Impact of Antenna Phase Centre Calibrations on Position Time Series: Preliminary Results

Advances in GPS error modelling and the continued effort of re-processing have considerably decreased the scatter in position estimates over the last decade. The associated reduction of noise in derived position time series has revealed the presence of previously undetected periodic signals. It has been shown that these signals have frequencies related to the orbits of the GPS satellites. A number of potential sources for these periodicities at the draconitic frequency and its harmonics have already been suggested in the literature and include, e.g., errors in the sub-daily tidal models, multipath and unresolved integer ambiguities. Due to the geometrical relationship between the observing site and the orbiting satellite, deficiencies in the modelling of electromagnetic phase centres of receiving antennas have the potential to also contribute to the discovered periodic signals. The change from relative to absolute type mean antenna/radome calibrations within the International GNSS Service (IGS) led to a significant improvement, but the use of individual calibrations could possibly add further refinements to computed solutions. However, at this stage providing individual calibrations for all IGS stations is not feasible. Furthermore, antenna near-field electromagnetic effects might outweigh the benefits of individual calibrations once an antenna is permanently installed. In this study, we investigate the differences between position estimates obtained using individual and type mean antenna/radome calibrations as used by the IGS community. We employ position time series derived from precise point positioning (PPP) as implemented in two scientific GNSS software packages. Our results suggest that the calibration differences propagate directly into the position estimates, affecting both sub-daily and daily results and yielding periodic variations. The sub-daily variations have periods close to half a sidereal day and one sidereal day with peak-to-peak amplitudes of up to 10 mm in all position components. The stacked power spectra of the daily difference time series reveal peaks at the GPS draconitic frequency and its harmonics with peak-to-peak amplitudes of up to 1 mm. Although these results are still preliminary, they confirm that small differences between individual and type mean antenna/radome calibrations propagate into position time series and may be partly responsible for the spurious signals with draconitic frequency and its harmonics.