Antenna Calibration FAQ

1) What is an "antenna calibration"?
2) Why do I need to use an antenna calibration?
3) Why are antenna calibration values always given for antenna code+radome code (antenna type)? Why are some antennas calibrated with different configurations?
4) What is an ARP?
5) What is a NRP?
6) Why are images or other metadata missing for some antenna types?
7) When ARP, NRP codes are not consistently labeled on the images and in the tables, which should I follow/use?
8) What is PCO?
9) What is PCV?
10) What is a relative calibration?
11) What is an absolute calibration?
12) What is the source of the NGS absolute antenna calibration values?
13) In what format(s) are the absolute antenna calibration values given? What is the difference between the ANTEX and ANTINFO formats?
14) What are the differences between the composite calibration files: ngs*.atx and ngs*.003?
15) Why has the description of some antenna codes changed?
16) Can my antenna be calibrated while still installed at a CORS station?
17) When did the absolute antenna calibration values start being used in CORS operational products?
18) How will new absolute antenna calibrations be added to this table/page?
19) Will calibration values ever change?
20) What if my antenna is not on the list of calibrated antennas?
21) What if a calibration exists for the antenna "model", but not for the exact "model+radome" I am using?
22) What if my antenna has calibration markings which are different from the published calibration?
23) Since NGS does not at this time do absolute calibrations where can I get an absolute calibration of my antenna?
24) Why did so many calibration values change in November 2022?
25) How are antenna calibrations related to a reference frame?

1) What is an "antenna calibration"?

An antenna calibration is the act of determining the point of reception of the Global Navigation Satellite System (GNSS) carrier phase signals. Antenna hardware such as the antenna elements and pre-amplifiers create signal phase advance and delay before passing the signal to the receiver. The phase advance/delay changes the range measurement, and will introduce error to position solutions. The point of signal reception of an antenna is not a physically measurable location, and varies depending upon the direction of the satellite signal being received. Therefore antenna calibrations create a map of phase advance and delay which depends on the satellite position in an antenna-centric frame (based on a specific ARP and NRP).

2) Why do I need to use an antenna calibration?

To obtain accurate coordinates at a point, the signal phase advance and delays (calibration values) that occur at the antenna must be applied when processing collected GNSS data. Non or misapplication of antenna calibration values can introduce errors at the centimeter level. Furthermore, because the calibrations are specific to a reference frame, processing GNSS data with the correct values is an essential step to accurately define and access positions consistent with the National Spatial Reference System (NSRS).

3) Why are antenna calibration values always given for antenna code+radome code (antenna type)? Why are some antennas calibrated with different configurations?

Near field effects are important to the calibration of an antenna. The antenna type explicitly indicates the configuration of the antenna for that calibration. The presence or absence of a radome, RF antenna or offset can change the PCO and PCV of an antenna.

4) What is an ARP?

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. Please consult the antenna diagram for precise information on the ARP for your antenna model, (See FAQ7 also). Common ARP names and descriptions are given below.

Name	Description
BAM	bottom of antenna mount
BCR	bottom of chokering
BDG	bottom of dome ground plane
BGP	bottom of ground plane
BPA	bottom of preamplifier
TCR	top of chokering
TDG	top of dome ground plane
TGP	top of ground plane
TOP	top of pole
TPA	top of preamplifier
UNK	unknown
	If the antenna webpages and diagrams do not designate an ARP, assume it is the point of attachment of the antenna to a monument. See also FAQ7

5) What is a NRP?

The north reference point (NRP) is a physical antenna feature which must be oriented toward the true north direction. Antenna calibration values assume that the NRP is properly oriented to true north. Please consult the antenna diagram for precise information on the NRP for your antenna model. Common NRP names and descriptions are given below.

