Novel LRI Level 0/1A Data Simulator

GRACE-FO was launched almost six years ago, following the successful GRACE mission. Measuring changes in the Earth's mass distribution is useful for climate research, such as monitoring melting ice caps or changes in groundwater storage. The Earth's gravitational field causes distance variations between the two twin satellites, which are measured by a Microwave Instrument (MWI) and a Laser Ranging Interferometer (LRI). This first inter-satellite laser interferometer provides reliable measurements with lower noise than the MWI and has therefore been selected as the main science instrument for future gravity missions.

Here we present the current status and results of a novel LRI Level 0/1A data simulator developed to support studies for future missions and to verify and improve the current LRI1B processing chain. The main inputs to the simulator are satellite orbit files with position and velocity and attitude data sets, which are transformed into LRI Level 0/1A data files, taking into account relativistic effects, phase wrapping, tilt-to-length coupling and many other noise sources. The output data format is equivalent to the raw phase or ranging data from GRACE-FO. The Level 1A data can then be further processed to Level 1B using the same software used for the GRACE-FO LRI flight data processing. We explain our strategy to validate the results and to identify and address limitations in the Level-1A simulator and in our Level-1B flight data processing chain, e.g. by using specially tailored orbit sets.

We conclude that the simulator is capable of deriving realistic LRI Level 0/1A data that can be further processed to Level 1B, and that the simulator has led to improvements in our GRACE-FO LRI Level 1B processing.