Direction-dependency of GNSS transmitter and receiver antenna group delays – group delay variations (GDV)
Not only GNSS phase observations, but also code observations are delayed by direction-dependent properties of the transmitter and receiver antennas. Those delays are called group delay variations (GDV). They can be determined by calibration in order to use them as code corrections. We do not carry out such special calibration measurements, but use GNSS observations of terrestrial reference stations or receivers onboard low earth orbiting (LEO) satellites to estimate GDV. Here, the separation of the GNSS satellite antenna GDV from those of the receiver antennas is a special challenge.
The by far largest GDV were and are shown by the BeiDou-2 satellites (meter level).
Due to these experiences, the transmitter system of the succeeding BeiDou-3 satellites was designed and tested more carefully. Their GDV are comparatively small. The satellite antennas of all other GNSS are also less affected. However, some GPS satellites show noticeably enlarged GDV (decimeter level). Concerning the receiver antennas, there are many types showing small GDV (decimeter level), especially at frequency bands L1 and L2. At other frequency bands it sometimes looks differently (many decimeters). Due to the independency of the GDV of two different frequencies, their effect is increased when the ionospheric-free linear combination is formed.
The correction of GDV becomes important whenever code observations are used for precise applications. This is for example the case, when phase ambiguities have to be fixed for precise point positioning (PPP) where code observations are usually required. GDV corrections also play an important role when code measurements are used as the primary observables, for example for the estimation of ionospheric total electron content or for GNSS time transfer.
References
Beer, S., Wanninger, L., Heßelbarth, A. (2021): Estimation of absolute GNSS satellite antenna group delay variations based on those of absolute receiver antenna group delays. GPS Solutions, 25:110. DOI: 10.1007/s10291-021-01137-8
Beer, S., Wanninger, L., Heßelbarth, A. (2020): Galileo and GLONASS group delay variations. GPS Solutions, 24: 23. DOI:10.1007/s10291-019-0939-7
Beer, S., Wanninger, L. (2018): Temporal Stability of GPS Transmitter Group Delay Variations. Sensors, 18(6):1744, DOI:10.3390/s18061744
Wanninger, L., Sumaya, H., Beer, S. (2017): Group delay variations of GPS transmitting and receiving antennas. Journal of Geodesy, 91(9):1099-1116, DOI:10.1007/s00190-017-1012-3
Wanninger, L., Beer, S. (2015): BeiDou satellite-induced code pseudorange variations: diagnosis and therapy. GPS Solutions, 19:639–648 DOI 10.1007/s10291-014-0423-3