COSMIC-2 Precise Orbit Determination with Multi-Constellation of Global Navigation Satellite System at NESDIS/STAR

The Constellation Observing System for Meteorology, Ionosphere, Climate (COSMIC) 2 mission is pivotal in enhancing global weather prediction, atmospheric studies, and space weather monitoring. Each COSMIC-2 satellite is equipped with a Tri-band GNSS (TriG) Receiver System (TGRS), which can receive radio occultation (RO) signals emitted from GPS (Global Positioning System), GLONASS (Global Navigation Satellite System), and GALILEO systems. The accuracy and utility of the COSMIC-2 RO measurements rely essentially on the precise orbit determination (POD) process.

Previously, COSMIC-2 POD has relied primarily upon GPS signals. The POD accuracy is in the order of 10 centimeters due to variations in the tracked GPS satellites. By harnessing the signals from multiple Global Navigation Satellite System (GNSS) satellites- namely GPS and GLONASS-the COSMIC-2 POD may achieve several distinctive advantages over a single GNSS constellation to improve the POD accuracy, such as 1) redundancy to ensure mission continuity and reliability in the event of a single constellation failure, 2) enhanced coverage to overcome the limitation in regions where a single constellation may offer limited coverage, 3) mitigation of multipath effects to minimizing inaccurate positioning, and 4) cross-validation among different GNSS satellites to enable comprehensive corrections for ionospheric and tropospheric effects.

Integrating signals from GPS and GLONASS requires accounting for their respective dynamics, including satellite orbits and frequency configurations. GPS signals make use of code-division multiple access encoding with the same frequencies for all satellites, while GLONASS uses frequency-division multiple access with satellites dependent frequencies. Inter-system and inter-frequency biases are critical to the accuracy and reliability of the orbit determination process, which require sophisticated algorithms to ensure compatibility among multi-frequency receivers.

This study concentrates on optimizing the COSMIC-2 POD performance by using both observations from the GPS and GLONASS. We focus on the calibration and alignment of measurements from these two constellations. Employing the latest BERNESE GNSS software (BSW 5.4) package, the enhanced modeling effort will include refined atmospheric models and ionospheric corrections to capitalize on the diversity of signals from both constellations. Instead of using a climatologic average as prior information, ECMWF-based troposphere data will be employed to adjust the zenith path delays (ZPD) and mapping functions for dry and wet components to improve accuracy. Before the POD estimation, necessary constraints will be applied according to the signal types and their respective biases to minimize impacts of the inter-system and inter-frequency biases on the final POD solutions.

Through comprehensive comparative analyses, the accuracy and precision of the multi-constellation COSMIC-2 POD solution will be performed to assess against the traditional single GPS method and UCAR approach. Furthermore, this study will demonstrate the impacts of different POD solutions on the excess phase and bending data quality for the COSMIC-2 RO events.