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ArtemEx: Artemis Experiment for Optical Feeder Links

Figure 1: Beam Wander . Figure 2: ESA-OGS 1 meter Telescope

ArtemEx is an ESA project (Artes 5.2) involving Cassidian Optronics (now a subsidary of Airbus Defence and Space) and DLR to verify an advanced transmission concept for ground-to-space optical links.
ARTEMIS satellite relayed in 2001 the first image form the LEO Satellite SPOT-4 to Earth (3) and since then has demonstrated successfully direct bidirectional links to the ESA OGS (Optical Ground Station) in Tenerife (Canary Islands - Spain) (1). The SILEX (Semiconductor-Laser Inter-satellite Link Experiment) test-bench in the ESA OGS, used to establish the links to the ARTEMIS satellite, combats atmospheric perturbations in uplink using spatial diversity. The transmitter diversity was achieved originally by splitting one single laser signal into up to four beams and transmitting them through the 1 meter OGS telescope (see Figure 2).  When transmitting several beams using a single laser source, a delay system is needed to ensure the incoherence among them. Although the several successful measurement campaigns, the first delay system installed in the OGS did not produce the expected results in terms of turbulence mitigation (2).
Under this project a new delay line system was developed by Cassidian Optronics and installed in the ESA Ground Station. In October 2012 and April 2013, DLR conducted, together with the collaboration of Cassidian Optronics and the IAC (Instituto de Astrofísica de Canarias), two measurements in the ESA OGS to verify the transmitter diversity with the new delay line system. A focus camera was integrated to the optical test-bench to characterize the strength of the atmospheric turbulence during the measurements. The quality of the signal transmitted from the ESA ground station on Tenerife and received by the satellite terminal was measured. These measurements are of great interest for the future development of optical GEO Feederlinks, because they characterize the uplink fading channel. The fading suffered by the satellite is due to scintillation and beam wandering (see Figure 1). Different transmitted laser beam configurations were tested and results are compared to theory. Also the uplink pointing quality was investigated using several tracking aperture positions, by masking the downlink signal in front of the tracking sensor.
 

1. Angel Alonso, Marcos Reyes, and Zoran Sodnik. Performance of satellite-to-ground communications link between artemis and the optical ground station. volume 5572, pages 372–383. SPIE, 2004.
2. Jose Romba, Zoran Sodnik, Marcos Reyes, Angel Alonso, and Aneurin Bird. Esa’s bidirectional space-to-ground laser communication experiments. pages 287–298, 2004.
3. Toni Tolker-Nielsen and Gotthard Oppenhauser. In-orbit test result of an operational optical intersatellite link between artemis and spot4, silex. volume 4635, pages 1–15. SPIE, 2002.