Abstract
Interferometric laser ranging is an enabling technology for high-precision satellite-to-satellite tracking within the context of earth observation, gravitational wave detection, or formation flying. In orbit, the measurement system is affected by environmental influences, particularly satellite attitude jitter and temperature fluctuations, demanding an instrument design, which has a high level of thermal stability and is insensitive to rotations around the satellite's center of mass. Different design approaches for a heterodyne dynamic laser ranging instrument have been combined to a new improved design concept that involves the inherent beam tracking capabilities of a retroreflector into a mono-axial configuration with nanometer accuracy. In order to facilitate the accommodation onboard a future satellite mission, the design allows for a continuously adjustable flexible phase center position. To cover large inter-spacecraft distances, the instrument design comprises an active transponder system, featuring a two-dimensional beam steering mechanism to align a local, strong laser to the (weak) input beam without affecting the measurement path. To this end, a dynamic laser ranging instrument is presented, which has compact dimensions and is fully integrated on a single Zerodur baseplate. The instrument performance will be evaluated in a dedicated test setup providing a flat-top beam simulating the laser beam received from a distant spacecraft, including a beam steering subsystem, which allows for monitoring of pathlength variations when the angle of incidence at the optical instrument is changing.
Highlights
Since its launch in March 2002, the Gravity Recovery And Climate Experiment (GRACE) provided a precise map of temporal variations in the Earth’s mass distribution and gravity field
A beam steering subsystem is included in the OGSE, which allows the measurement of pathlength variations while the angle of incidence at the optical instrument is changed
Up to six external devices such as lower frequency analog-to-digital converters (ADCs) and digital-to-analog converters (DACs) communicate directly with the field programmable gate array (FPGA) via a custom interface board connected to the general purpose input/output header
Summary
Since its launch in March 2002, the Gravity Recovery And Climate Experiment (GRACE) provided a precise map of temporal variations in the Earth’s mass distribution and gravity field. Several studies have already been carried out in this context such as e/e2-motion [2,3] and NG2 [4], deriving mission requirements and providing enhanced optical instrument designs. In terms of ranging accuracy, the future gravitational wave observatory Laser Interferometer Space Antenna (LISA) necessitates an improvement of three orders of magnitude and requires a different instrument architecture, several design and manufacturing approaches investigated during the extensive LISA research are valid for a future gravity mission. An instrument design that allows to measure absolute distances while not relying on a large inter-spacecraft distance would enable formation flying, e.g. using several spacecraft in order to synthesize for example a large optical aperture, investigated within the DARWIN mission concept
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