Laser link acquisition demonstration for the GRACE Follow-On mission

Abstract

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.

Test environments for the GRACE follow-on laser ranging interferometer

“…This implies that SC jitter, which is a varying misalignment of the SC, could be simulated with an amplitude of up to 225 µrad by shifting the set-point of the DPA control-loop. The static part of the SC misalignment (which is in a range of a few mrad [14]) is introduced by deliberately misaligning the RX and TX beam assemblies by using their motorized rotation mounts. However, larger amplitudes of the SC jitter could also be introduced by synchronously varying the tilt of the RX and the TX assemblies with respect to the SC.…”

Shipborne Acquisition, Tracking, and Pointing Experimental Verifications towards Satellite-to-Sea Laser Communication

“…The divergence angle of very narrow communication beams requires both communicating sides to maintain extremely high dynamic alignment accuracy. With the ultimate aim of realizing a global-scale laser communication for practical use, many significant achievements have been made in the past decades, such as the demonstration of links for building-to-building communication [3,4], airborne applications [5][6][7], inter-satellite applications [8,9], satellite-to-ground [10][11][12][13][14][15][16], and ship-to-ship communication [17][18][19]. However, currently, most of the experiments only involve space and ground communication links or short-distance communication between ships.…”

Towards solid-state beam steering using a 7-emitter 1550 nm optical phased array

“…For example, the Laser Lunar Communication Demonstrator by MIT and NASA used a 2-axis gimbal system 1 to maintain the transmitting aperture pointing towards earth. The pair of satellites used in the GRACE-FO mission by NASA used a piezo-actuated fast steering mirror 2 for the link acquisition scan between the satellites at a distance of over 200km. Light Detection And Ranging (LiDAR) systems also requires high speed beam steering devices to rapidly and precisely interrogate a field of regard.…”