The
European Space Agency (ESA) has programmes underway to place Satellites carrying
optical terminals in GEO orbit within the next decade. The first is the ARTEMIS
technology demonstration satellite which carries both microwave and SILEX (Semiconductor
Laser Intro satellite Link Experiment) optical interorbit communications terminal.
SILEX employs direct detection and GaAIAs diode laser technology; the optical
antenna is a 25cm diameter reflecting telescope. The
SILEX GEO terminal is capable of receiving data modulated on to an incoming laser
beam at a bit rate of 50 Mbps and is equipped with a high power beacon for initial
link acquisition together with a low divergence (and unmodulated) beam which is
tracked by the communicating partner. ARTEMIS will be followed by the operational
European data relay system (EDRS) which is planned to have data relay Satellites
(DRS). These will also carry SILEX optical data relay terminals.Once
these elements of Europe's space Infrastructure are in place, these will be a
need for optical communications terminals on LEO satellites which are capable
of transmitting data to the GEO terminals.
A wide range of LEO space craft is
expected to fly within the next decade including earth observation and science,
manned and military reconnaissance system. The
LEO terminal is referred to as a user terminal since it enables real time transfer
of LEO instrument data back to the ground to a user access to the DRS s LEO instruments
generate data over a range of bit rates extending of Mbps depending upon the function
of the instrument. A significant proportion have data rates falling in the region
around and below 2 Mbps. and the data would normally be transmitted via an S-brand
microwave IOLESA initiated a
development programme in 1992 for LEO optical IOL terminal targeted at the segment
of the user community. This is known as SMALL OPTICAL USER TERMINALS (SOUT) with
features of low mass, small size and compatibility with SILEX. The programme is
in two phases.
