Optical Satellite Communication full report
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20-01-2010, 01:38 PM

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Satellite crosslinks 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 intersatellite links.
And establishing an optical cross link requires first the initial
acquisition and cracking of the veacon by the transmitting satellite
followed by a pointing of the LASER beam after which data can be
modulated and transmitted.
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
Fig. 1
Inter orbit link for relaying data from LEO space craft to
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
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
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 project and implimentationed 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.
3.1 Wave length and polarization.
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.
3.2 Link budgets for an asymmetric link
The requirement to transmit a much higher data rate on the
return link than on the forward link implies that the minimum
configuration is one with a large telescope diameter at GEO ie maximise
the light collection capabilities and a smaller diameter telescope at
Leo. A smaller telescope at LEO has the disadvantages of reduced light
collection hut the advantage of reduced pointing loss due to wider beam
width. The smaller telescope on LEO facilitates the design of a small user
terminal. For SILEX the telescope diameter in 25 cm but it is highly
desirable k a telescope with less than 10 cm aperture in the user
terminal. The design process begins with the link budgets to ensure
that adequate link margins is available at end of life too the chosen
telescope diameters and laser powers.
3.3 Pointing, Acquisition and Tracking
The narrow optical beam width gives rise to a need to perform
the following critical pointing factions.
3.3.1 Pointing
The LEO terminal must be able to point in the direction of the
GEO terminal around a large part of the LEO orbit. Pointing error do
occur some time and it is determined by the accuracy with which the
transmitting satellite can illuminate the receiving satellites. This is
turn depends on
1. accuracy to which one satellite knows the location of the other
2. accuracy to which it knows its own attitude and
3. accuracy to which it can aim its beam knowing the required
3.3.2 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 f 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
3.3.4 Point ahead
This is needed because of the relative orbital motion between
the satellites which calls for the transmitted beam to be aimed at a
point in space where the receiving terminal will be at the time of
arrival of the beam. The point ahead angle is calculated using the
Point ahead angle 2Vt /c where
Vt = transverse Velocity component of the satellite.
C = Speed of light
The point ahead angle is independent of the satellite cross
link distance.
The block diagram for a generic direct detection optical
terminal is shown.
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 (EPA) 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.
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.
Small optical user terminal configuration
In the figure the SOUT configuration head unit is shown. The
REM is not shown and the supporting structure and terminal control
hardware have been removed for clarity. The terminal head performs the
critical functions of generating and pointing the transmit laser beam
and acquiring and tracking the received beacon and tracking beams.
There is fixed head unit with a periscopic course pointing
assembly (CPA) on top of the telescope. The telescope with the CPA is
referred to as the optical antenna. The head unit is soft mounted to
the satellite by a set of three anti vibration mounts arranged in a
triangular geometry. This fillers out high frequency microvibrations,
originating from the space craft. Inclusion of the soft mount has a
major impact on the terminal fine pointing loop design and structural
configuration as described below. All of the optical components and
mechanisms needed for transmit and receive functions except for the
telescope and CPA are mounted on the double sided optical bench. The
head unit also includes an electronics package (CPEM) which contains
electronics required to be in close proximity to the sensors and
pointing mechanisms.
Key elements of the head unit are the integrated transmitter
comprising diode laser and point ahead assembly (PAA) optical antenna
comprising telescope and coarse pointing assembly, fine pointing loop
comprising acquisition and tracking sensor (ATDU) and fine pointing
assembly (FPA) and optical bench.
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.
The integrated transmitter is shown schematically below.
This consists of a prime/redundant pair of laser modules, a
redundancy switch, and a point ahead assembly (PAp). The lasers are
connected to the PM by a single mode polarisation. This allows grater
layout flexibility on the optical bench and simplifies redundancy
switching. Each laser module contains a laser diode, collimating
lenses, cylindrical le and focusing lense for coupling light into the
fiber. Coupling effiency into the fiber is expected to exceed 70%.
The point ahead angular is ±200 prad for both polar orbiting
and equatorial LEO orbits. The PAA is used in calibration mode to
coalign the transmit and receive paths. The PAA is a piezoelectricaily
actuated device which translates the optical fibre from the selected
laser source in the focal plane of a collimating lens so as to
introduce the required angular offet to the transmit beam direction.
Orthogonal piezos provide for two dimensional pointing of the beam
Capacitive, sensors measure the relative position of the fibre and lens
enabling pointing bias and noise levels of less than 2 micoral and less
than 0.4 microrad respectively to be realised. The redundancy switching
is implemented by a paraffin actuator which translates the required
fibre into the focal point or the PAA collimating lens.
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.
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.
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 Coaligtnment 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.
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
Europeâ„¢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.
1. Editorial: IEEE Processing - Optoelectronics - Vol 141 - Dec
2. GATENBY, P and GRANT.M : Optical intersatellite links for space
3. ROBERT GAGUADE: Sattelite Communication
4. WITTIG.M and OPEN HAUSER.G: Performance of optical
intersatellite links.
I extend my sincere thanks to Prof. P.V.Abdul Hameed, Head of
the Department for providing me with the guidance and facilities for
the Seminar.
I express my sincere gratitude to Seminar coordinator Mr. Berly C.J, Staff in charge, for their cooperation and guidance for
preparing and presenting this seminar and presentation.
I also extend my sincere thanks to all other faculty members of
Electronics and Communication Department and my friends for their
support and encouragement.
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