SILEX is a free-space optical communication system which consists of two optical communication payloads to be embarked on the ESA Artemis (Advanced Relay and TEchnology MIssion Satellite) spacecraft and on the French Earth-observation spacecraft SPOT-4. It will allow data transmission at 50Mbps from low Earth orbit (LEO) to geostationary orbit (GEO) using GaAlAs laser-diodes and direct detection.
The SILEX Phase A and B studies were conducted around 1985, followed by technology breadboarding and predevelopment of the main critical elements which were tested on the so-called System Test Bed to verify the feasibility of SILEX. A detailed design phase was carried out in parallel with the System Test Bed activities up to July 1989. At that time, the development of SPOT-4 Phase C/D was agreed with an optical terminal as passenger. This was an important decision since it made a suitable partner satellite available for the ESA data-relay satellite project; the stage was therefore set to start the main SILEX development effort in October 1989.
In March 1997, a major milestone was reached in the SILEX programme: both terminals underwent a stringent environmental test programme and are now ready for integration with their host spacecraft. However, due to the agreed SPOT-4 and Artemis launch dates, it is likely that the in-orbit demonstration of the overall system will not start before mid-2000. Consequently, the GEO terminal will need to be stored after the completion of the spacecraft testing. The first host spacecraft (SPOT-4) is planned for launch in February 1998. The launch of Artemis on a Japanese H2A is delayed for non-technical reasons until February 2000. Apart from launching Artemis, Japan is participating in the SILEX programme with its own laser terminal, LUCE (Laser Utilizing Communications Equipment), to be carried onboard the Japanese OICETS satellite (Optical Inter-orbit Communications Engin-eering Test Satellite), set for launch in summer 2000.
Optical ground station on Tenerife As part of the SILEX in-orbit check-out programme, ESA started to construct an optical ground station on the Canary Islands in 1993 (Fig. 2). This station, which will be completed by the end of 1997, simulates a LEO optical terminal using a 1 m telescope, allowing the performances of the GEO optical terminal on Artemis to be verified. The optical ground station will receive and evaluate the data transmitted from Artemis and will simultaneously transmit data at optical wavelengths towards Artemis. In addition to its primary objective as the SILEX in-orbit check-out facility, the optical ground station will also be used for space-debris tracking, lidar monitoring of the atmosphere and astronomical observations.