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Precise orbit determination (POD) of low earth orbiters (LEO) using data of the Global Positioning System (GPS) and the computation of GPS network solutions are usually performed in separate processes. Typically, orbits and clock corrections of GPS satellites, which have been previously determined as part of a network solution, are introduced as fixed in a LEO POD. However, various studies have shown that the inclusion of GPS observations of LEOs in a global network solution can be advantageous, particularly in terms of the quality of resulting GPS products and certain geodetic parameters, such as the Earth’s center of mass coordinates. Since a few years more and more GPS data from LEO CubeSats are available, such as from the SPIRE satellites which are equipped with dual-frequency GPS receivers and may be used as part of specific scientific investigations. The SPIRE satellite constellation is a group of remote sensing satellites that are used to measure and map the earth's surface and atmosphere.In the present study, we aim to determine the impact of GPS observation data from specific SPIRE satellites when they are included in the computation of a global network solution. Code and phase observations received by GPS receivers on board of selected SPIRE satellites, along with those from ground stations of the International GNSS Service (IGS), are processed together in a joint least-squares adjustment process to obtain a combined GPS-LEO solution. This determines orbit parameters of the SPIRE satellites together with GPS orbit parameters and geodetic parameters, such as station coordinates, Earth rotation parameters, and the Earth's center of mass coordinates. To assess the influence of the SPIRE LEOs more closely, various solutions are analyzed in which GPS observations from different SPIRE satellites and from scientific LEO missions are included. Given the different orbit characteristics of the selected SPIRE satellites, specifically the orbital inclinations, it is also possible to examine the impact of this on the resulting solution. The global solutions are compared to each other and to a solution using only data from terrestrial GPS stations. Quality characteristics are analyzed to assess the quality of the combined GPS-LEO solution, which are the resulting GPS orbit solutions in terms of orbit overlaps at arc boundaries, the quality of geodetic parameters, and the determined formal errors of the estimated parameters. The resulting SPIRE orbits of the combined solutions are also compared to those of a separate POD.

Global Navigation Satellite Systems such as the Global Positioning System (GPS) are a unique tool for deriving very precise orbits of Low Earth orbiting (LEO) satellites equipped with onboard GPS receivers. LEO precise orbit determination (POD) requires the proper modeling of antenna phase center variations (PCVs) for both the GPS transmitter and the LEO receiver antennas. While for the GPS antennas the nadir-dependent values from the official absolute antenna phase center model igs14.atx of the International GNSS Service (IGS), consistent with the underlying GPS orbit and clock products, are used, official PCV maps are usually not available for the LEO receiver antennas. If these variations are not considered, however, this may result in systematic errors in the derived LEO orbits. LEO PCV maps can be determined and exploited in different ways. One possibility is to use the PCV maps from ground calibrations provided by the manufacturer, which usually do not reflect, however, the influence of error sources which are additionally encountered in the actual spacecraft environment, e.g., near-field multipath. Alternatively, one can make use of GPS measurements and POD results to estimate the PCV map empirically, as it is done in this study.In this study, the influence of different attitude modes on Jason-3 POD using GPS observations and PCV map estimation is investigated. As Jason-3 in an altimetry satellite, its main objective is to measure global sea-level rise. Therefore, it is of particular importance to precisely determine the radial component of the orbit and proper PCV modeling is of high importance. As Jason-3 is experiencing different attitude modes, yaw-steering and fixed-yaw attitude with either the positive or negative x-axis pointing in the direction of flight, PCV maps are expected to be better disentangled from other error sources. In this study, we are analyzing PCV maps determined from residual stacking using GPS data from the different attitude modes and from different orbit parametrizations. First results indicate that PCV maps estimated from time spans of different attitude modes differ and systematic orbit differences are occurring in a reduced-dynamic POD.

A classical reduced-dynamic GPS-based Precise Orbit Determination (POD) strategy for Low Earth Orbit (LEO) satellites is often based on a limited explicit modelling of satellite dynamics and modelling deficiencies are compensated by numerous empirical parameters. With better gravitational models and the advances in satellite surface force modeling, uncertainties in the satellite dynamics are significantly reduced. Furthermore, single-receiver ambiguity resolution allows for more robust POD as well. Therefore, a dynamic POD strategy using significantly fewer estimated empirical parameters can be implemented to generate dynamic orbits, which allow for force modeling sensitivity analyses and evaluating potential errors in the adopted GPS antenna reference points or phase center offsets, etc. This presentation outlines the recent dynamic POD methodology developments at the Astronomical Institute of the University of Bern (AIUB) and investigates the POD performances for a few dedicated space geodesy satellite missions (Swarm, GRACE-FO, Sentinel-1, Sentinel-2, Sentinel-3 and Jason-3) that are operated at altitudes ranging from 430 to 1350 km. The focuses will be on satellite gravitational and non-gravitational force modeling, satellite dynamics parametrization, and orbit validations for different types of satellites. Results reveal that the dynamic POD strategy is flexible and robust to generate high-quality orbits for those satellites, showing reliable agreements with the independent ambiguity-fixed kinematic orbits and the external Satellite Laser Ranging (SLR) measurements.

In this article we analyze the benefit of computing a combined solution from individual orbit solutions for the low Earth orbiting satellite Sentinel-3A. The selected combination scheme for calculating the combined solution is Variance Component Estimation (VCE). Before a combination is calculated, the individual orbit solutions are analyzed to identify systematic differences and characteristics. Simulation studies are performed to show under which conditions a combined solution of superior quality may be expected. The combined solution is validated by Satellite Laser Ranging (SLR) measurements and compared with SLR validations of the individual solutions. The result of this investigation shows that VCE, implemented as an iterative procedure, is suitable to obtain an orbit solution of superior quality for Sentinel-3A.