSS positioning. The technique applies the difference between a GNSS-derived position with its known coordinates at a known point (often called the “base”) to the difference between a simultaneously derived GNSS-derived position and its true position at an unknown point (the “rover”).
- Earth-Centered, Earth-Fixed (ECEF)
A geographic and Cartesian coordinate system. It represents positions as X, Y, and Z coordinates, with the origin at the center of mass of Earth. Its axes are aligned with the international reference pole (IRP) and international reference meridian (IRM) that are fixed with respect to Earth’s surface. The z-axis extends through true north, which does not coincide with the instantaneous Earth rotational axis. The slight “wobbling” of the rotational axis is known as polar motion. The x-axis intersects the sphere of the earth at 0° latitude (the equator) and 0° longitude (prime meridian in Greenwich).
- Ephemeris
An electronic data file containing information about the position of a satellite with respect to time. Typically, each satellite retains the latest copy of its ephemeris onboard the satellite, that it broadcasts to the receivers to provide information about its position relative to Earth. This is called the “broadcast ephemeris,” and for GPS satellites, the ephemeris files maintained by the US Space Force are uploaded to the satellites from the Master Control Station every 2 hours. The International GNSS Service (IGS) computes additional ephemerides based on both collected and modeled data. Ultra-rapid ephemerides are very similar to the broadcast ones, and include predicted positions in real time. Rapid ephemerides are available with a 17-41 hour latency, and offer slightly improved accuracy. Final ephemerides are considered the best available, and are available with a typical 12-18-day latency. The IGS considers all of its ephemerides “precise.”
- Elevation Cutoff
This number, expressed in degrees above the horizon, specifies the angle below which data from satellites will be ignored. The default for new projects is a conservative but common value of 15 degrees. By eliminating low elevation satellites, some of the problematic effects of the troposphere and ionospheric delays may be avoided. Pre-planning the GNSS observations on each user mark by preparing a ‘mark obstruction diagram’ will produce a cleaner data set for each observation. Trees, buildings, hilly topography, etc. reduce satellite visibility, and may cause cycle slips because of temporary losses of signal from a satellite. Most survey grade receivers allow the elevation cutoff to be specified for an individual observation. A common strategy is to set the receiver’s elevation cutoff mask at a lower value, such as 10 degrees, then apply a higher elevation cutoff mask for individual session processing.
- G-file
An ASCII text file generated as an output of network adjustment that contains vector components, standard deviations, correlations, and processing information, in standard NGS “Bluebook” format (https://geodesy.noaa.gov/FGCS/BlueBook/pdf/Annex_N.pdf).
- Geometric Reference Point (GRP)
The physical point on an antenna mount that defines the placement of the antenna’s reference point, or ARP. Like the ARP, the GRP coincides with the axis of attachment of the antenna to a monument or surveying instrument. When a GNSS antenna is properly installed, the GRP and ARP are coincident in space. See “Antenna Reference Point” for more information on the ARP and how it relates to antenna calibrations.
- Geoid
Equipotential surface of Earth’s gravity field which best fits mean sea level and the continuation of that same surface onto land. It is typically used as the zero reference surface for orthometric heights.
- GNSS
Global Navigation Satellite System uses satellites to provide autonomous geo-spatial positioning. It allows electronic receivers to determine their location (longitude, latitude, and altitude (elevation) to high precision (within a few centimeters to metres) using time signals transmitted along a line of sight by radio from satellites.
- GPS
Global Position System is a satellite-based radionavigation system owned by the United States government. It is one of the Global Navigation Satellite Systems (GNSS) that provides geolocation and time information to a GPS receiver anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites.
- GPSCOM
This is a program for the combined adjustment of multiple GPS data sets initially processed by the program PAGES. GPSCOM is a simple Helmert Blocking, normal equation processor which combines multiple GPS data sets that have initially been processed by the program PAGES to form and partially reduce normal equations, eliminating numerous nuisance parameters which are not generally of interest in a large global adjustment.
- GRP
See “Geometric Reference Point”
- GVX
GNSS Vector eXchange is a standard file format for exchanging GNSS vectors derived from a variety of GNSS survey methods and manufacturer hardware. GVX supports GNSS vectors derived as part of a real-time kinematic (RTK) survey or from baseline post-processing. More information is available online (https://geodesy.noaa.gov/data/formats/GVX/index.shtml).
- Hub
In GNSS network survey design, the hub is a point observed with GNSS which is preferentially selected for inclusion in baseline processing. In other words, an included user mark that is a “hub” is more likely to be connected to other user marks.
