Diapositiva 1

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Intersatellite link for Earth Observation
Satellites constellation
SPACEOPS 2006 19 23 June 2006, Roma, ITALIA
LtC P. Maurizio De Carlo, Maj G. Marano (IT
MOD),G. Francesco De Luca (ASI) Michelangelo
LAbbate, Domenico Oricchio, Paolo Venditti
(AAS-I) Maria Rosaria Santovito (CORISTA)
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Background/Introduction
This paper is devoted to investigate the
possibility to implement a data Intersatellite
Link between an the satellites of an Earth
Observation constellation and a dedicated TLC GEO
satellite. An EO system is mainly composed of
two segments (space and ground) connected by a
complex network of communications that permits
to manage the operations of the constellation.
Due to the high performances in terms of imaging
capabilities of the radar/optical payloads a
corresponding high capacity to download data to
ground is needed. To guarantee that the system
is able to download to the ground station, in a
proper time, all the images taken from the
imager during the mission an external ground
station, located in a polar zone, have been
foreseen. Polar stations offer a service needed
to provide the requested images to the users in a
near real-time manner. An alternative
approach, using an Intersatellite link system
(ISLs) instead of a polar stations, is
presented. A high speed two way optical link,
between LEO and GEO terminals, in the frame of
satellites constellation, should permit to avoid
to use the polar stations while still ensuring
the achievement of the system end-to-end
performances, in a costsaving approach.
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Mission Scenario
The communication system allows the optical link
between two terminals mounted respectively on EO
satellite which is in a low earth orbit (LEO)
and a dedicate satellite which is in a
geostationary one (GEO)
The mission scenario foresees an hybrid
communication pack optical link from LEO to GEO
and RF link (and/or optical link) from GEO to
ground station. A fiber optic network between
the ground facilities can realize in a future a
very fast connection, without bottleneck,
between space and ground components.
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Simulated Operation Scenario (1)
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Simulated Operation Scenario (2)
OISL link simulated scenario
Areas for direct image acquisition (example for
Italian and Polar stations)
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Simulated Operation Scenario results (3)
Scenario A satellite data receiving facilities
located in Italy centre, Fairbanks and Kiruna
Polar Stations)
16 days visibility period with one satellite
data volume from one satellite
Scenario B Satellite data reciving facilities
located in. Italy centre OISL is considered
between the LEO satellite and a TLC GEO satellite
(assuming the same LEO current data rate from
TLC GEO satellite)
16 days visibility period with one satellite
data volume from one satellite
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Simulated Operation Scenario results (4)
The OISLs arrangement is applicable to all the
station located within the footprint of TLC GEO
satellite
GEO footprint for data downlink
The total access duration is calculated
considering an ideal condition without link
blockage effects due to satellite
appendixes. In a first approximation, we can
considering a reduction of 70 for the data
link duration.
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OISL System Response-time Impact

• reaction time time between the user request
  acquisition and the imaging
• information age time between the imaging and
  the product availability to the users

Impacts performing the complete uplink of the
mission plan and its activation in one single
passage, with consequent reduction of the command
uplink delay time reducing the information age
delay, in particular the contribution of the
On-Board Data Latency and the down-load at
X-band Data Acquisition Station
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OISL System Response-time Impact
Payload data latency (T7) is defined as the time
interval between image sensing time and related
data down-load start time. This time interval is
equal (In the worst case) to the max visibility
gap for the given receiving stations
configuration. Scenario A this time has been
evaluated equal to 11h 30m for National Station
only, located in Italy centre 1h 19min for
Kiruna Fairbanks ground station scenario
Scenario B in case of using a TLC GEO satellite
configuration the maximum visibility gap will be
reduced to about half an orbit (i.e. about 45
minutes) As result, the Maximum information age
will be heavily reduced with respect to the
values given for the National station only (i.e.
about 45m, against 11h 30min ). In the scenario
with Kiruna-Fairbanks the T7 is one half (about
40min).
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OISL System Architecture
The general OISLs system will function
essentially like a conventional RF system. The
main components are a two-axis gimbaled
telescope, an optical bench with a fine pointing
system, a communication sensor and laser diodes,
a thermal control system for precision
temperature control. One of the major differences
between RF and optical ISLs is the pointing
requirements. RF system have pointing accuracy
of mrads, while optical one have µRad and/or
nRad.
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Technological considerations

• Smaller size of the terminal
• Lower weight of the terminal
• Less transmitter power
• Higher data rate
• Higher immunity to Electromagnetic Interferences
• Higher immunity from intercept of data
  communication due
• to the smaller transmitter beam divergence
  angle

Advantages of optical link
Drawbacks of optical link

• New technology for optical space communication

• D.W. Dreisewerd et al., A high data rate LEO to
  GEO optical crosslink design, Military
  Communications Conference 1992,
• Communications-Fusing Command Control and
  intelligence, IEEE 11-14 Oct. 1992, pp.
  1163-1169.
• D.W. Dreisewerd et al., A GEO to GEO high data
  rate optical crosslink approach, Military
  Communications Conference 1992,
• Communications-Fusing Command Control and
  intelligence, IEEE 11-14 Oct. 1992, pp. 183-187.

