A new approach for LEO receiver bias estimation and TEC calibration for LEO-GNSS paths

Many Low Earth Orbiting (LEO) satellites have been/will be launched with Global Navigation Satellite System (GNSS) receivers aboard, especially for precise orbit determination (POD) purpose and occasionally also for radio occultation measurements (e.g., CHAMP/GRACE/COSMIC-1,-2/SAC-C/TerraSAR-X/Metop- A/Jason1, 2 ...). Space based TEC (total electron content) observations, especially ionospheric radio occultation (IRO) observations have shown a great potential in ionospheric data assimilation [3] for better nowcast, forecast, and ionospheric driver estimation because of global coverage and high vertical resolution. Having a large number of POD related slant TEC observations makes the 3-D imaging of ionosphere/plasmasphere attractive [4]. However, big TEC errors will make the data useless. Therefore, it is important to know the quality of TEC data used in data assimilation. One of the main sources of TEC error is the inter-frequency LEO receiver bias (e.g., Differential Code Bias DCB) estimation. We applied a new approach for bias estimation of GPS receivers installed on board the COSMIC satellites and calibration of ionospheric radio occultation measurements, i.e., TEC estimation for LEO-GPS paths. For doing this, we used the daily estimates of differential GPS satellite biases that are publicly available from the CODE (Center for Orbit Determination System). Additionally, high quality CODE post processed vertical TEC maps are used together with a multi-layer mapping function approach [1] to initialize TEC at the highest elevation angle ray-path in the LEO-GPS phase-connected arc. Moreover, an exponential decay of electron density with altitude is used to model the plasmaspheric electron content above the LEO satellite height