GLONASS R720 transmit power loss

All GLONASS satellites transmit navigation signals in the L1 and L2 frequency band whereas newer generations (M+ and K1) also utilize the L3 band. Previous studies have shown that the transmit power in the individual frequency bands can significantly differ for dedicated satellites. These differences are visible in the carrier-to-noise density ratio (C/N₀) of geodetic GNSS receivers and can be measured with a high-gain antenna. Whereas C/N₀ allows for a continuous monitoring, high-gain antenna measurements are only performed on an irregular basis.

In April 2019, a drop in C/N₀ could be observed for the GLONASS-M satellite R720. Measurements of the R720 equivalent isotropically radiated power (EIRP) with the 30 m high-gain antenna of the German Aerospace Center show a reduction by up to 9 dB for L1 and 1.5 dB for L2 compared to earlier measurements obtained in June 2017. The 2019 EIRP measurements also show an asymmetry of ascending and descending arcs that was not present before the power loss and indicate a change in the apparent gain pattern of the R720 transmit antenna. The transmit power change is accompanied by discontinuities in the estimated satellite antenna phase center offsets (PCOs) by about 15 cm in the transmit antenna plane and inter-frequency differential code biases (DCBs) by up 0.6 ns. Several GLONASS PCO and DCB changes were already reported by Dach et al. (2019) but they could not show a direct relation to transmit power changes.

This contribution analyzes the impact of the R720 transmit power loss on C/N₀ and high gain antenna measurements as well as PCO and DCB estimates. The current transmit antenna gain pattern is reconstructed based on repeated high-gain antenna measurements. Differential gain pattern are obtained from C/N₀ measurements of geodetic receivers and low-gain GNSS antennas before and after April 2019. Furthermore, the impact on precise orbit determination is evaluated as transmit power affects the modeling of antenna thrust.