title: Generation and Analysis of Satellite Laser Ranging Normal Points for Geodetic Parameter Estimation
Creators: name: Geisser, Linda
Publisher: name: Universität Bern
Year: 2024
Subjects: subject: 520 Astronomy

description: Satellite Laser Ranging (SLR) is a well-established space geodetic technique measuring the round-trip time-of-flight of ultra-short laser pulses emitted by a terrestrial ground station to satellites, ideally equipped with retro-reflectors, and back to the station with an accuracy of few millimeters. The dedicated spherical SLR satellites with small area-to-mass ratio, which minimizes the impact of non-gravitational perturbing forces and simplifies the orbit modeling, are well suited to determine geodetic parameters, e.g., station and geocenter coordinates, Earth rotation parameters and the very low-degree Spherical Harmonic (SH) geopotential coefficients. As a consequence, SLR is essential for the realization of long-term stable terrestrial reference frames. In addition, the reliable estimation of the low-degree SH geopotential coefficients C₂₀ and C₃₀, makes SLR an indispensable contribution to the determination of the Earth's time variable gravity field.
SLR observations are sparse and the global coverage is poor due to the limited SLR network of the International Laser Ranging Service (ILRS) consisting of around 45 active stations, which are mainly located on the Northern hemisphere with a main cluster in Europe. Furthermore, the SLR Normal Point (NP) data, which are the basis for all geodetic SLR analyses, result from inhomogeneous data compression performed by the individual SLR stations and, therefore, may introduce systematic errors in the SLR analyses. Since 2018, most of the SLR stations also provide their non-compressed full-rate data, which allows to homogenize the data compression process and to analyze different screening techniques. The first results, where NP data from the Swiss Optical Ground Station and Geodynamics Observatory Zimmerwald (SwissOGS) are analyzed, indicate a potential impact on the parameter estimation, when the SLR NP data are generated homogeneously for all SLR stations.
The full scientific potential of SLR is investigated by studying weekly multi-satellite SLR combinations of spherical SLR satellites (LAGEOS-1/2, LARES, LARES-2, Starlette and Stella) with different orbital properties, e.g., inclination or altitude, are performed. The combination of observations to different satellites is challenging due to differences in the number of observations, sensitivity to a parameter or the stochastic behavior. In this work, two different combination approaches, i.e., Variance Component Estimation (VCE) and the Helmert's 'simple' estimator, are analyzed. To improve the individual contribution of each satellite, the optimal orbit parametrization and modeling, especially for low-flying satellites which experience a more variable environment, e.g., a higher density of the atmosphere and feature an increased sensitivity to the SH geopotential co-efficients, are studied. The increased sensitivity gained by multi-satellite SLR combinations allows to extend the estimated parameter space but also introduces new correlations. Consequently, the orbit parametrization is adapted to again perform a reliable parameter estimation. As an example, the SH geopotential coefficient C₃₀ can only be estimated by a LAGEOS-1/2 and LARES SLR combination, when the once-per-revolution sine accelerations in along-track are omitted in the orbit parametrization.
Finally, to strengthen the ILRS contribution to the realization of further terrestrial reference frames, multi-satellite SLR combinations based on LAGEOS-1/2, LARES and LARES-2 are discussed. Although the LARES-2 mission at this point in time was in orbit for less than half a year, the first analysis already confirm the high quality of the SLR observations to LARES-2.