04-08-2014, 12:56 PM
SOUT
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INTRODUCTION
• 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
• 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.
• 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 (like 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.
• Optical beam widths are much less than for microwaves, leading to very high antenna gains on both transmit and receive.
• This enables low transmitter (i.e., 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
SOUT
• 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-band 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
GENERAL OPTICAL TERMINAL
The block diagram for a generic direct detection optical terminal is shown in figure
• In this system a nested pair of mechanism which perform the course pointing and fine pointing functions is used.
• The former is the coarse pointing assembly (CPA) and has a large angular range but a small band width while the latter, the fine pointing assembly (FPA) has a small angular range and large band width.
• These form elements of control loops in conjuction with acquisition and tracking sensors which detect the line of sight of the incoming optical beam.
• A separate point ahead mechanism associated with the transmitter sub system carries out the dual functions of point ahead and internal optical allignment.
4.1 Communications performance
• A property of free space links is the occurrence of burst errors. A burst error results when the instantaneous bit error rate (BER) drops below a defined value.
• This is caused by beam mispointing which reduces the optical power collected by the receiving terminal.
• For SOUT, the probability of a burst error occurring must be less than 10-6.
OPTICAL ANTENNA
• The optical antenna comprises the telescope and coarse pointing assembly. The telescope is a refractive keplerian design which does not have the secondary mirror obscurration loss associated with reflective systems.
• The CPA uses stepping motors together with a conventional spur gear and planetary gear.
• The total height of the optical antenna is a major contributor to the height of the CPA above the platform which affects LEO and GEO link obscurration by solar arrays, antennas and other space craft appendages.
OPTICAL BENCH
• The diplexer, quarter wave plate and other lens system required too acquisition and tracking are all placed in the optical bench.
• The diplexer has a dietetric multilayer coating which provides efficient transmission of one type polarised light at the transmit wavelength (848 nm) and rejects another type poiarised light at the receive wavelength (800 nm).
• A quarter wave plate (QWP) converts the transmit light to circular polarisation state prior to the telescope.
• The PAA, lasers, and redundancy switching mechanisms are on one side while the diplexer, receive paths and calibration path are on the other side of the optical bench.
CONCLUSION
• Optical intersatellite communications promises to become an important element in future space infrastructure and considerable development effort is currently underway in Europe and elsewhere.
• There will be a need for small optical terminals for LEO space craft once Europe’s data relay satellites are in orbit within the next five years.
• The small official user terminal (SOUT) programme funded by ESA seeks to fill this need for data rate around 2Mbps.
• Detailed design and modelling of the SOUT fight configuration has been carried out and has provided a high confidence level that the unique terminal design can be built and qualified with a total mass around 25 Kg.
• The next phase of the programme will be to integrate and test a bread board terminal which is representative of the flight equipment.
• This breadboard will be used to test the performance of the PAT subsystem and to verify the structural and optical configuration for the SOUT.