Systematic errors in Satellite Laser Ranging validations of microwave-based orbit solutions

Abstract

Satellite Laser Ranging (SLR), i.e., the optical distance measurement to satellites equipped with laser retroreflectors, has become an invaluable core technique in numerous geodetic applications. SLR measurements to active satellites in Low Earth Orbit (LEO) are up to now mostly used for an independent validation of orbit solutions, usually derived by microwave tracking techniques based on Global Navigation Satellite Systems (GNSS) or Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS). This allows for the analysis of systematic orbit errors (e.g., originating from poorly known satellite center of mass locations or sensor offsets) not only in radial direction, but in three dimensions. A high level of radial orbit reliability is key, e.g., to satellite altimetry applications. For many geodetic applications a mm accuracy and 0.1 mm/year stability is required or at least desired. Unavoidable SLR station biases and coordinate uncertainties are a major error source and obstacle to reach the aforementioned accuracy and stability goals when relying on SLR data. Among the stations of the International Laser Ranging Service (ILRS) there is a large diversity of biases and measurement qualities, and the calibration of these biases for all stations is key to further exploit SLR data for present and future geodetic applications. It has recently been demonstrated that the analysis of SLR data to active LEO satellites with fixed microwavederived orbit solutions is a promising means to analyze SLR biases and their stability. For this, a combined analysis of numerous different satellites and a high-quality modeling of gravitational and non-gravitational forces is a prerequisite. Nevertheless, different uncertainties in various dynamical models and offsets remain, potentially affecting also SLR station-related calibration parameters. In this presentation we address the question on how both station- and orbit-related parameters can be simultaneously derived from SLR analyses to active LEO satellites. Based on a consistently produced set of orbit solutions for 9 different LEO missions (Sentinel-3A/B, Sentinel-6A, Swarm-A/B/C, GRACE-FO C/D and Jason-3) we explore different possibilities to compute parameters that reflect corrections to individual orbit solutions, next to station calibration parameters. A special focus is on how to put constraints that are needed to decorrelate the different parameter sets, as well as their impact on the results. These investigations will help to disentangle station- from orbit-related systematic errors, allowing, e.g., a better characterization of the latter in particular in altimetry applications.