- Integer Ambiguities
The unknown number of wavelengths spanning the distance between a GNSS satellite when a signal is emitted and a GNSS receiver on the surface of Earth.
- Mark
The term OP uses to describe a point observed with GNSS and whose GNSS observations are brought into the project via the OPUS upload page. See also “User Mark.”
- Multi-Year CORS Solution (MYCS)
Periodically, new coordinates and velocities are computed for all CORS that are part of the NOAA CORS Network (NCN). This massive computation effort combines data from prior solutions with new CORS data available since then. This processing/reprocessing is made to coincide with the adoption of a new realization of the International Terrestrial Reference Frame (ITRF).
- Network Design
A reflection of the choices made in selecting baselines to compute in GNSS-based session processing and network adjustment.
- NGS 59
The abbreviated name commonly used to refer to NOAA Technical Memorandum NOS NGS 59, which outlines procedures to obtain orthometric height network accuracies of 5 cm and orthometric height local accuracies of 2 cm or 5 cm using GNSS.
- NGS Datasheets (“Datasheets”)
The expression of the authoritative geodetic control from the NGS database (the Integrated Database, or “IDB”) in the form a report containing the published NSRS time dependent, coordinates on a survey mark, as well as subsidiary information and metadata such as superseded coordinates, descriptions and recovery history of the survey mark.
- NGS IDB
The NOAA National Geodetic Survey’s “Integrated database” containing authoritative, high accuracy positioning on survey marks throughout the nation and its territories. Access to these authoritative information is through datasheets, available online (https://geodesy.noaa.gov/datasheets/index.shtml).
- NOAA CORS Network
The NOAA CORS Network (NCN) is a multi-purpose cooperative network of GNSS stations from over 200 government, academic, and private organizations. Its primary objective is to enable GNSS users by providing precise positioning relative to the U.S. National Spatial Reference System (NSRS).
- Non-NCN CORS
A continuously operating GNSS reference station that is not part of the NOAA CORS network (NCN) or NGS integrated database (NGS IDB).
- PAGE-NT
A Microsoft Windows Graphic User Interface (GUI) that facilitates the running of the PAGES program.
- PAGES
An NGS-developed GNSS baseline processing engine that uses double-differencing to solve for geodetic coordinates and uncertainties of unknown marks relative to marks with known coordinates. PAGES can perform simultaneous baseline processing, allowing for determination of correlations between all GNSS receivers during a session. In determining the solutions, PAGES solves for integer ambiguities and determines tropospheric parameters. Output is provided in Solution INdependent EXchange (SINEX) format and as GNSS vectors (used in the ADJUST program). PAGES currently only uses GPS observables, but other GNSS constellations will be added in a future version.
- PDOP
Position Dilution of Precision is a unitless value that expresses how measurement errors may affect the final 3D position estimate. A general rule of thumb is that the lower the PDOP value, the more precise the final position will be.
- Peak-to-Peak (“P2P”)
An expression of the range among the different positional solutions coming from OPUS post-processed results. For single OPUS (static) solutions, these numbers express the range among the different positional solutions as a result of post-processing GNSS observations through OPUS (expressing the variability among the different baselines to the CORSs). For OPUS Projects, these numbers express the range among multiple least squares adjustments (e.g. comparisons among the different session processing).
- Post-processing
The geodetic computations applied to static GNSS observation data, after the observation files are taken, which take advantage of simultaneous GNSS observations at known reference stations to resolve errors that degrade the precision and accuracy of GNSS-derived positions. OPUS and OPUS Projects are examples of post-processing software.
- Project Tracking ID
To gain access to certain capabilities of OP, an NGS-supplied Project Tracking ID must be entered. This ID is provided to you by NGS after we have reviewed the information submitted via the NGS Survey Project Proposal page. For any project you intend to submit to NGS, you must submit the Project Proposal to receive an assigned ID. If you intend to use OP for your own purposes you can utilize the trial ID of “0000” (four zeros). Note that if you initially enter the trial ID with the intent to later enter an assigned ID, you will need to enter your assigned ID before performing any Network Adjustments or you will experience problems with resultant files generated by OP. The most typical reason a user would choose to enter the trial ID is to access the integrated ADJUST software.
- Publish
The action of NGS providing to the public the official, National Spatial Reference System (NSRS) time-dependent geodetic coordinates set on a survey mark.
