Characterization of Multi-GNSS Receiver Biases and their Temperature-Induced Variations in Low Earth Orbit

In support of full interoperability between Global Navigation Satellite Systems (GNSSs) constellations, Bobcat-1, a 3-unit CubeSat, was deployed with the primary objective of evaluating the feasibility of estimating system-to-system time offsets (XYTOs) from low Earth orbit (LEO). Bobcat-1, developed by Ohio University, was deployed in November 2020 from the International Space Station (ISS) and deorbited in April 2021 after a successful 17-month mission. The maximum orbit altitude, after deployment, was about 440 km and was above 380 km for more than a year. At these altitudes, the ionospheric effect on GNSS measurements is lower than those experienced by terrestrial users but still present; the multi-GNSS receiver onboard Bobcat-1 provides multi-frequency measurements, enabling dual-frequency ionospheric corrections. However, ionospheric corrections are affected by receiver-specific inter-frequency bias (IFB) as well as by satellite differential code biases (DCBs). These biases need to be calibrated in order to form accurate XYTOs estimates. In addition, given the significant temperature variations experienced by Bobcat-1 in orbit, the effect of temperature on the receiver-specific IFB shall be taken into account. In this paper, the receiver-specific IFB calibration applied to Bobcat-1 is described and the in-lab IFB temperature calibration is validated over multiple in-orbit data collections, spanning a six-month time interval. Results shown in this analysis focus on Galileo E1C – E5b and GPS L2P(Y) (semi-codeless) – L5 IFB, with plans to expand to other GNSSs and frequency/signal combinations in future work. In order to separate the receiver-specific IFB from the ionospheric effect, a zero-total electron content (TEC) method is applied, resulting in residual errors whose upper bound can be defined using global TEC maps. More sophisticated TEC estimation techniques will enable more accurate IFB calibration in future work. As expected, the result shows that the temperature is the dominant effect on IFB variations, and it is seen that the calibration holds throughout data collected in-orbit.