Impact of incorporating SPIRE CubeSat GPS observations in a global GPS network solution

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.