Data Analysis for Laser Ranging from GRACE-FO to NGGM

The technology demonstrator instrument, laser ranging interferometer (LRI), aboard the two-satellite constellation GRACE-FO mission has been highly successful in measuring the changing range between the spacecraft. The phase data measured by the optical electronics of the LRI is filtered by the Laser Ranging Processor (LRP) and transmitted to the onboard computer (OBC), which then beams it to the ground stations.

We will discuss the LRI data processing on-ground for GRACE-FO and the development of technology and data analysis for future gravity missions.

The post-processing, on ground, involves calculating one-way range retrieval, range rate, range acceleration, and the scale factor. It employs various algorithms to detect non-gravitational events in the data, including phase jumps and single-event upsets. The LRI's one-way range sensitivity can be shown to be 1000 times better than that of the conventional microwave ranging instrument (MWI/KBR). This success has led to all future gravity-field retrieval missions to have an LRI-equivalent as their primary ranging instrument.

The NGGM is the first twin-satellite gravity mission by the European Space Agency (ESA), estimated to launch in 2032. The ranging instrument onboard NGGM is called the Laser Tracking Instrument (LTI). All components of this instrument are designed, implemented and tested in Europe.

Our group is primarily involved with developing and testing the engineering model (EM) of the Instrument Control Unit (ICU). This instrument is similar to the LRP of GRACE-FO. The ICU consists of an RTG4 FPGA and a GR740 processor. We developed the software and firmware to enable the ICU to measure the phase from the optical bench in various operating modes and control the laser. The processor within the ICU is responsible for filtering and decimating the phase data and packaging it into ECSS standard PUS telemetry packets. To communicate with the ICU, an onboard computer simulator (OBC-SIM) was developed. This OBC-SIM can send telecommands to and receive telemetry from the ICU over a SpaceWire interface. This telemetry can be recorded and analysed with similar post-processing to the LRI.

All in all, the presentation will provide an overview of the data processing for GRACE-FO and the subsequent activities in the development of the software, OBC-SIM, and post-processing of the data for the ICU EM.