Name	Description
BAT	battery compartment door release
BTD	bottom of tear drop shape (wide end)
CAC	nonspecific cable connector (only allowed for legacy calibrations)
CMP	mounted compass
CTC	external controller connector
DAC	data cable connector (for data collectors besides external receivers)
DIS	display/digital readout (DIS is a specific type of MMI)
DRY	cap or cover for drying agent
MMI	man-machine interface
NOM	north orientation mark (placed on antenna by manufacturer)
PCS	PC card slot
PWC	power port; external power connector
RTC	RTK connector; UHF connector for RTK broadcasting antenna
RXC	receiver connector (connect antenna to external receiver)
TMT	tape measure tab or notch for slant height pole
UNK	unknown
	If the antenna webpages and diagrams do not designate an NRP, it is unknown. See also FAQ7

6) Why are images or other metadata missing for some antenna types?

Images have not always been archived by NGS, this is especially true of legacy antennas. Furthermore, legacy antennas often did not have a standardized ARP/NRP assigned. NGS is reviewing these calibrations and making updates to images, diagrams, and metadata tables on a periodic basis. If you need any clarification, please contact the Antenna Calibration team at ngs.antcal @ noaa.gov.

7) When ARP, NRP codes are not consistently labeled on the images and in the tables, which should I follow/use?

The use of fully standardized ARP and NRP codes was only introduced in 2015. NGS is slowly updating older images to match the codes shown on the web page. The calibration values are with respect to the ARP/NRP values shown in the table. If these values are blank, then follow the indications on the images. If neither is available, use for ARP the lowest physical point on the antenna, and orient the antenna cable attachment point to True North.

8) What is PCO?

The initial phase center offset (PCO) for a particular frequency, given in north-east-up components relative to the antenna reference point (ARP). PCO is considered the average point of signal reception if the satellite signal direction is not taken into account.

9) What is PCV?

Phase center variations (PCV) capture the component of an antenna calibration which depends upon the direction of the incoming signal. PCV may be provided as a function of elevation angle in the antenna frame (1D), or elevation and azimuth angle in antenna frame (2D). Like the PCO, the PCV is dependent upon the GNSS signal frequency.

10) What is a relative calibration?

In a relative calibration, all antenna offsets (PCO) and phase center variations (PCV) are computed with respect to a reference antenna which is normally assigned zero PCV values. For NGS relative calibrations, the reference antenna is the Dorne Margolin choke ring antenna, type T (AOAD/M_T NONE). A relative calibration is therefore biased by the phase advance/delay experienced by the reference antenna. A file with NGS relative calibrations conducted to date is available on request, please contact ngs.antcal @ noaa.gov.

11) What is an absolute calibration?

To conduct an absolute calibration, the antenna being tested is moved via a robot so that a particular satellite is received at different angles by the test and reference antennas. This angular separation enables cancellation of the reference antenna effects, leaving behind only the antenna offsets (PCO) and phase center variations (PCV) of the test antenna.

12) What is the source of the NGS absolute antenna calibration values?

NGS sources calibration values from a combination of IGS absolute calibrations and NGS relative calibrations. NGS starts with the most recent published IGS ANTEX file and incorporates all the antennas listed in this file. For additional antennas with NGS relative calibrations that are not in the IGS ANTEX, NGS has converted these additional antennas using the absolute antenna calibration values for the reference antenna AOAD/M_T NONE to create an "absolute from relative" absolute calibration. NGS is in the process of completing development of an absolute antenna calibration technique; this is being developed by the Geosciences Research Division and conducted by the Instrument and Methodologies Branch of the Geodetic Services Division at NGS’s Testing and Training Center in Corbin, VA.

13) What is the difference between the ANTEX and ANTINFO formats?

Calibrations are given in two formats: ANTEX established in 2003 by the IGS Antenna Calibration Working Group; ANTINFO established by NGS in the late 1990s. The ANTINFO format is a legacy format, used primarily for relative antenna calibrations, as it supports only GPS L1/L2 and only ground based antennas with no azimuthal dependencies. The newer ANTEX format is more comprehensive and flexible, capable of handling multiple Satellite Navigation Systems, multiple signal frequencies, satellite and ground calibrations, azimuthal dependencies, etc. Although NGS will continue to support the ANTINFO format for legacy software users, it is clear that to improve accuracy in processing GNSS data the ANTEX format should be used instead. NGS in all its GNSS processing software, including OPUS, uses the ANTEX formatted file.

14) What are the differences between the composite calibration files: ngs*.atx and ngs*.003?

