Download Optical Communication in Space

Optical Communication in Space
An artist’s impression of the satellites OICETS and ARTEMIS communicating
via a laser link while they orbit earth.

Roland Amofa & Bert Declercq

Studieopdracht voor het vak “Optische Communicatie”

Prof. J. Engelen

Academiejaar 2004-2005
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Table of contents
Introduction……………………………………………………………………………………3
Problems and difficulties in space optical links……………………………………………….4
Losses………………………………………………………………………………………...4
Absorption…………………………………………………………………………………..4
Scattering……………………………………………………………………………………4
Scintillations………………………………………………………………………………...4
Visibility…………………………………………………………………………………….4
Geometrical loss…………………………………………………………………………….4
Background light……………………………………………………………………………4
Losses in free space…………………………………………………………………………5
Difficulties……………………………………………………………………………………5
Pointing and tracking system………………………………………………………………..5
Vibrations…………………………………………………………………………………...5
Basic components of a space optical communication system…………………………………5
Transmitter…………………………………………………………………………………...5
Receiver………………………………………………………………………………………6
Tracking and pointing system………………………………………………………………..6
The history of optical communications in space………………………………………………7
Future projects…………………………………………………………………………………8
OICETS………………………………………………………………………………………8
MLCD………………………………………………………………………………………..9
Conclusion……………………………………………………………………………………10
Some definitions……………………………………………………………………………...11
References…………………………………………………………………………………….12
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Introduction
In this paper we will discuss the use of optical or laser communication in space. Since space
exploration took off in the 1960s, we have launched a big number of satellites that are orbiting
our planet. We can divide them into three categories according to how big their orbit is: low
earth orbit (LEO), medium earth orbit (MEO) and geostationary satellites (GEO). All these
satellites have to communicate with the earth and with eachother. Not only satellites are
involved in space communication, there are also many spacecrafts that are much further away
from the earth, for the exploration of other planets for example. These vehicles gather
enormous amounts of data (e.g. pictures, scientific data,…) that have to be sent to the earth.
Since the beginning of space communications, all satellites and other space vehicles relied on
radio or microwave links to beam the data home. However, as the radio spectrum becomes
increasingly congested and the amount of data grows, scientists have been busy exploring an
alternative – laser-based links.
An optical link offers many advantages over rf links. First of all, laser-based links have a
much larger bandwith, which offers the possibility of transferring many gigabits of data per
second. Further, lasercom systems are small, lightweight and have compact dimensions; and
they have low power consumption. This results in big cost savings. Finally, the optical
domain is not hindered by any regulatory constrictions, this means there are no licencing
requirements and no tarifs are required for their use.
However, there are also a few drawbacks or problems. These problems include very large
losses in the atmosphere, low laser transmitter powers of only a few watts compared to tens of
watts for microwave transmitters, and the large distances between receiver and transmitter.
Also, the very narrow laser beams require more precise aiming and tracking of the
spacecrafts.
An optical link offers many possibilities and prospects, like for example the construction of a
high-bandwith space network, much more efficient data relay, and beaming high-definition
streaming video and data-rich measurements back to Earth.
First we will look at the problems and difficulties of free space optics (FSO) in general. FSO
is an other name for fibreless optics or optical wireless transmission, but it only covers the
communication between two stations on Earth. The losses of FSO are mainly caused by the
atmosphere, so if we look at optical links between space and the Earth, these links have the
same problems. The losses caused by free space (in the meaning of ‘de ruimte’), are
mentioned after this.
After that, we review the basic components of a space optical communication system, namely
transmitter, receiver and pointing and tracking system.
After the theoretical part of this paper, we take a look at some highlights of the history of
optical communication in space, followed by two promising projects scheduled in the near
future.
Finally, we give a definition of a few basic components.
References are put in bold and in superscript and refer to the title with the same number in the
reference section on page 12.
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Problems and difficulties in space optical links
A) Losses
The losses involved in space optical links are mainly due to losses in the atmosphere.
