Communication links between space crafts is an important element of space infrastructure, particularly where such links allow a major reduction in the number of earth stations needed to service the system. An example of an inter orbit link for relaying data from LEO space craft to ground is shown in the figure below
Interorbit link for relating data from LEO spacecraft to ground
Inter orbit link for relaying data from LEO space craft to ground.
The above figure represents a link between a low earth orbiting (LEO) space craft and a geostationary (GEO) space craft for the purpose of relaying data from the LEO space craft back to the ground in real time. The link from the GEO Satellite to ground is implemented using microwaves because of the need to communicate under all weather conditions. However, the interorbit link (IOL) can employ either microwave or optical technology. Optical technology offers a number of potential advantages over microwave.
I. The antenna can be much smaller. A typical microwave dish is around 1 to 2m across and requires deployment in the orbit, An optical antenna (le a telescope) occupies much less space craft real estate having a diameter in the range of 5 to 30 cm and is therefore easier to accommodate and deploy.
II. Optical beam widths are much less than for microwaves, leading to very high antenna gains on both transmit and receive. This enables low transmitter (ie laser) powers to be used leading to a low mass, low power terminal. It also makes the optical beam hard to introsept on fan leading to convert features for military applications, consequently there is a major effort under way in Europe, USA and Japan to design and flight quality optical terminals
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 Europeâ„¢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 IOL
ESA 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. Phase I was to produce a terminal flight configuration and perform detailed subsystem design and modelling. Phase 2 which started in september 1993 is to build an elegant bread board of the complete terminal.
The link from LEO to ground via the GEO terminal is known as the return interorbit link (RIOL). The SOUT RIOL data rate is specified as any data rate upto 2 Mbps with bit error ratio (BER) of better than 106. The forward interorbit link (FIOL) from ground to LEO was a nominal data rate of (34 K although some missions may not require data transmissions in this directions. Hence the link is highly asymmetric with respect to data rate.
The LEO technical is mounted on the anti earth face of the LEO satellite and must have a clear line of sight to the GEO terminal over a large part of the LEO orbit. This implies that there must be adequate height above the platform to prevent obstruction of the line of sight by the platform solar arrays, antenna and other appertages. On the other hand the terminal must be able to be accommodated inside the launcher fairing. Since these constraints vary greatly with different LEO platforms the SOUl configurations has been designed to be adaptable to a wide range of platforms.
The in-orbit life time required for a LEO mission in typically 5 years and adequate reliability has to be built into each sub-systems by provision of redundancy improved in recent years. and GaAIAs devices are available with a projected mean time to failure of 1000 hours at 100 MW output power.
The terminal design which has been produced to meet these requirements includes a number of naval features principally, a periscopic coarse pointing mechanism (CPA) small refractive telescope, fibre coupled lasers and receivers, fibre based point ahead mechanism (PAA), anti vibration mount (soft mount) and combined acquisition and tracking sensor (ATDU). This combination has enabled a unique terminal design to be produced which is small and lightweight These features are described in the next sections.
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