Abstract
The gravity field maps of the satellite gravimetry missions Gravity Recovery and Climate Experiment (GRACE ) and GRACE Follow-On are derived by means of precise orbit determination. The key observation is the biased inter-satellite range, which is measured primarily by a K-Band Ranging system (KBR) in GRACE and GRACE Follow-On. The GRACE Follow-On satellites are additionally equipped with a Laser Ranging Interferometer (LRI), which provides measurements with lower noise compared to the KBR. The biased range of KBR and LRI needs to be converted for gravity field recovery into an instantaneous range, i.e. the biased Euclidean distance between the satellites’ center-of-mass at the same time. One contributor to the difference between measured and instantaneous range arises due to the nonzero travel time of electro-magnetic waves between the spacecraft. We revisit the calculation of the light time correction (LTC) from first principles considering general relativistic effects and state-of-the-art models of Earth’s potential field. The novel analytical expressions for the LTC of KBR and LRI can circumvent numerical limitations of the classical approach. The dependency of the LTC on geopotential models and on the parameterization is studied, and afterwards the results are compared against the LTC provided in the official datasets of GRACE and GRACE Follow-On. It is shown that the new approach has a significantly lower noise, well below the instrument noise of current instruments, especially relevant for the LRI, and even if used with kinematic orbit products. This allows calculating the LTC accurate enough even for the next generation of gravimetric missions.
Highlights
The twin Gravity Recovery and Climate Experiment (GRACE) satellites observed Earth’s gravity field and, more importantly, the monthly time variations of it from the launch in 2002 until their reentry in 2017
In order to verify the equations for the light propagation time and our implementation of the software code, we performed a closed-loop simulation using reduced-dynamic orbit data of both GRACE FollowOn satellites in the Geocentric Celestial Reference Frame (GCRF) from 5th February 2019 (GNI1B Release 04)
The Laser Ranging Interferometer aboard GRACE FollowOn demonstrated for the first time laser ranging between satellites in a gravimetric satellite mission
Summary
The twin GRACE satellites observed Earth’s gravity field and, more importantly, the monthly time variations of it from the launch in 2002 until their reentry in 2017. The novel LRI and conventional KBR are operated in parallel and, since both should measure the same Euclidean distance variations after some post-processing corrections that are described below, intercomparisons and cross-calibrations can be performed in order to characterize the instruments and their behavior. Both instruments rely on the transmission of electromagnetic radiation, back and forth, between the satellites. We compare our results for the LTC against the results from official datasets for GRACE and GRACE Follow-On in Sect. 10, while Sect. 11 addresses further potential improvements in the light time correction calculation
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