COSMIC-2 Precise Orbit Determination

We present initial results for GNSS orbit and clock estimation strategies implemented for the FORMOSAT-7/COSMIC-2 (Constellation Observing System for Meteorology, Ionosphere, and Climate) constellation. The six COSMIC-2 satellites launched on June 25, 2019 into a 24 deg inclination, ~725 km circular orbit. Over time, all satellites will be lowered to an operational altitude of ~520 km. The primary COSMIC-2 science payload is the JPL designed Tri-GNSS Radio-occultation Receiver System (TGRS), which tracks GPS and GLONASS signals on two upward looking choke-ring precise orbit determination (POD) antennas facing the forward- and anti-velocity directions. We first present basic metrics including tracking arc coverage and constellations/satellites tracked on both antennas. We then evaluate two operational GPS-only POD strategies applied at the UCAR COSMIC Data Analysis and Archive Center during early mission phases: a basic strategy that is robust but less accurate, and a higher accuracy reduced dynamic approach tuned for larger, stable orbiters. We also analyze research-grade solutions using tracking data from one and two antennas concurrently, and explore the the addition of GLONASS observations to GPS in the POD estimation. For these analyses, we measure POD solution quality by way of postfit residuals, formal errors, as well as internal orbit and clock overlaps. We furthermore compare the performance of the six orbiters, and compare quality metrics at the higher and lower orbit altitudes.