31-07-2014, 02:17 PM
optical satellite communication
[attachment=66426]
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.
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
LINK DESIGN
• The transmit and receive wavelengths are determined by the need for interoperability with future GEO terminals such as SILEX which are based on GaAIAs laser diodes.
• Circular polarisation is used over the link so that the received power does not depend upon the orientation of the satellite.
• The transmit and receive beams inside the terminal are arranged to have orthogonal linear polarisation and are separated in wave length.
• This enables the same telescope and pointing system to be used for both transmit and receive beams since the optical deplexing scheme can then be used
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.
OVERVIEW OF THE SOUT TERMINAL
• The SOUT terminal consists of two main parts: a terminal head unit and a remote electronics module (REM).
• The REM contains the digital processing electronics for the pointing acquisition and tracking (PAT) and terminal control functions together with the communications electronics.
• This unit is hard mounted to the space craft and has dimensions 200 by 200 by 150mm.
• The REM will have the advantage of advanced packaging ASIC and technologies to obtain a compact low mass design.
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.
FINE POINTING LOOP
• The fine pointing loop (FPL) is required to attenuate external pointing disturbances so that the residual mispoint angle is a small fraction of the optical beam width.
• The closed loop tracking subsystem consists of a tracking sensor which determines the direction of the incoming communications beam with an angular resolution around 5% of the optical beam width and a fine pointing mirror assembly (FPA) which compensates beam mispointing effects. The SOUT FPL is used to compensate for frequencies upto 80 HZ.
• A three point antivibration mount (soft mount) acts as a low pass filter to form an isolating interface between the satellite microvibration environment and the SOUT thereby reducing the bandwidth requirements of the FPL.
• This also removes any concerns about uncertainities in the vibration spectrum of the user space craft. The EPA is implemented by a pair of orthogonal mirrors. The EPA for the SOUT is based on a dual axis tilting mirror mechanism. This employs a single mirror and a permanently excited DC motor.
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.
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=66426]
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.
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
LINK DESIGN
• The transmit and receive wavelengths are determined by the need for interoperability with future GEO terminals such as SILEX which are based on GaAIAs laser diodes.
• Circular polarisation is used over the link so that the received power does not depend upon the orientation of the satellite.
• The transmit and receive beams inside the terminal are arranged to have orthogonal linear polarisation and are separated in wave length.
• This enables the same telescope and pointing system to be used for both transmit and receive beams since the optical deplexing scheme can then be used
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.
OVERVIEW OF THE SOUT TERMINAL
• The SOUT terminal consists of two main parts: a terminal head unit and a remote electronics module (REM).
• The REM contains the digital processing electronics for the pointing acquisition and tracking (PAT) and terminal control functions together with the communications electronics.
• This unit is hard mounted to the space craft and has dimensions 200 by 200 by 150mm.
• The REM will have the advantage of advanced packaging ASIC and technologies to obtain a compact low mass design.
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.
FINE POINTING LOOP
• The fine pointing loop (FPL) is required to attenuate external pointing disturbances so that the residual mispoint angle is a small fraction of the optical beam width.
• The closed loop tracking subsystem consists of a tracking sensor which determines the direction of the incoming communications beam with an angular resolution around 5% of the optical beam width and a fine pointing mirror assembly (FPA) which compensates beam mispointing effects. The SOUT FPL is used to compensate for frequencies upto 80 HZ.
• A three point antivibration mount (soft mount) acts as a low pass filter to form an isolating interface between the satellite microvibration environment and the SOUT thereby reducing the bandwidth requirements of the FPL.
• This also removes any concerns about uncertainities in the vibration spectrum of the user space craft. The EPA is implemented by a pair of orthogonal mirrors. The EPA for the SOUT is based on a dual axis tilting mirror mechanism. This employs a single mirror and a permanently excited DC motor.
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.
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.