- RINEX
The Receiver-INdependent EXchange format is an internationally accepted, ASCII-based GNSS observation file format which enables observations files from many different receiver types to be shared, uploaded, and post-processed.
- RMS
Root Mean Square is a computation of error derived on the square root of the sums of squared difference terms.
- RTK
Real-time kinematic positioning is a differential GNSS technique that uses carrier-phase and pseudorange measurements to generate real-time measurement corrections relative to a single reference station (single-base RTK) or from a network of reference stations (network RTK), providing centimeter-level position solution accuracy.
- RTN
Real-time networks consist of strategically located permanent reference stations whose measurements and well-known positions are used to provide corrections in real-time from a central server to rover receivers via a wireless data link. Raw GNSS data logged at each station are commonly available for download from a network manager.
- Serfil
A text file which associates each user mark with both its OPUS 4-character ID and the sequential serial number (SSN) used in the WinDesc description file associated with the project
- Session
A group of files associated with nearly simultaneous observations of a group of marks. OP automatically creates sessions based on the start/stop times in the submitted files and the limits selected in Manager’s Preferences. A session by itself is not the same as a solution.
- Session processing
The post-processing of two or more GNSS data files whose contents were collected with substantial time (observation) overlap to estimate the coordinates of the marks at which the data were collected with respect to one or more marks chosen to act as reference stations.
The act of releasing to NGS the observations (via OPUS), metadata and results of geodetic surveys tied to the NSRS for public dissemination.
- Shift
The amount by which a coordinate of a given mark has changed or is different compared to some other coordinate for the same mark. This is a common term when comparing the input coordinates to the output coordinates in an adjustment, or the published coordinates to the adjusted output coordinates.
- Sigma
An expression error defined as the sum of squared differences, typically between observations and either the mean of a sample or a (known) parametric value. Sigmas are identified by the greek letter σ.
- SINEX
Solution INdependent EXchange format: an internationally recognized data file containing the solutions of GNSS baseline processing. The NGS software PAGES produces SINEX files. For more information, see https://www.iers.org/IERS/EN/Organization/AnalysisCoordinator/SinexFormat/sinex.html
- Solution
A solution is the result of a computation based on the observation files in a given session and the processing choices selected by the user. Such user choices include which marks to include or exclude, hub selections, constraints (loose/normal/tight), etc. Therefore, many solutions can be computed from a single session. Each solution can be named. Users should include only one solution per session in a network adjustment.
- SSN
Station Serial Number: For the purpose of identifying the station on the observation records in a concise manner, each survey point is assigned a project specific station serial number (SSN) in the range 0001 to 9999.
- Submission to NGS
The act of a user releasing to NGS the observations, metadata and results of geodetic surveys tied to the NSRS for the express purpose of NGS evaluating the survey and, if appropriate, including the descriptions and observations in the authoritative national database of survey control, and making the information available through NGS datasheets.
- Tropospheric Modeling
Water vapor and the gas components of the dry air cause a delay in the GNSS signals arriving at the receiver. To correct for this delay, tropospheric models are computed, based on a number of tropospheric parameters. Models used by OPUS include a global climatological model (Boehm et al. 2007), a global mapping function for mapping the “dry” component of the tropospheric model (Boehm et al. 2006) and another global mapping function to model the “wet” component of the troposphere (Boehn et al. 2006). Finally, OPUS uses a correction to the “wet” component of the tropospheric model using the GNSS data itself. The estimation of the correction relies on the tropospheric interval.
- Tropospheric Parameters
A number of variables are required to model tropospheric delays affecting GNSS signals. Variables include latitude, longitude, height, day-of-year, relative humidity, air temperature, and air pressure.
- Tropospheric Interval
The time interval over which tropospheric parameters are interpolated and corrected from model output. OP allows the user to specify not only the time interval, but also the manner in which the interpolation occurs (piecewise linear vs. step-offset.
- User Mark
This is the term used throughout this OP user manual to refer to a point observed with GNSS, and whose GNSS observations have been uploaded into the project by the user through the OPUS upload page. Typically, this means a point which was directly observed by the user with GNSS, but it could also refer to a reference point with continuous GNSS observations whose data the user has brought into the project via the OPUS upload page. Within OP, user marks are referred to more simply as “Marks” and are differentiated from “CORS,” whose data are brought into the project automatically by OP through CORS selection.
- Vertical Sigmas
Vertical uncertainties (sigmas) for vertically constrained marks or CORSs in OP are based on values and relationships established by NGS, according to the source of the height on the datasheet. For an explanation of these values, please see Appendix D.