Gaps in Real-Time GNSS Satellite Clocks and Their Impacts on LEO Satellite POD

Abstract

Low Earth Orbit (LEO) satellite orbits are required in real-time with high accuracy to enable the LEO augmentation to Global Navigation Satellite Systems (GNSSs) for its use in Positioning, Navigation and Timing (PNT). To guarantee high accuracy in realtime LEO-augmented PNT services, real-time GNSS orbits and clocks computed utilizing GNSS measurements need to be provided with high accuracy, and among them, the high-sampling clock products play an essential role in the GNSS Signal-In-Space Range Error (SISRE). In case of gaps in the GNSS satellite clocks, their interpolation is often performed to allow for the usage of the observations during the gaps. The introduced interpolation biases, however, could lead to larger degradations in the results than those when sacrificing these observations. In this contribution, an approach is first introduced to stabilize the time reference of the realtime GNSS satellite clocks to reduce the interpolation errors. Taking the CNES real-time products as an example, the resulting combined orbital and interpolated clock errors are significantly reduced, e.g., from more than 1 dm to below 5 cm for gaps of 390 s. Secondly, the study attempts to search for an appropriate maximal allowed gap length for clock interpolation for the kinematic and the reduced-dynamic Precise Orbit Determination (POD). During testing, an observation availability of 90% is ensured with gaps generated with different lengths, i.e., from 20 to 1800 s. It was found that clock interpolation over, e.g., 300 s, is important to guarantee proper pre-processing. In the final POD, clock interpolation over 60 s already leads to a 13% degradation in the kinematic orbital accuracy. Considering also the availability of the solutions, it is suggested to allow for a clock interpolation of 30 s in the kinematic POD. The differences of the reduced-dynamic orbits are generally small allowing or not allowing for clock interpolation over different gap lengths up to 1800 s, i.e., at the sub-mm level in the Orbital User Range Error (OURE).