Therefore we discuss the losses of free space optical systems (FSO) [3], which in general
involve wireless optical links on Earth. The additional losses that optical links in space will
encounter are mentioned in an extra paragraph.
Absorption
There are many types of gases in the atmosphere that can cause absorption, leading to a
reduction in the power level of the laser light. In the wavelength region of the laser light
used, the dominant type of gas in the atmosphere that contributes to absorption is water
vapour. So by staying out of the 'water' windows and keeping the path lengths short,
absorption can largely be ignored.
Scattering
Scattering of the laser light is also another problem with FSO. There are two types of
scattering mechanisms: Rayleigh and Mie scattering. Rayleigh scattering is really only
significant for very long paths. Scattering by particles, or Mie scattering, is a different story.
This is especially true as the size of the particles approaches the wavelength of the transmitted
light. The amount of scattering depends on the particle size distribution and the density of the
particles. Wavelengths near the particle size are scattered very effectively (i.e. thick fogs or
clouds look white).
Scintillations
The performance of FSO systems is also limited by scintillations. Scintillation is caused by
small-scale fluctuations in the index of refraction of the atmosphere on small spatial scales.
The major effect of scintillation is signal fading, due to phase changes in the wavefront of the
signal arriving on the receiver causing both null and high signal receive levels. Unless the
receiver has a very high dynamic range, or the aperture is large enough to average out the
scintillation spots, this can have an extremely detrimental effect on the signal.
Visibility
FSO performance is associated with visibility because the infrared laser sources used in an
FSO system propagate through the atmosphere in the same way as visible light. This can give
one an intuitive feel for the relative importance of fog, snow and rain in preventing FSO
operation. Fog can be extremely thick, with attenuation values of 350 dB/km or more
reducing visibility of the light drastically.
Geometrical Loss
This is the fraction of laser power that actually reaches the receiver in the absence of
atmospheric losses.
Background light
The sun contains significant energy in its spectrum at all wavelengths of FSO interest. This
requires some sort of filter to reject this energy at the receiver.
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Losses in free space [2]
These are mainly due to particulates that scatter and absorb the laser radiation. Sources of
particulates in free space include interstellar clouds of particulates, crumbled meteorites
(meteorite belt), wakes of comets, gas and particulate exhaust from spacecraft engines, and
explosions in space.
B) Difficulties
Pointing and tracking [1]+[6]
Laser communications systems are extremely sensitive to mechanical impacts and therefore it
is necessary to accurately steer the transmitted carrier beam in the direction of the receiver
and to direct the receiver field of view to the transmitter. Because of the great distances and
velocity differences between space vehicles and earth, even at the great speed of light, the
laser beams must point ahead of the receiving vehicle to be received, just as a rifle must point
ahead of a moving target for a bullet to hit the target. This requires a good pointing and
tracking system.
Vibrations [6]
Satellite platform vibrations (jitter) cause significant displacements of the laser beam at the
receiver. To compensate for the vibration effects, the system needs a steering system with a
bandwith more than 2 to 3 kHz. Two possible steering mechanisms are a fast steering mirror
(FSM) and an optical phased array (OPA) antenna.
Basic components of a space optical communication system [1]
In this section, we review the basic components of a space optical communication system,
namely the transmitter, receiver and the tracking and pointing system. This model is not
entirely general, because we have already made a choice for some components. For example,
as a steering mechanism we use an optical phased array (OPA) antenna, where a fast steering
mirror (FSM) was also an option.
A detailed transmitter/receiver configuration for an inter-satellite link is presented in the
figure on the next page.
Transmitter
The transmitter model for an on-off keying ( OOK) modulation format includes a laser,
modulator, and telescope. The messages arrive at the input of the transmitter and then the
transmitter converts electrical signals to optical signals using the laser. Finally, a transmitter
telescope with a beam expander (BE) collimates the laser beam in the direction of the receiver
satellite.
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Fig. 1: A transmitter/receiver configuration for an intersatellite link.
