Abstract
We experimentally demonstrate an inter-satellite laser link acquisition scheme for GRACE Follow-On. In this strategy, dedicated acquisition sensors are not required-instead we use the photodetectors and signal processing hardware already required for science operation. To establish the laser link, a search over five degrees of freedom must be conducted (± 3 mrad in pitch/yaw for each laser beam, and ± 1 GHz for the frequency difference between the two lasers). This search is combined with a FFT-based peak detection algorithm run on each satellite to find the heterodyne beat note resulting when the two beams are interfered. We experimentally demonstrate the two stages of our acquisition strategy: a ± 3 mrad commissioning scan and a ± 300 μrad reacquisition scan. The commissioning scan enables each beam to be pointed at the other satellite to within 142 μrad of its best alignment point with a frequency difference between lasers of less than 20 MHz. Scanning over the 4 alignment degrees of freedom in our commissioning scan takes 214 seconds, and when combined with sweeping the laser frequency difference at a rate of 88 kHz/s, the entire commissioning sequence completes within 6.3 hours. The reacquisition sequence takes 7 seconds to complete, and optimizes the alignment between beams to allow a smooth transition to differential wavefront sensing-based auto-alignment.
Highlights
The Gravity Recovery and Climate Experiment (GRACE) satellites have been monitoring key aspects of our climate since their launch in 2002 [1, 2]
A new mission, GRACE Follow-On (GFO), is planned to include a laser ranging instrument based on a laser interferometer as a technology demonstrator to improve the inter-satellite displacement measurement by up to a factor of 25 [10, 11]
Laser interferometry is the preferred candidate for satellite ranging and will likely be utilized in future space missions, such as for gravity observations/geodesy missions (e.g. GRACE Follow-On [10], GRACE-II [12]) and space-based gravitational wave detectors (LISA [13], BBO [14])
Summary
The Gravity Recovery and Climate Experiment (GRACE) satellites have been monitoring key aspects of our climate since their launch in 2002 [1, 2]. These twin satellites fly 200 km apart in a polar, low-Earth orbit and employ a microwave instrument to continuously track changes in their separation with micron-level sensitivity. These displacement measurements (along with GPS and accelerometer data) have been used to produce monthly models of Earth’s gravity field [3]. Laser interferometry is the preferred candidate for satellite ranging and will likely be utilized in future space missions, such as for gravity observations/geodesy missions (e.g. GRACE Follow-On [10], GRACE-II [12]) and space-based gravitational wave detectors (LISA [13], BBO [14])
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