Impact of GNSS Clock Instability on Radio Occultation Retrievals from the COSMIC-2 Mission

Abstract:

The positive impacts that COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) radio occultation (RO) profiles are having on weather forecasting, climate and space weather applications has prompted U.S. agencies to execute a COSMIC follow-on mission (called COSMIC-2) with Taiwan that will put twelve low Earth orbit satellites with GNSS (Global Navigation Satellite System) RO payloads into orbit on two launches in the 2016-19 time frame. COSMIC-2 will make use of an advanced RO receiver with an innovative beam-steering antenna design, and will produce at least 10,000 high-quality atmospheric profiles per day from GPS and GLONASS signals to support operational and research applications. The raw RO measurements of interest are the tracked high-rate (i.e. 50-100 Hz) carrier phase and amplitude of the L1 and L2 signals transmitted from the GNSS satellites as the ray tangent points descend from ~100 km altitude to the Earth’s surface. The fundamental RO observable is the atmospheric excess phase due to the signal’s propagation through the atmosphere (in excess to vacuum). The computation of accurate atmospheric excess phase requires geodetic quality knowledge of the orbital motion and clock estimates for both the transmitting and receiving satellite systems. Allan deviations of the GPS and GLONASS clocks were investigated in previous studies from a single ground receiver, and showed the GLONASS clocks are less stable than GPS clocks on time scales critical for RO profiling, i.e., approximately 0.5 to 20 seconds. In this study we estimate GPS and GLONASS Allan deviations from a global network of GNSS receivers tracking at 1 Hz sampling rate and verify the results with the previous studies. We then estimate the expected impact that these clock instabilities will have on future COSMIC-2 RO retrievals. This presentation will give a short overview of the COSMIC-2 mission, and then discuss details of the GNSS clock processing and how transmitter clock instability impacts the RO retrievals.