Receiver
The receiver telescope includes a beam concentrator BC, which concentrates an incoming
laser beam onto the receiver OPA antenna aperture. The receiver contains an optical detector
(photodiode) in the direct detection mode. Generally, there are two types of detectors used, a
PIN photodiode or an avalanche photodiode (APD). APD detectors are generally used
because they have internal gain that greatly increases the sensitivity of the sytem [3]. An
optical preamplifier in the receiver amplifies the received optical signal before it enters the
detector, thus improving receiver sensitivity. An optical filter is used in the receiver to
remove background radiation and amplifier spontaneous emission (ASE) noise, so that only
the wavelengths of the signal pass through. The transimpedance amplifier TIA amplifies the
electric signal to provide voltage output. The decision circuit determines the nature of the bits
of information based on the time of arrival and the amplitude of the pulse.
Tracking and pointing system
To keep the transmitter and the receiver aligned when jitter is present, a tracking and pointing
system is implemented in both of them. Coarse beam pointing is performed by the gimbaloperated steering mirrors, and the fine beam pointing by the OPA antenna. The satellites use
the Ephemerides data (the position of the satellite according to the orbit equation) for coarse
pointing. The method of tracking between satellites includes use of a beacon signal on one
satellite and a matrix CCD detector and tracking system on the other. The tracking angle is
evaluated by the satellite computer from the output signal of the matrix CCD detector.
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The history of optical communications in space [5]
The following is a brief overview of the highlights of laser communication in space. We
won’t go into details about the technology used, except for SILEX, because that was the
technology used in the first optical intersatellite link.
1992: Galileo probe
In December 1992 there was a historic uplink laser beam transmission to the Galileo
spacecraft during its flyby of the Earth at a distance of 6 million kilometers. The probe
received the pulses using its solid-state imaging camera as an optical receiver.
1994-1996: GOLD
The Japanese Engineering Test Satellite VI (ETS-VI) participated in an experiment called
Ground/Orbiter Lasercomm Demonstration (GOLD) involving a 1 Mbit/s link with ground
stations operated by JPL and Japan's Communications Research Laboratory (CRL). However,
the satellite was not launched into its planned geostationary orbit, and as a result tests were
hard to make and maintain.
November 2001: SILEX
A 50 Mbit/s optical link was demonstrated between the ESA's satellite ARTEMIS in a
parking orbit at 31 000 km and the French Earth-observation satellite SPOT-4 (Satellite
Probatoire d'Observation de la Terre) which was orbiting the planet at an altitude of 832 km.
The experiment was heralded as a great success, and boasted highly reliable data transfer with
a bit-error-rate of just 1 part in 109, for periods between 4 and 20 min.
The 30 000 km link used transmission equipment named the Semiconductor Laser InterSatellite Link Experiment (SILEX), which was built by Astrium in France and installed on
both satellites.
Fig. 2: The SILEX system.
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SILEX uses a semiconductor laser, i.c. a 60 mW GaAlAs laser diode that emits light at around
800 nm as source. Because of the limited power output of GaAlAs laser diodes, this type of
long-range, high-rate communication link is only feasible because of the extremely high
antenna gain possible with optical frequencies. This in turn means the use of very narrow
beams, with divergence of no more than 2 arc seconds. [8]
No satellite today can provide this type of directional stability, which means that beam
pointing, acquisition and tracking are essential. Parallel search spatial acquisition is done by
using a silicon charge-coupled device (CCD). Another CCD is used for the tracking sensor. A
fast steering mirror (FSM) is used to take out small but fast spacecraft mechanical
disturbances. [11]
An EGG silicon avalanche photo detector (APD) is used for the direct detection receiver.
Detection sensitivity on the order of 100 received photons per bit can be achieved. [11]
2001: GEOLite
A few months before the success of the ARTEMIS communication experiment, a US satellite
called GEOLite in a geosynchronous orbit also allegedly demonstrated a successful optical
link to a ground station on Earth. However, the military nature of the project means that
details of the link have not been made public.
