Inter-Satellite laser interferometry

Abstract

Subtle gravitational e ects can be measured by precisely monitoring the position of a test mass. Often this is done by measuring the displacement between two or more test objects. The Gravity Recovery and Climate Experiment (GRACE) satellites do just this, by continuously tracking changes in their separation with micron-level sensitivity. These displacement measurements are used to infer the gravitational potential of the Earth, which has enabled scientists to monitor key aspects of our climate since their launch in 2002. It is planned that the GRACE Follow-On satellites will include a laser ranging instrument as a technology demonstrator to improve the displacement measurement. Before science operation commences and measurements can begin, the laser on each satellite needs to be precisely pointed towards the opposite satellite, and thus the satellites must undergo an initial acquisition scan after launch to establish the laser link. This thesis is concerned with developing technology for the GRACE Follow-On laser ranging instrument and exploring interferometric techniques for future satellite missions. In the following chapters, we experimentally demonstrate an acquisition system with GRACE Follow-On-like parameters, requiring no additional hardware but relying on the photodetectors and signal processing equipment already required for science operation. This strategy was developed with multiple collaborators over several years led by C. Mahrdt at the Albert Einstein Institute. To establish the laser link, ve degrees of freedom must be optimized (pitch and yaw for each beam, and the frequency di erence between the two lasers). Laser steering and frequency scanning patterns are combined with a fast Fourier transform-based peak detection algorithm run on each satellite to nd the signal. We successfully demonstrate both stages (commissioning and reacquisition) of the proposed acquisition strategy. One of the core components needed for the GRACE Follow-On laser ranging instrument is the triple mirror assembly (TMA), a modi ed corner cube that symmetrically routes the laser beam around existing hardware about the satellite's center of mass. A prototype triple mirror assembly was designed and constructed by local and international collaborators, and we present optical tests demonstrating three of the performance requirements of the prototype. The path length stability of a beam traveling through the TMA was measured in a test bed resembling the measurement con guration of the GRACE Follow-On interferometer. The parallelism between the incoming and outgoing beams to/from the TMA is measured to the arc second level. Additional measurements quantify changes in the parallelism as the TMA prototype is heated and cooled. Finally, we give a brief overview of digitally-enhanced interferometry, a developing technique for optical metrology which has signi cant advantages over a conventional heterodyne system and could be employed for future space missions. We present an experimental demonstration of the multiplexing capability of the technique, showing an improved displacement sensitivity between measurement points when information from several sensors is combined to suppress errors due to laser frequency noise. We discuss an option for the technique to be applied to future inter-satellite measurement architectures and examine possible simpli cations to the optical bench layout.