Composite calibration files combine individual calibrations into a single file; the files use different data sources, formats, and reference frame and reference systems. The file suffix indicates the format: *.atx for the ANTEX format, and *.003 for the ANTINFO format. Files beginning with ngs* contain calibrations from IGS as well as NGS. See FAQ12 and FAQ25 for more information.

15) Why has the description of some antenna codes changed?

To improve and standardize the nomenclature of antenna codes and improve identification of antennas by users, NGS has modified, and we believe improved, the descriptions. The descriptions of antenna types are established by the antenna manufacturer, IGS, and NGS to ensure they are unique and identifiable.

16) Can my antenna be calibrated while still installed at a CORS station?

No. Antenna calibration methods require collecting data with the antenna in a laboratory environment. Many research groups are working on in-place calibration methods, but no single method has yet been developed and accepted by the geodetic GNSS community.

17) When did the absolute antenna calibration values start being used in CORS operational products?

NGS's CORS group began using absolute antenna calibrations on 30 June 2012, upon the release of the new CORS coordinates in IGS08 epoch 2005.00 and NAD 83(2011,MA11,PA11) epoch 2010.00.

18) How will new absolute antenna calibrations be added to this table/page?

New entries are added to this page after they have been published by NGS or IGS. Note that when both IGS and NGS values are available for an antenna, the IGS values will supersede the NGS values, appearing on the website as well as in the composite absolute calibration files (ngs*.atx, ngs*.003) and in OPUS. See FAQ12 and FAQ20 for additional information.

19) Will calibration values ever change?

NGS strives to not change calibration values after publication; for the majority of antenna types, calibration values will not change. A notable exception is when a reference frame update occurs, in which case many calibrations may change at once (see FAQ24). During periods when there are no changes to the reference frame, in rare cases, calibrations will be updated if errors are found or improved calibrations are available. More commonly, new IGS values will supersede any previously published NGS values (see FAQ18).

20) What if my antenna is not on the list of calibrated antennas?

If an antenna is not listed, it has not been calibrated by NGS nor has a calibration been provided to the IGS. As NGS does not solicit antennas for calibration, this generally means that no calibration has been requested. Calibrations may be requested by manufacturers or other eligible providers in accordance with the NGS Antenna Calibration Policy. See Request Calibration.

21) What if a calibration exists for the antenna "model", but not for the exact "model+radome" I am using?

Antenna calibrations are provided for antenna model + radome. It is known that the choice of radome can affect calibration values at the mm level, and the choice of antenna model can affect calibration values by up to several cm. Therefore, if your exact model + radome is not listed, your next best choice is to select model + NONE (no radome) as this will affect the calibration by a few mm. However, as stated in other FAQ, NGS recommends calibration of exact model + radome combinations for users to achieve the best positioning results possible.

22) What if my antenna has calibration markings which are different from the published calibration?

Some manufacturers conduct independent calibration of their antennas, and print this information onto the antenna's label. Calibrations served up by NGS are determined in a specific reference system, and will yield positioning results in the specified reference system.

23) Since NGS does not at this time do absolute calibrations where can I get an absolute calibration of my antenna?

Although at this time NGS does not provide absolute calibration services, IGS does have a list of authorized institutions. Upon request, absolute antenna calibrations completed by these institutions can be added to the IGS absolute calibration list and will then be added to the NGS list of calibrations.

24) Why did so many calibration values change in November 2022?

On 2 October 2022, the IGS released a new reference frame, IGS20; NGS incorporated these changes into our database in November 2022. As part of the IGS20 reference system, the absolute calibration values for many GNSS satellites and receiver antennas were updated with improved estimates or additional GNSS frequencies. In all, the calibrations for approximately 50 receiver antennas in the NGS database changed in November 2022. A complete list of antennas which changed with the IGS20 update is here.

25) How are antenna calibrations related to a reference frame?

Calibrations are one part of a reference system composed of a reference frame, GNSS satellite orbits, and antenna calibrations. Calibrations should be used only with orbits and CORS coordinates in the same system. The reference system is denoted in the file name. For example, ngs20.atx should only be used with orbits and CORS coordinates which were determined in the IGS20 reference system.