Future projects
We will look at two projects that are scheduled in the near-future: OICETS (scheduled for
2005) and MLCD (scheduled for 2009). For each mission we will look at the goal, at the
details of the optical link and at the current state of the project.
Optical Inter-Orbit Communications Engineering Test Satellite (OICETS)
OICETS is a Japanese low-earth-orbit (LEO) satellite that will be equipped with LUCE (Laser
Utilizing Communications Equipment). It’s goal is to communicate with the European Space
Agency’s geostationary (GEO) satellite ARTEMIS and a ground station in Japan. The tests
between ARTEMIS and OICETS will primarily focus on the beam-pointing, tracking and
acquisition, which will be important for future space laser links. [5]
LUCE offers a 2-50 Mbit/s link. The transmitter consists of a 847 nm, 200-mW GaAlAs laser
diode. It uses an avalanche photodiode (APD) as a receiver, a charge-coupled device (CCD)
with 450 X 350 pixels for acquisition, and a quadrant detector for tracking. [14] The link from
Artemis to LUCE will be at 2.048 Mbps with a pulse position modulation (PPM) format. The
return link will be at the same rate or at 49.37 Mbps with an on-off modulation (OOK) format.
In September 2003 there has been an experiment to confirm pre-launch optical adaptability of
OICETS with ARTEMIS by conducting two-way optical communications between
ARTEMIS now in geostationary orbit and an optical communication equipment engineering
model (the same as the flight model), which was installed at ESA's Optical Ground Station
(OGS) in Tenerife. The successful result of this experiment verified the adaptability between
LUCE and SILEX (mounted on ARTEMIS), which was one of the important pre-launch
verification items for OICETS.
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Fig. 3: Structure of optical adaptability test between ARTEMIS and
LUCE
The current situation of OICETS is not very clear. Basically OICETS is now waiting its
launch, which is expected at 2005, but at the website of the Japan Aerospace Exploration
Agency (JAXA), the launch schedule says ‘undecided’. [13]
Mars Laser Communications Demonstration System (MLCD)[5]+[15]
The Mars Laser Communication Demonstration (MLCD) Project will demonstrate laser
communication between Earth and Mars. MLCD will be one of the communications payloads
on the Mars Telecommunications Orbiter (MTO), scheduled for launch in 2009. MTO will
provide communication services between Earth and missions exploring Mars.
The MTO will feature the world’s first laser communications link from deep space to Earth.
Because of the much larger distance between two terminals, compared with an intersatellite
link (400 million km vs. 30 000 km !!!), the technology suitable for near-Earth use does not
easily extend to deep space requirements.
As shown in the figure below, Mars incurs almost 80 dB additional space loss when compared
to GEO links[15]. Thus, assuming that an organization was successful in building a 10
Gbits/second class terminal that could operate from GEO to the ground, simply transporting
such a terminal from Earth to Mars would result in a data rate of only 100 bits/second! An
improvement of 50 dB is required to provide 10 Mbits/second from Mars.
Fig. 4: Deep Space Lasercom Compared to Geosynchronous Earth Orbit Systems
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MLCD aims to support a 1-30 Mbps link across the enormous distance between Earth and
Mars, so all transmitter, receiver and pointing and tracking technology will need to be
optimized.
Instead of the semiconductor laser transmitter used in previous experiments such as the
OICETS-ARTEMIS link, the MLCD will use an amplified fibre laser as a source. The most
likely candidate is a 1.06 μm fibre master oscillator power amplifier (MOPA) configuration
which uses a distributed feedback (DFB) ytterbium (Yb) fibre laser connected to a high-power
doped fibre amplifier. As modulator, a Mach-Zehnder lithium niobate modulator will be
used.
For the pulses to survive the journey, it is very important to use a highly robust and efficient
encoding scheme, in order to be able to operate as close as possible to the channel-capacity
limit (= Shannon limit). The MLCD will use an encoding technology called 64-PPM.
