Processing and validation of FORMOSAT-7/COSMIC-2 GLONASS Total Electron Content observations

The Formosa Satellite-7/Constellation Observing System for Meteorology, Ionosphere, and Climate-2 (FORMOSAT-7/COSMIC-2, F7/C2) Tri-GNSS Radio Occultation System observes both Global Positioning System (GPS) and GLObalnaya NAvigazionnaya Sputnikovaya Sistema (GLONASS) slant total electron content (TEC). Space-based TEC observations have historically relied on GPS signals, and the processing methodologies and data quality of GLONASS absolute TEC observations are thus less well established. We present a description of the differences in the processing for the F7/C2 GLONASS absolute TEC observations. This primarily entails estimation of a paired receiver-transmitter differential code bias, which is needed due to the GLONASS usage of frequency-division multiple access. We additionally perform a validation of the F7/C2 GLONASS absolute TEC observations through comparison with colocated F7/C2 GPS absolute TEC observations. Based on this comparison, we estimate the GLONASS absolute TEC error to be similar to 2.6 TEC units (TECU), which is similar to previous estimates of the F7/C2 GPS absolute TEC error (similar to 2.5 TECU). This demonstrates that the F7/C2 GLONASS absolute TEC observations are generally similar in quality to the F7/C2 GPS absolute TEC observations, and are suitable for use by the operational and scientific communities.