High-rate GNSS satellite clock estimation: implications for radio occultation bending angle precision

Abstract

The Global Navigation Satellite System (GNSS) radio occultation (RO) technique plays a vital role in collecting data for meteorological and space weather prediction. It is exemplified by the COSMIC-2 low-Earth-orbit (LEO) satellite constellation, which collects the GNSS signals from an elevation angle of 90° to below the horizon. Those GNSS observation data above 5° elevation angle are used for the precise orbit determination of satellites, while those GNSS observation data below 5° are used for the RO processing. A key part of the RO processing is to estimate the bending angle due to the atmospheric refraction, which requires an accurate information of the positions and clock offsets of both the transmitter (i.e., GNSS satellite) and the receiver (i.e., COSMIC-2 satellite). Previous research at University Corporation for Atmospheric Research (UCAR) [1] indicates a notable reduction in the intrinsic uncertainty of GLONASS radio occultation when employing higher-rate GNSS satellite clock products (e.g., from 30-second sampling interval to 2-second sampling interval). However, that work only analyzed one day of dataset. To analyze multiple days of dataset, I have developed a software program that can automatically generate high-rate GNSS clock products by using a GNSS toolkit called GINAN [2]. This program is also important to the future UCAR's RO postprocessing and near-real-time processing. To be specific, it first downloads, merges, and decimates 1-second GNSS-receiver data from 50 worldwide ground stations, and then runs the GINAN software to generate clock products. I have validated the clock products generated by the program by comparing to International GNSS Service (IGS) analysis centers' clock products – the standard deviation of the time difference between our clock products and the clock products published by the Center for Orbit Determination in Europe is as small as ~ 0.1 nanoseconds. Using one week of 2-sec clock products generated by the program, I have run the standard RO processing and found that the bending-angle uncertainty of the GLONASS RO has been reduced by ~ 34%, as compared to if using the existing 30-sec clock products. Admittedly, there is no obvious improvement for the GPS RO because the GPS satellite clocks are stable at a short term of <= 30 seconds. By pushing down the noise of the RO technique, we can possibly observe the atmosphere at an unprecedented precision which could benefit the research of atmosphere modelling, the operation of weather monitoring and forecast, and even the study of space weather.