A new approach for receiver bias estimation and TEC calibration for SWARM-GPS paths

Kurzfassung

The GPS receivers on-board three identical SWARM satellites provide dual-frequency carrier-phase and code-pseudorange measurements primarily for orbit determination. Since the ionosphere is a dispersive medium, i.e. refraction effects are frequency dependent. Hence, by combining measurements at two or more frequencies the propagation delay or ionospheric total electron content (TEC) can be estimated along the SWARM-GPS paths. The phase derived TEC is precise and smooth but biased by an unknown phase ambiguity constant whereas the code derived TEC is not ambiguous but noisy and less precise. Therefore, we use the low-noise carrier phase derived relative TEC to smooth the code-derived relative TEC. However, the differential carrier phase and code measurements are biased by receiver and satellite instrumental delays. Here we present a new method for bias estimation of GPS receivers installed on board the SWARM satellites for slant TEC calibration. Used are the daily estimates of GPS satellite biases that are publicly available from the Center for Orbit Determination in Europe (CODE) in Bern. Additionally, high quality CODE post processed vertical TEC maps are used together with a multi-layer mapping function approach to initialize TEC for the highest elevation angle ray-path in the SWARM-GPS connected arc. An exponential decay of electron density with altitude is used to model the plasmaspheric electron content above the SWARM satellite height. We successfully tested and validated the method for the GPS receiver bias estimation on board COSMIC satellites. Although the SWARM satellites (~460 - 530km) fly far below of the orbit of the COSMIC satellites (~700 - 800km) the initial investigation shows that the method can also be applied for receivers onboard lower satellite orbit height due to usage of actual vertical TEC maps and advanced mapping function approach.