18-09-2014, 10:57 AM
optical satlite communication
[attachment=68088]
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
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 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 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-systems by provision of redundancy improved in recent years and GaAlAs 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.
Acquisition
• The transmitted beam cannot be pointed at the communicating pointer in the open loop made with sufficient accuracy because of uncertainties in the attitude of the space craft, pointing uncertainties in the terminal and inadequate knowledge of the location of the other satellite.
• Consequently before communication can commence, a high power beam laser located on GEO end has to scan over the region of uncertainty until it illuminates the GEO terminal and is detected.
• This enables the user terminal to lock on to the beacon and transmit its communication beam back along the same path.
• Once the GEO terminal receives the LEO communication beam it switches from the beacon to the forward link communication beam.
• The LEO and GEO terminals then track on the received communication beams, thereby foaming. a communication link between the LEO and GEO space craft.
3.3.3 Tracking
• After successful acquisition, the LEO and GEO terminals are operating in tracking mode In this mode the on-board disturbances which introduce pointing fitter into the communication beam are alternated by means of a fine pointing control loop (FPL) to enable acceptable communications to be obtained.
• These disturbances are due to thruster firings, solar arrays drive mechanisms, instrument harmonics and other effects.
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.
STRUCTURAL CONFIGURATION
• The SOUT has a novel structural and thermal design which satisfies the unique demands imposed by the various sub-systems.
• The main structural elements are a truss frame assembly which supports the optical antenna orthogonal to the optical bench, a triangular plate which forms the lower truss support and carries the soft mounts, optical bench and electronic units.
• Key design drivers for the structure are the optical bench pointing stability, soft mount constrains and base-bending moments associated with the telescope CPA.
• There has to be a high degree of Coalignment between the transmit and receive beam paths on the optical bench in order that the transmit beam can be pointed towards the GEO terminal with an acceptably small pointing loss.
• The height of the terminal above the space craft depends upon the mounting interface; options include mounting through a hole in the side wail of the space craft (Suitable for large platforms), external mounting on a support frame, mounting on a deployment mechanism.
• The head unit occupies an area of about 40 by 40cm depending upon the platform interface.
Mass and Power
• The base-line SOUT has a total mass (including REM) of around 25 Kg and a dynamic mass of 3.7kg due to the motion of the CPA.
• The maximum power dissipation is around 65 W.
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.
[attachment=68088]
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
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 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 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-systems by provision of redundancy improved in recent years and GaAlAs 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.
Acquisition
• The transmitted beam cannot be pointed at the communicating pointer in the open loop made with sufficient accuracy because of uncertainties in the attitude of the space craft, pointing uncertainties in the terminal and inadequate knowledge of the location of the other satellite.
• Consequently before communication can commence, a high power beam laser located on GEO end has to scan over the region of uncertainty until it illuminates the GEO terminal and is detected.
• This enables the user terminal to lock on to the beacon and transmit its communication beam back along the same path.
• Once the GEO terminal receives the LEO communication beam it switches from the beacon to the forward link communication beam.
• The LEO and GEO terminals then track on the received communication beams, thereby foaming. a communication link between the LEO and GEO space craft.
3.3.3 Tracking
• After successful acquisition, the LEO and GEO terminals are operating in tracking mode In this mode the on-board disturbances which introduce pointing fitter into the communication beam are alternated by means of a fine pointing control loop (FPL) to enable acceptable communications to be obtained.
• These disturbances are due to thruster firings, solar arrays drive mechanisms, instrument harmonics and other effects.
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.
STRUCTURAL CONFIGURATION
• The SOUT has a novel structural and thermal design which satisfies the unique demands imposed by the various sub-systems.
• The main structural elements are a truss frame assembly which supports the optical antenna orthogonal to the optical bench, a triangular plate which forms the lower truss support and carries the soft mounts, optical bench and electronic units.
• Key design drivers for the structure are the optical bench pointing stability, soft mount constrains and base-bending moments associated with the telescope CPA.
• There has to be a high degree of Coalignment between the transmit and receive beam paths on the optical bench in order that the transmit beam can be pointed towards the GEO terminal with an acceptably small pointing loss.
• The height of the terminal above the space craft depends upon the mounting interface; options include mounting through a hole in the side wail of the space craft (Suitable for large platforms), external mounting on a support frame, mounting on a deployment mechanism.
• The head unit occupies an area of about 40 by 40cm depending upon the platform interface.
Mass and Power
• The base-line SOUT has a total mass (including REM) of around 25 Kg and a dynamic mass of 3.7kg due to the motion of the CPA.
• The maximum power dissipation is around 65 W.
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.