Analysing Sporadic Short Excitations in LRI Ranging Data: Single Event Upsets and Momentum Transfer Events

Abstract

The Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) mission is a joint USGerman collaboration to continue the record of Earth’s gravity field since the decommissioning of GRACE (2002-2017). GRACE-FO is a two-satellite constellation that orbits the Earth in a near-polar orbit at an altitude of 490 km. The satellites maintain a distance of 220 km ± 50 km between them as they fly along the same orbit, one behind the other. Each spacecraft is affected by the Earth’s varying mass distribution, which changes the inter-satellite range. Two satellite instruments measure this changing range to determine the gravity field: the laser ranging interferometer (LRI) and the microwave instrument (MWI). The LRI is a novel technology that uses a laser link between the satellites and determines the range through heterodyne interferometry of the Doppler-shifted laser beams. The range calculated from this instrument has nanometre-level accuracy, 1000 times lower than the conventional MWI. This thesis analyses the LRI ranging data to study certain fleeting and intermittent disturbances: single event upsets (SEUs) and momentum transfer events (MTEs). SEUs are temporary errors within the memory of an electric device caused by interaction with charged particles. Within the architecture of the laser ranging processor (LRP), two FIR filters offer a certain number of registers where such SEUs can cause bit upsets that eventually affect the four-channel phase data. Simulations are generated to examine the SEU signatures in the data. The unique patterns observed are stored in a pair of look up tables (LUTs) and used in a maximum likelihood method (MLM) pattern-matching algorithm. The goal was to detect SEUs, characterise them and remove the model from the ranging data. The algorithm successfully detects 32 bitflip events over five years. They are equally distributed between both spacecraft and also among the four channels. They are modelled and removed from the phase data with residuals in the order of 10−6 cycles. The geographical location of the events over the map of the Earth shows more than half of them occurring in the South Atlantic anomaly (SAA), where the magnitude of the Earth’s magnetic field is the lowest, and the charged particles’ concentration is thus high. The more frequently occurring MTEs are caused by the physical impact of orbital debris on the GRACE Follow-On (GRACE-FO) spacecraft. The LRI measures these disturbances as accelerations along the line-of-sight. By cross-correlating these accelerations with those measured by the onboard accelerometer, 265 such MTEs are detected. MTE impacts are caused by natural or artificial sources. The natural sources are, for example, the constant meteoroid background and yearly meteoroid streams. The artificial sources comprise orbital debris accumulated around the Earth since the beginning of space exploration, especially concentrated in low earth orbit. The events detected coincide with the period of the β-angle of the sun.