Intersatellite laser crosslinks provide significantly higher data throughput than traditional RF communication systems. They take advantage of lasers' small wavelengths and highly directed beams to transmit data over long distances with minimal losses. A laser crosslink system consists of a transmitter, receiver, acquisition subsystem to initially align the links, and a tracking subsystem to maintain the alignment during data transmission. Optical communications offer wide bandwidths and high data transmission rates well beyond what is possible with RF technologies. Various laser crosslink systems are discussed to understand their subsystems and implementations.

1. I. INTRODUCTION
Intersatellite laser crosslinks (ISLs) provide a
Intersatellite Laser Crosslinks method of communication that has significantly
increased the data throughput that can be managed
over typical RF communication systems. The data rate
growth potential is well beyond the few gigabit per
second range of RF technology. The use of lasers in
JOHN E. MULHOLLAND, Senior Member, IEEE transmitting optical data takes advantage of its small
Villanova University wavelength and low beam divergence.
SEAN ANTHONY CADOGAN The ISL is subdivided into five major subsystems.
Martin Marietta Corp.
The transmitter is typically a semiconductor laser, or
laser diode. The receiver has a design very dependent
on the method of communication, and transmitter
construction. The acquisition subsystem is responsible
Intersatellite laser crosslinks (ISL) provide a method
for aligning the transmitter and receiver to prepare
of communication that has significantly increased the data for communication. The tracking subsystem must
throughput that can be managed over typical RF communication maintain the link with the stability necessary to allow
systems, and has significant growth potential. Optical for reliable data transmission. The communication
communications offer very wide bandwidths which can be subsystem is responsible for encoding and decoding
effectively utilized with wavelength division multiplexing the data to be sent between satellites.
techniques. The data rate growth potential is well beyond the few The RF atmospheric coefficient of attenuation is
gigabit per second range of RF technology. The use of lasers in very low, which results in RF signals slowly losing
transmitting optical data takes advantage of its small wavelength strength in the atmosphere and can therefore travel
and low beam divergence to send highly directed signals over long distances, including over the horizon. On the
significant distances with controlled losses in intensity. The contrary, laser signals are highly directional, permit
high directivity of the laser aids in resistance to jamming
large bandwidths, and are attenuated to a significant
extent by the atmosphere. This, in addition to the fact
communications between satellites, or between satellites and
that they are line-of-sight [1], causes some important
ground stations.
design problems that must be addressed.
Various intersatellite laser optical crosslink systems are
Various ISL systems are discussed in order to
discussed including the Massachusetts Institute of Technology’s
display the various subsystems which comprise a laser
Laser Intersatellite Transmission Experiment (LITE), the crosslink, and their implementations. Discussion on
McDonnell Douglas Electronic Systems Company Laser the strengths of laser communications is provided, and
Crosslink System, and The Ball Aerospace Optical Intersatellite related to RF technology.
Link, in order to display the various subsystems and their Background is provided on earlier system
implementations. Link budget calculations are performed on the architectures and methods of laser communication,
most commonly used modulation formats to determine system as well as presently implemented systems. Optical link
parameters necessary to close the crosslink. budget calculations are performed for various methods
Background is provided on primal system architectures and of communications. The author provides some insights
methods of laser communication, as well as presently implemented on where intersatellite laser optical crosslink systems
systems. The authors provide some insights on where ISL systems
have opportunity to increase their data throughput and
reduce acquisition time.
have opportunity to increase their data throughput and reduce
acquisition time.
II. INTERSATELLITE LASER CROSSLINKS
A. Why Satellite?
McDonnell Douglas Electronic Systems Company
Manuscript received June 18, 1994; revised March 27, 1995. (MDC) was chosen by the U.S. government in 1981
IEEE Log No. T-AES/32/3/05872. to bring laser communications into production by
Authors’ addresses: J. E. Mulholland, Dept. of Electrical and
developing a satellite-to-satellite crosslink. The system
Computer Engineering, Villanova University, Villanova, PA was to be installed on an already existing satellite.
