Satellite cross links generally require narrower bandwidths for increased power concentration. We can increase the power concentration by increasing the cross link frequency with the same size antenna. But the source technology and the modulation hardware required at these higher frequency bands are still in the development stage. Use of optical frequencies will help to overcome this problem with the availability of feasible light sources and the existence of efficient optical modulation communications links with optical beams are presently being given serious considerations in inter-satellite links. And establishing an optical cross link requires first the initial acquisition and cracking of the beacon by the transmitting satellite followed by a pointing of the LASER beam after which data can be modulated and transmitted.
[Keywords.:- LASER, Acquisition, Modulation, Optical Cross Link]
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.
Fig. 1:-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 inter orbit link (IOL) can employ either microwave or optical technology.
The European Space Agency (ESA) has programmed 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 inter orbit 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 un-modulated) 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 has 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 programmed 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 program is in two phases. Phase I was to produce a terminal flight configuration and perform detailed subsystem design and modeling. 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 inter orbit link (RIOL). The SOUT RIOL data rate is specified as any data rate up to 2 Mbps with bit error ratio (BER) of better than 106. The forward inter-orbit 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 faces 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 apertures. 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 SOUT 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-system 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 periscope coarse pointing mechanism (CPA) small refractive telescope, fiber coupled lasers and receivers, fiber 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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