Link acquisition and optimization for intersatellite laser interferometry

Abstract

The topic of this thesis is the initial link acquisition process for intersatellite laser interfer- ometers. Furthermore, a technique was studied that can be applied to enhance the signal to noise ratio of the established laser link. This work was carried out in the context of the Gravity Recovery And Climate Experiment Follow-On (GRACE-FO), Next Generation Gravity Field Mission (NGGM) and Laser Interferometer Space Antenna (LISA) satellite missions. Link acquisition is one of the most critical steps during the commissioning phase of an intersatellite laser interferometer. It is required to calibrate unknown pointing offsets of the laser beams that are caused by manufacturing and alignment tolerances of the instrument, structural distortions of the spacecraft (S/C) due to temperature changes and the different gravity levels on ground and in orbit, as well as insufficient knowledge about the attitude of the S/C. Furthermore, the frequencies of the lasers on the different S/C have to be matched to within the bandwidth of the opto-electronics readout chain. This thesis is split into two parts. The first part focuses on the Laser Ranging Interfer- ometer (LRI) on-board the GRACE-FO mission, while the second part is dedicated to future missions: LISA and NGGM, a possible successor to GRACE-FO. In the first part, laboratory tests that verified the robustness of the GRACE-FO link acquisition procedure are presented. These tests were carried out using a realistic mock-up LRI and a proper experimental test bed that allowed for the introduction of MHz Doppler frequency shifts, pW received (RX) laser powers and flat-top RX beams. Also shown in this part are analysis results of the actual in-orbit link acquisition process that was carried out in June 2018. Unambiguous laser beam pointing offsets below ±1 mrad and a frequency offset, also in the expected range, were obtained. Small alignment errors of the LRI’s triple mirror assemblies (TMAs) were studied by analyzing in-orbit data. A trade-off study is presented which shows how these errors can be optimally compensated by introducing dedicated beam pointing offsets. In the second part of this thesis, the design, construction and characterization of an ex- perimental test bed that simulates the intersatellite laser link of LRI-like instrument is presented. The test bed faithfully recreates the RX laser power as function of the trans- mitted beam pointing angles with an error below 10 %, even for fast spatial scans of the TX beam. Furthermore, dedicated acquisition sensors were studied in the context of LISA and NGGM. A position sensitive photodetector (PSPD) and an indium gallium arsenide (InGaAs) camera with 256×320 pixels were investigated. The PSPD required a minimum laser power of ∼10 nW. The ratio of the achieved resolution, in terms of beamwalk on the sensor, to the sensor size was 0.2 %. Tests of the InGaAs camera were carried out with a laser power of 1 pW. The achieved resolution in relation to the sensor size was 0.065 %. Hence, both sensors fulfill the preliminary NGGM requirements, but only the InGaAs sensor satisfies all considered LISA requirements.