19085-1681; S. A. Cadogan, Martin Marietta Corp., Management Therefore to minimize any impact to the satellite, the
and Data Systems, King of Prussia, PA. laser crosslink needed to be a stand-alone, bolt-on
package, which provided terminal control, a despun
0018-9251/96/$10.00 ° 1996 IEEE
c line of sight, and could operate from raw spacecraft
IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS VOL. 32, NO. 3 JULY 1996 1011

2. power [2]. The profitability of communications by
satellite becomes evident when reviewing the key
features of the MDC laser crosslink subsystem:
1) reduction of the reliance on foreign ground stations,
2) survivability, 3) jam resistance, 4) low probability of
data intercept, and 5) field of view (FOV) limited only
by gimbal location relative to the sensor.
With one centralized U.S. ground station which
takes inputs from multiple satellites, the dependence
on multiple foreign ground stations is greatly reduced.
This alleviates the time delays and increased factors of
error associated with the distributive nature of multiple
foreign ground stations. Survivability is especially
important in times of natural disaster, war, or other
events which can be detrimental to low altitude
communication devices (i.e., air craft systems) and
ground-station-to-ground-station communications. Jam
resistance and low probability of error, features of the
Fig. 1. Synchronous range crosslink aperture.
MDC laser crosslink system, are results of the narrow
beamwidth used. The high altitude of the satellites
leads to a much more expansive FOV which is only advantage over the best achievable communication
limited by the gimbal location relative to the sensor. in the RF spectrum. The extremely low beam
divergence minimizes signal loss and a narrow receiver
FOV makes it extremely difficult to jam. The short
B. Why Optical?
wavelength of lasers offers the opportunity to modulate
The Optical Communications Group at M.I.T. at very high data rates. Laser communication offers
Lincoln Laboratory has been investigating and 1) low probability of data intercept, 2) jam resistance,
developing the technologies required to make high and 3) high bandwidth capabilities.
to very high data rate optical intersatellite crosslink The highly directional nature of lasercom makes
a reality for over ten years. According to Boroson it difficult to intercept and jam communication. The
[3], optical communications allows the use of high directivity arises from the short wavelengths of
comparatively small antenna (telescope) packages visible and nearly infrared energy. Lasercom sidelobes
because of its very short wavelength. RF technology, are also generally much lower than RF or millimeter
even in the upper EHF region over 60 GHz, requires wave sidelobes, resulting in an inherent resistance
antenna apertures on the order of several feet in to interception or jamming [1]. There are many
diameter to support links with capabilities of more constraints which must be taken into account when
than a few tens of megabits per second. Fig. 1 choosing a laser subsystem. Some of these constraints
compares the package apertures for 40,000 km links are identified in a later section, which discusses the
which quantifies the difference in aperture size for RF transmitter of a laser crosslink system.
versus optical communications at various data rates.
With the utilization of satellites, special attention must III. SYSTEMS APPROACH
be taken to payload constraints on size and weight
added by the communications subsystem. A. General Parameters
Optical communications also offers very
There are many parameters which the system
wide bandwidths, especially when utilizing
designer must consider in the development of an ISL.
wavelength-division multiplexing techniques. RF
For instance, in order to get maximum use of the
technology, on the contrary, does not have data rate
relatively low power of the laser diodes, the designer
growth potential beyond a few gigabits per second,
must pay particular attention to beam pointing
especially in a network where frequency reuse may not
and tracking, wavefront quality, package rigidity,
be possible.
point-ahead accuracy, and maintenance of these
properties through the temperature and vibrational
C. Why Laser? extremes of the lifetime of a satellite. In order to
arrive at a successful lasercom design, all of these
The development of laser communication began constraints must be fulfilled simultaneously in a full
at MDC in the late 1960s under both U.S. Air Force system context. The lasercom should be compact,
and company sponsorship. Laser communication lightweight, and have a relatively simple package
at short wavelengths theoretically holds a great design as a result of the solution with these constraints.
