Antenna Calibrations

Antenna Calibration FAQ

1) What is an "antenna calibration"?

An antenna calibration is the act of determining the point of reception of the GNSS carrier phase signals. Antenna hardware such as the antenna elements and pre-amplifiers create signal phase advance and delay before the signal is passed to the receiver. The phase advance/delay change the range measurement and will introduce error to position solutions. The point of reception is not a physically measureable location on the antenna, and the point of reception 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.

2) Why do I need antenna calibration?

A wide variety of Global Navigation Satellite System (GNSS) antennas are available and in use. In order to properly utilize the wide variety of antennas, the characteristics of these antennas must be accurately and consistently measured, so that the physical position of the points being positioned may be unambiguously determined. NGS conducts these calibrations as an essential service for the surveying, mapping, and engineering infrastructure of the U.S. These calibrations are an essential component of GNSS data processing and are used by vendor-supplied software as well as NGS' Online Positioning User Service (OPUS).

NGS publishes coordinates that were computed using a particular set of antenna calibrations. Current CORS coordinates are all defined using relative antenna calibration values e.g. ITRF00 epoch 1997.00, NAD 83(CORS96) epoch 2002.00, NAD 83(2007) epoch 2002.00. The new CORS coordinates (http://geodesy.noaa.gov/CORS/coords.shtml) e.g. IGS08 epoch 2005.00 and NAD 83(2011) epoch 2010.00 are defined using absolute antenna calibrations.

3) What is an ARP?

The antenna reference point (ARP) is the physical point on the antenna, often the middle bottom of the antenna, to which all antenna calibrations are referenced.

4) 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.

5) 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 PCO, the PCV is dependent upon the GNSS signal frequency.

6) 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). A relative calibration is therefore biased by the phase advance/delay experienced by the reference antenna. http://www.ngs.noaa.gov/CORS/Articles/MaderGPS-Sol-1999.pdf

7) What is an absolute calibration?

In an absolute calibration, all antenna offsets and phase center variations are independent of the reference antenna. 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 cancelation of the reference antenna effects, leaving behind only the antenna offsets and phase center variations of the test antenna. http://www.ngs.noaa.gov/CORS/Articles/Bilich-and-Mader_ION2010.pdf

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

NGS has taken the published IGS08.atx file and incorporated all antennas listed in this file into its list of calibrations for the particular antenna type. The absolute antenna calibrations listed in the IGS08 are selected by the IGS Antenna Working Group http://igscb.jpl.nasa.gov/projects/antenna/ and are established from a few facilities operated by commercial, government and academic groups. For the additional antennas which are available in the NGS relative antenna calibration file ant_info.003, but not in the IGS08.atx, NGS has converted these additional antennas using the IGS08.atx absolute antenna calibration for the reference antenna AOAD/M_T 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 the calibration facility in Corbin, VA.

9) What is the difference between the old NGS "absolute from relative" calibration file (ant_info.abs) and the new absolute antenna calibrations (ngs08.003)?

The ant_info.abs file was created by taking all the NGS relative antenna calibration values and converting them to "absolute" by subtracting an anechoic chamber calibration for the AOAD/M_T antenna used as the relative calibration reference. The "absolute from relative" values have only minor differences from the new NGS absolute antenna calibrations; The new absolute antenna calibrations (ngs08.003 and ngs08.atx) instead includes mostly truly absolutely calibrated antennas.

10) In what format(s) are the absolute antenna calibration values given?

The international community under the guidance of the International GNSS Service (IGS) has established a format called ANTEX (ftp://igscb.jpl.nasa.gov/pub/station/general/antex14.txt). NGS provides data in this format and calls its file ngs08.atx. NGS also provides the antenna calibration in the ANT_INFO format (http://geodesy.noaa.gov/ANTCAL/documents/format.txt) traditionally used by NGS. The ANT_INFO formatted file contains a subset of the information included in the ANTEX formatted file, namely: no azimuth dependencies, limited to GPS only and ground based antennas. This file is called ngs08.003.

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

The ANTINFO format is a format used by NGS and other groups for sharring antenna calibration data. This format has worked successfully for many years, but suffers from a number of restrictions/omissions. The format was and is used primarily for relative antenna calibrations, it can only support a single satellite navigation system with up to two frequencies and supports only ground based antennas with no azimuthal dependencies. A new format ANTEX was proposed back in 2003 under the auspices of IGS's Antenna Calibration Working Group (http://igscb.jpl.nasa.gov/projects/antenna/). This format addressed a slew of issues including: multiple Satellite Navigation Systems, 2 or more signal frequencies, satellite and ground calibrations, azimuthal dependencies, etc. NGS in its PAGES software used to compute satellite orbits and processing CORS data uses the ANTEX format and it is clear that this is the format of the future. Although the ANTINFO format will continue to be supported for GPS only calibrations by NGS to provide backward compatibility it is clear that to improve accuracy in processing GNSS data the ANTEX format should be used instead.

12) Why are antenna calibrations values always given for antenna model and radome?

The presence or absence of a radome can introduce substantial changes to the PCO and PCV of an antenna. All antenna calibration values include in their name explicitly whether they were calibrated with a radome (4-character id) or not (NONE).

13) Why has the description name of some antenna types changed?

To improve and standardize the nomenclature of antenna names 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.

14) 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.

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

NGS's CORS group began using absolute antenna calibration upon the release of the new CORS coordinates in IGS08 epoch 2005.00 and NAD 83(2011,MA11,PA11) epoch 2010.00. For more information on the new coordinates see:
http://geodesy.noaa.gov/CORS/coords.shtml

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

No change will be made to any published antenna calibration value listed on this page after this page becomes operational in early July 2011. Only new entries will be added. If an error is found or an improved set of calibrations for a particular antenna+radome is made this will not be added to this table. By table it means the set of calibrations associated with IGS08 and NAD 83(2011) which will be called ngs08.atx and ngs08.003. Those improvements/corrections will be included in the next version of this table which will be released along with the next IGS reference frame and/or NAD 83 realization.

17) What if my antenna is not on the OPUS antenna type list / antenna calibration list?

If an antenna is not listed, it has not been calibrated by either NGS or the IGS. Under most circumstances NGS can calibrate new antennas and add them to the antenna calibration list - see the AntCal Procedures for more information, and request a calibration of your antennas via the NGS AntCal Request page.

When processing data for an unlisted antenna, it is *not advised* to choose a different antenna as calibration values can differ significantly between models and even within revisions of the same model. NGS recommends using the "NONE - no antenna selected" default zero calibration values when calibration values are not available.