Reference Frame from the Combination of a LEO Satellite with GPS Constellation and Ground Network of GPS Stations

Abstract

In this section we demonstrate the combination of a LEO satellite with the satellites of the GPS constellation and the ground networks of space geodesy techniques (GPS, SLR, DORIS) in the generation of reference frame parameters. We show clear improvements in terrestrial reference frame parameters after the combination of the GPS constellation in MEO with spaceborne GPS, DORIS and SLR measurements from the Jason-2 satellite in LEO orbit, including station coordinates, tropospheric zenith delays, Earth rotation parameters, geocenter coordinates and GPS satellite orbit and high-rate clock parameters. We analyze the impact of the LEO data on the terrestrial reference frame parameters and possible improvements they could bring. (See also (Svehla et al. 2010b).) This is a continuation of the work performed with the GPS data from the Jason-1 satellite, where the strong impact of the LEO data on the global parameters has already been demonstrated by means of simulated GPS measurements and variance-covariance analysis (Švehla and Rothacher 2006a).

Terrestrial reference frames are usually defined by a set of station coordinates that are estimated over a long period of time using a combination of different space geodesy techniques. However, in the case of Precise Point Positioning (PPP) of a GPS receiver on the ground or kinematic or dynamic POD of LEO satellites using GPS, reference stations on the ground are not directly used to estimate the orbit of a LEO satellite or coordinates of a GPS receiver on the ground. The PPP of a ground station or POD of LEO satellites is based on an intermediate reference frame defined by the GPS satellite orbits and epoch-wise estimates of GPS satellite clocks. Any error in the GPS satellite orbits and clocks, or in this intermediate space-based reference frame (that is highly temporal in nature), will map directly into the LEO kinematic/dynamic orbit and gravity field determination (CHAMP, GRACE, GOCE), altimetry results (Jason-2, Sentinel-3, etc.) or coordinates of a ground station. Therefore, an instantaneous terrestrial reference frame can be defined as a frame created by the epoch-wise solution of GNSS orbit and clock parameters supported by other space geodesy techniques such as SLR, DORIS and VLBI. In the next section we introduce the concept of phase clocks in order to consistently bridge the gap between ground-based and space-based terrestrial frames and show how a terrestrial frame can be transferred to the LEO orbit avoiding biases associated with the code GPS measurements.

At the end we give an insight into the generation of an instantaneous reference frame from different GPS frame solutions (e.g., provided by IGS ACs) by means of least-squares collocation using a so-called intermediate reference sphere in LEO or GNSS orbit. The use of a simple weighted average, which is often used in the combination of GNSS solutions from different IGS ACs without taking into account correlations in time (and space) of each individual solution, will always introduce systematic effects that are not equally distributed over an imaginary sphere at the GNSS orbit height.