The telescope of the transmitter will have a 30.5 cm-diameter, and the beam will have a
divergence of 3.5 μrad. [5]
Two major problems which require the use of accurate tracking and pointing technology, are
vibration and drift. Because these occur at a wide range of frequencies, a hybrid scheme will
be applied: vibration isolators to eliminate high frequencies, a fast steering mirror (FSM) to
compensate for medium frequencies, and an uplink beacon to provide a reference for low
frequencies. [5]
To detect the pulses, it has been calculated that a collector with an aperture 3-5 m diameter
will be required. Two different sites will be used: the 5-metre Hale Telescope in southern
California and an array of four 0.8-metre telescopes whose location has yet to be determined.
If the weather is overcast at one location, astronomers can try the next. Both telescopes
would be equipped with highly sensitive avalanche photodetectors (APDs) and very narrow
(0.1 nm bandwidth) optical filters centred on the signal wavelength to screen out as much
background light as possible. It is possible to further decrease the amount of background light
seen by the receiver, by operating above the atmosphere, or at least operating at an extreme
altitude to get above the scattering medium (e.g., 25 000 km). The Mars Lasercom Study
showed that putting a receiver in space would reduce the aperture size required by
approximately 10 dB compared to a ground-based receiver[15]. Finally, a solar filter will also
be used, because if a telescope is pointed at the sun, as much as 1 kW of optical power could
be focused into its sensitive imaging electronics, which would get damaged by that. [5]
The new Mars laser, allocated $270 million from NASA, will undergo a design review in
early 2005 and will fly on NASA's Mars Telecommunications Orbiter in 2009.
Conclusion
Optical communication in space certainly offers many prospects. Since the 1990’s the
technology has evolved enormously, with the first optical intersatellite link in 2001 as a big
breakthrough. Now that new projects start to focus at communication with deep space (the
Mars mission for example), existing technologies will have to optimised further and new
technologies developed to overcome the enormous distances between transmitters and
receivers. So there are still many challenges in this relatively young field.
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Some definitions
Avalanche photodiode (APD)[17]
A photodiode that operates with a reverse-bias voltage that causes the primary photocurrent to
undergo amplification by cumulative multiplication of charge carriers. As the reverse-bias
voltage increases toward the breakdown, hole-electron pairs are created by absorbed photons.
An avalanche effect occurs when the hole-electron pairs acquire sufficient energy to create
additional pairs when the incident photons collide with the ions, i.e., the holes and electrons.
Thus, a signal gain is achieved.
Charge-coupled device (CCD)[18]
A charge-coupled device (CCD) is a light-sensitive integrated circuit that stores and displays
the data for an image in such a way that each pixel (picture element) in the image is converted
into an electical charge the intensity of which is related to a color in the color spectrum.
Mach-Zehnder modulator[19]
It is customarily used as an intensity modulator for typical systems making use of the non
return-to-zero (NRZ) or return-to-zero (RZ) modulation formats, and has recently
demonstrated its potential for phase modulation in future systems making use of the
differential phase-shift keying (DPSK) format. Such modulators are made from an electrooptic crystal (typically lithium-niobate, LiNbO3), whose refractive index depends on the
electric field, hence voltage, that is applied to it. The electrical data can thus modulate the
refractive index of the crystal, hence the phase of the incoming lightwave. Incorporating the
crystal into an interferometric structure (Mach-Zehnder interferometer) in turn converts the
phase modulation into intensity modulation.
Master Oscillator Power Amplifier (MOPA)[20]
For a transmitter to be stable, its oscillator must not be loaded down. This means that its
antenna (which can present a varying impedance) must not be connected directly to the
oscillator circuit. The rf oscillations must be sent through another circuit before they are fed to
the antenna for good frequency stability to be obtained. That additional circuit is an rf power
amplifier. Its purpose is to raise the amplitude of rf oscillations to the required output power
level and isolate the oscillator from the antenna. Any transmitter consisting of an oscillator
and a single-amplifier stage is called a master oscillator power amplifier transmitter.
Fig. 5: Block diagram of a master oscillator power amplifier transmitter
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References
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http://www.newscientist.com/article.ns?id=dn1603
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