LoginGNSS Antenna Phase Center Variations
Motivation
Until November 2006, receiver antenna phase center corrections relative to the reference antenna AOAD/M_T from field calibrations were used within the International GNSS Service (IGS). Such calibrations suffer from an uneven distribution of observations, an increased observation noise at low elevations, and the arbitrary assumption that the reference antenna is free from phase center variations (PCVs). In contrast, chamber calibrations as well as field calibrations using a robot capable of rotating and tilting the antenna minimize these drawbacks and thus, provide absolute PCVs.
However, as long as the behavior of the transmitting GNSS antennas was ignored, the use of absolute phase center corrections in global solutions resulted in a considerable scale bias compared to solutions from very long baseline interferometry (VLBI) or satellite laser ranging (SLR). In order to generate consistent corrections for the satellite antennas, GFZ and TUM reprocessed more than 10 years of data fixing the terrestrial scale from VLBI and the absolute receiver antenna corrections from robot calibrations. For this effort, that also implicated the reestimation of troposphere, orbit and Earth rotation parameters, two different software packages were used (EPOS.P.V2, Bernese GPS Software).
Results
Fig. 1 shows the results for four different GPS satellite antenna types together with the corresponding values for the transmitting antennas onboard the GLONASS satellites estimated from about one year of data by CODE.
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| Fig. 1: Antenna PCVs w.r.t. the nadir angle (station seen from the satellite) for the different generations of GPS and GLONASS satellites. |
Whereas nadir-dependent PCVs are already part of the official IGS model igs05_wwww.atx (together with satellite-specific z-offsets), azimuth-dependent PCVs are still an open issue. Fig. 2 shows the results of a TUM solution based on a few days of data only and JPL values based on a residual approach. The comparison between the two independent approaches looks very promising.
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| Fig. 2: Azimuth-dependent satellite antenna PCVs for a given nadir angle of 14° (red: TUM solution, blue: JPL solution). | |
Benefit
Due to the improvement in the modeling of the GNSS antenna phase center position several GNSS parameters should benefit, especially those that are highly correlated with the phase center corrections. Among those are the troposphere parameters, the station heights, and, consequently, the global terrestrial scale. Fig. 3 shows how that scale changes with time due to changes in the satellite constellation, errors in the station velocity model, or other modeling deficiencies. Switching to absolute PCVs, the scale drift can be considerably reduced from 0.34 ppb/y to 0.15 ppb/y.
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| Fig. 3: Terrestrial scale of daily global GPS solutions compared to IGb00. |
One possible indicator for the quality of troposphere parameters is the difference between estimates of independent techniques. Fig. 4 shows the bias between GPS and VLBI for a large number of stations where the GPS antenna and the VLBI telescope are located close to each other. Therefore, the two instruments should provide more or less identical results. However, as long as relative PCVs are used, the mean bias is of the order of +6 mm. With the transition to absolute PCVs the mean bias can be reduced to about -1 mm. The remaining large biases at several individual stations are related to unmodeled radome effects in most cases.
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| Fig. 4: Troposphere bias between GPS and VLBI, corrected for the height difference between the GPS antenna and the VLBI telescope. |
Publications
Dach, R.; Schmid, R.; Schmitz, M.; Thaller, D.; Schaer, S.; Lutz, S.; Steigenberger, P.; Wübbena, G.; Beutler, G.: Improved antenna phase center models for GLONASS; GPS Solutions, 2010, DOI: 10.1007/s10291-010-0169-5
Steigenberger, P.; Rothacher, M.; Schmid, R.; Rülke, A.; Fritsche, M.; Dietrich, R.; Tesmer, V.: Effects of different antenna phase center models on GPS-derived reference frames; in: Drewes, H. (ed.) Geodetic Reference Frames, IAG Symposia, Vol. 134, pp 83-88, Springer, ISBN 978-3-642-00859-7, 2009, DOI: 10.1007/978-3-642-00860-3_13
Schmid, R.; Steigenberger, P.; Gendt, G.; Ge, M.; Rothacher, M.: Generation of a consistent absolute phase center correction model for GPS receiver and satellite antennas; Journal of Geodesy, Vol. 81, No. 12, pp 781-798, 2007, DOI: 10.1007/s00190-007-0148-y, free version
Schmid, R.; Rothacher, M.; Thaller, D.; Steigenberger, P.: Absolute phase center corrections of satellite and receiver antennas, Impact on global GPS solutions and estimation of azimuthal phase center variations of the satellite antenna; GPS Solutions, Vol. 9, No. 4, pp 283-293, 2005, DOI: 10.1007/s10291-005-0134-x, free version
Schmid, R.; Mader, G.; Herring, T.: From relative to absolute antenna phase center corrections; in: Meindl, M. (ed.) Proceedings of the IGS Workshop and Symposium 2004, Astronomical Institute, University of Bern, 2005
Schmid, R.; Rothacher, M.: Estimation of elevation-dependent satellite antenna phase center variations of GPS satellites; Journal of Geodesy, Vol. 77, No. 7-8, pp 440-446, 2003, DOI: 10.1007/s00190-003-0339-0
Links
- IGS Antenna Working Group
- Absolute IGS phase center correction model igs08_wwww.atx
- Geo++ PCV database
- NGS antenna calibrations
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