GNSS Antenna Calibration
Research question: Can we precisely determine the GNSS phase location of a GNSS antenna?
Accurate positioning supports many daily activities. Commercial and recreational transportation at sea, land, air, and space depend on Global Navigation Satellite Systems (GNSS) receivers to provide public services, such as safe navigation, road construction and maintenance, and precision agriculture to increase land use efficiency. In order to achieve such high accuracy, calibration of survey grade GNSS antennas is mandatory. Without proper calibration, these survey grade antennas are able to determine heights typically to accuracies greater than 10 cm.
Over the past decade, NOAA’s National Geodetic Survey (NGS) has been investigating different approaches for accurately determining the offsets between the GNSS signal point of reception and the physical antenna structure. The result of this research was an innovative calibration approach using a robotic arm that rotates a GNSS antenna in all directions while collecting GNSS signals. With this three-dimensional mapping allowing NGS to determine the GNSS antenna offsets to the base of the antenna or a connector, survey accuracy can be determined to the 1cm level (a tenfold increase).
Antenna calibrations are referenced to a location on the outside of the antenna. This location is defined as the Antenna Reference Point, which is typically an easily accessible point on the lowest non-removable horizontal surface of the antenna. The actual location measure is defined as the phase center. The average Phase Center Offset is a few centimeters with respect to Antenna Reference Point and is expressed using a North (N), East (E), Up (U) vector. The exact location of the phase center depends on the azimuth and elevation angles of the incoming GNSS signal. These Phase Center Variations can range up to 4-5 mm.
The challenge is that there are multiple manufacturers with different GNSS antenna types. As such, NGS’s Absolute (3D) Antenna Calibration Service is a first in North America and is one of only five antenna calibration sites in the world to provide such a service (https://geodesy.noaa.gov/ANTCAL/). This NOAA’s robotic arm at NGS Training and testing facility at Corbin, VA calibration capability has been also recognized by the International GNSS Service (IGS), where NGS participates in their Antenna Committee that aggregates calibration data and metadata for both ground and satellite antennas. This is a prestigious opportunity for NOAA to contribute and collaborate on an international level.
The Antenna Calibration research complements NOAA’s research on space weather and ray-path geometry of the GNSS signals. All will enable the establishment of more accurate reference systems and geospatial infrastructures on national and global scales. The GNSS Antenna Calibration also supports NOAA’s strategic goal of “Local/In-Situ Distributed Observations” by improving the calibration of GNSS-based observing systems. This has major implications for a myriad of precise positioning applications, including autonomous navigation, precision agriculture, civil surveying, early warning systems for hazards, and improved floodplain mapping.
Peer Review Publications and Conference Presentations
Bilich, A. 2021. “GNSS Antenna Calibrations at NGS. NGS Webinar Series,” January 14, 2021. https://geodesy.noaa.gov/web/science_edu/webinar_series/gnss-antenna-calibration.shtml
Bilich, A., B. Erickson, and C. Geoghegan. 2018. “6-axis Robot for Absolute Antenna Calibration at the US National Geodetic Survey.” Presentation at 2018 IGS Workshop, Wuhan, China. https://www.ngs.noaa.gov/web/science_edu/presentations_archive/files/igs2018poster.pdf
Bilich, A. and G. Mader. 2014. “Relative vs. Absolute Antenna Calibrations: How, when, and why do they differ? A Comparison of Antenna Calibration Catalogs.” Presentation at 2014 IGS Workshop, Pasadena, CA. https://geodesy.noaa.gov/web/science_edu/presentations_archive/files/agu2013poster_antcalcompared
.pdf
Bilich, A. and G. Mader. 2010. “GNSS Absolute Antenna Calibration at the National Geodetic Survey,” ION GNSS conference proceedings. https://geodesy.noaa.gov/pub/abilich/papers/ABilich_ION2010official.pdf
Kersten, T., I. Sutyagin, A. Bilich, and S. Schön. 2024. “Uniting Global Efforts to Calibrate GNSS Antennas,” IGS Symposium and Workshop, Bern Switzerland, July 1-5. https://geodesy.noaa.gov/web/science_edu/presentations_library/files/igs24-ringcalval-poster.pdf
Ray, J. and Z. Altamimi, Z. 2005. “Evaluation of co-location ties relating the VLBI and GPS reference frames,” J Geodesy 79, 189–195. https://doi.org/10.1007/s00190-005-0456-z
