Abstract
The GRACE Follow-On satellites will use, for the first time, a Laser Ranging Interferometer to measure intersatellite distance changes from which fluctuations in Earth's geoid can be inferred. We have investigated the beam steering method that is required to maintain the laser link between the satellites. Although developed for the specific needs of the GRACE Follow-On mission, the beam steering method could also be applied to other intersatellite laser ranging applications where major difficulties are common: large spacecraft separation and large spacecraft attitude jitter. The beam steering method simultaneously coaligns local oscillator beam and transmitted beam with the laser beam received from the distant spacecraft using Differential Wavefront Sensing. We demonstrate the operation of the beam steering method on breadboard level using GRACE satellite attitude jitter data to command a hexapod, a six-degree-of-freedom rotation and translation stage. We verify coalignment of local oscillator beam/ transmitted beam and received beam of better than 10 μrad with a stability of 10 μrad/ √Hz in the GRACE Follow-On measurement band of 0.002...0.1 Hz. Additionally, important characteristics of the beam steering setup such as Differential Wavefront Sensing signals, heterodyne efficiency, and suppression of rotation-to-pathlength coupling are investigated and compared with analysis results.
Highlights
Since the Gravity Recovery and Climate Experiment (GRACE, see e.g. [1,2,3]) was launched in 2002, it has been successfully monitoring the spatial and temporal variations of Earth’s geoid, proving the feasibility of low-orbit satellite-to-satellite tracking
Developed for the specific needs of the GRACE Follow-On mission, the beam steering method could be applied to other intersatellite laser ranging applications where major difficulties are common: large spacecraft separation and large spacecraft attitude jitter
We demonstrate the operation of the beam steering method on breadboard level using GRACE satellite attitude jitter data to command a hexapod, a six-degree-of-freedom rotation and translation stage
Summary
Since the Gravity Recovery and Climate Experiment (GRACE, see e.g. [1,2,3]) was launched in 2002, it has been successfully monitoring the spatial and temporal variations of Earth’s geoid, proving the feasibility of low-orbit satellite-to-satellite tracking. The LRI employs an active receiver-transponder principle This means that the weak incoming RX beam is “amplified” by a strong local oscillator (LO) beam which is sent back to the distant spacecraft via retroreflection by the Triple Mirror Assembly (TMA, [12,13,14]). On-axis concepts have not shown such a simple capability of closed-loop beam steering For this reason, we would consider the “racetrack” configuration a promising candidate architecture even for a new mission design in which the line-of-sight would be available. The RX beam that is received from the distant spacecraft is clipped at an aperture on the LRI optical bench and overlapped with the local oscillator (LO) beam on a beam splitter (BS, nominally 90% reflective, 10% transmissive). While the steering mirrors on both spacecraft perform angular scan patterns, the laser frequency on one spacecraft (“slave”) is tuned to find the steering mirror positions for each spacecraft and the laser frequency on the slave spacecraft that produce the largest heterodyne signal amplitude [22]
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