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Technological considerations
High level block diagram of a terminal for
optical satellite communication system
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Technological considerations
The development of satellite optical
communication is carried out mostly by Europe,
Japan and United States. More than twenty years
of technology endeavours, sponsored by ESA and
other European space agencies, has put Europe in
a leading position in the domain of space laser
communications. The most visible result of this
effort is SILEX (Semiconductor Laser
Intersatellite Link Experiment), the world's
first launch-ready civilian laser communication
system.

• Edelson et al., Laser satellite communications.
  Program, technology and applications, IEEE-USA
  Aerospace policy
• committee rep., Apr. 1996.
• H.P. Lutz, Optical Communications in Space
  Twenty Years of ESA Effort, ESA Bulletin 91, pp.
  25-31, 1997.
• Y. Suzuki, Recent RD activities for optical
  data link in NASDA, CRL Int. Topical Workshop on
  Space Laser Communication-Current Status and
  future perspectives, Tokio, Japan, March 10-11
  1997, pp. 29-33.

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Technological considerations
The SILEX Program

• The first-ever transmission of an image by laser
  link from a satellite to another one
• An ESA project
• The SILEX design
• based on diode laser
• SILEX program showed high reliability with life
  time higher than 100000 hours
• Today, it appears hardly as an alternative to RF
  systems

• H.P. Lutz, Optical Communications in Space
  Twenty Years of ESA Effort, ESA Bulletin 91, pp.
  25-31, 1997.
• G. Planche et al., SILEX in-orbit
  performances, Proceedings of the 5th
  International Conference on Space Optics, 30
  March- 2 April 2004, Toulouse, France.
• A. Biswas, High data-rate laser transmitters for
  free-space laser communications, JPL Technical
  report

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Technological considerations
Direct and Coherent detection techniques
For a given link distance the sensitivity of a
coherent terminal can be used to reduce the
telescope size, the overall dimension and the
mass of the terminal. Nevertheless the
implementation loss for coherent systems and the
higher complexity negates theoretical advantage
while direct detection in the space is easier
and lower risk based on current technology thanks
to terrestrial heritage.
EDFA and heterodyne detection have
similar performance EDFA pre-amplified direct
detection receiver lends itself to much easier
implementation and has to be favourite for
optical satellite communication systems

• R.S. Bondurant, High rate space laser
  communication, LEOS 99, San Francisco, CA.
• E. Rochat et al., Fiber amplifiers for coherent
  space communication, IEEE Journal on selected
  topics in quantum electronics, vol.7, no.1,
  January/February 2001.
• S. Shimer et al., High bandwidth optical
  intersatellite link technologies, International
  topical meeting on Microwave photonics 1999,
  vol.1, pp.101-104
• R.A. Cryan, Space communications employing
  optical pulse position modulation, ECSC, 3rd
  European Conference on Satellite Communications,
  November 2-4 1993, pp. 192-195.
• V.W.S. Chan, Optical space communication, IEEE
  Journal on selected topics in quantum
  electronics, vol.6, no.6, Nov/Dec 2000, pp.
  959-975

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Technological considerations
The Erbium Doped Fiber Amplifier (EDFA)

• Used to boost the intensity of optical signals
• Many of the lost dBs can be recovered
• High reliability with lifetime higher than 106
  hours
• High data rate capability (several Gbps)
• High output power
• Simple current modulation up to several Gbps
• Drawbacks relative low electrical/power
  efficiency (5 to 10)

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Technological considerations
Preliminary non-redounded optical architecture
for EDFA
The transmitter is a DFB laser directly modulated
with a doped fiber amplifier. A direct detection
scheme is selected.
The architecture appears particularly innovative
thanks to the use of 1550 nm wavelength on
satellite
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Technological considerations
Preliminary estimated enveloping dimensions and
weights of an optical communication subsystem
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Technological considerations
Worldwide Space Agencies have developed programs
to increase the satellite links data rates and,
as shown in a study performed by NASA, the
current X-band links are almost saturated and the
plan is to use the Ka band and optical links.
In conclusion, band saturation and the
advantages to obtain more bandwidth with smaller
antenna size and radiated electromagnetic power
are the main reasons to investigate more and more
in this new technical field of space
communication.
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Conclusions
The mission of the OISL system is to support an
Earth Observation satellites constellation for
telemetry, control (TTC) and mission data
requirements. The purpose is to investigate the
GEO Data Relay Satellite in order to determine
whether or not it can support and/or augment the
capabilities of space/ground link connectivity.
In particular, if it is possible and /or useful
to replace the use of polar station with a
Dedicate TLC GEO satellite. Based upon the
results of this study, it can be concluded that
the use of a TLC GEO Satellite can extremely
augment the contacts duration with national
stations (e.g. Italy centre about 800), with
respect a system without OISL system. It should
be noted that to address the use of an OISL
system, instead of a polar station, is
significant and could to be a more cost effective
approach to support the implementation of EO
satellites constellation.
From the technological point of view band
saturation and the advantages to obtain more
bandwidth with smaller antenna size and radiated
electromagnetic power are the main reasons to
investigate more and more in this new technical
field of space communication.