1012 IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS VOL. 32, NO. 3 JULY 1996

3. Mass, prime power, and volume estimates
for reliable ISL payloads were performed in a
telecommunications system that provides a full-duplex
interconnection of three wideband transponders
between two spacecraft separated by 60 deg along
the geostationary arc by R. Marshalek of the Ball
Aerospace Systems Group and D. Paul of COMSAT
Laboratories [5]. The following conclusions related to
transmitter laser choice were made.
1) The CO2 system demands excessive laser
redundancy and large payload mass to support a 10-yr
Fig. 2. LITE engineering model block diagram. high reliability (0.9) mission.
2) Redundancy increases payload weight by about
20 to 30 kg int he Nd:YAG, InGaAsP, and GaAlAs
It must be noted that there are many different ways systems.
of configuring an ISL, and along with this, different 3) GaAlAs systems entail lower payload mass
system parameters which must be considered. and prime power, and are recommended for a
In general, the total weight of the transmitter, telecommunications ISL.
receiver, acquisition, tracking and communications
subsystems should be within the range of 200—300 lb.
C. Receiver
It was also noted that state of the art systems transmit
at approximately 300 Mbit/s. Using Fig. 1, for a data Many different types of receivers can be utilized
rate on the order of 100 Mbit/s, a 0.1 W laser requiresin the lasercom system. Some of these receivers
an aperture diameter of about 0.4 ft, and a 1 W laser are introduced in a general nature. In general, the
of only about 0.2 ft. receiver or detector must be able to transform light
into electrical signals. Many, but not all, have some
B. Transmitter amount of built-in gain to better detect the incoming
signal. Depending on the operating wavelength, there
There are many considerations in designing a will be different materials used. The receiver is also
transmitter. The laser used must not only be powerful application dependent. If direct modulation is used as
enough to transmit the necessary beam over a a communications method, a detector and amplifier is
specified distance, but it must pass a screening test needed. If a synchronous detection method (such as
designed to select lasers with acceptable operating RF links) is employed, a local oscillator (laser) must
temperature, narrow linewidths, acceptable optical be used. This is the heterodyne case. The detector
properties, reasonable FM responses, and prospects of choice for Nd:YAG and GaAs wavelengths is the
for long life [4]. A laser must qualify for space avalanche photodiode (APD). An APD can be used
usage in a satellite crosslink system. For example in the tracking and communication phase, which is
gas lasers (i.e., He-Ne) are not practical in space discussed in a later section.
due to their relatively low efficiency and large size. The system impact of resonant laser receivers for
Inability to maintain uniformity of the vapor in the free-space laser communications has been studied,
discharge region has ruled out metal vapors such and the major advantage of the resonant receiver
as Zn, Hg, Sn, and Pb which have displayed laser design approach is that it enables laser communication
transition in the visible spectrum [11]. Therefore a link closure for many applications, by using an
solid state or semiconductor laser is the device of available 14 cm aperture and existing compact diode
choice. Semiconductor lasers, particularly the GaAlAs laser sources (for acquisition and high-data-rate
family, are good candidates for the laser source communication). The problem of having to close the
in a heterodyne system. Semiconductor lasers are communication link with a reasonably sized aperture
compact, have high power conversion efficiency (with has now been circumvented. This alleviates the
prime-to-optical output power conversion efficiency problem that previously existed with the traditional
between 20% and 50%), architectural simplicity, direct-detection approach to laser communications
and utilize single-frequency operation. As a part of with a reliable, high-power high-beam-quality (Strehl
the Laser Intersatellite Transmission Experiment ratio) transmitter that controls system mass and
(LITE) project [3], M.I.T. has built many laboratory beam-pointing requirements [6]. The resonant receiver
communications links based on commercially available design approach also has immunity to large optical
30 mW GaAlAs lasers with wavelengths between 0.83 background interference, while not overloading
and 0.86 ¹m. These lasers were adequate for crosslinks requirements on transmitter frequency, thermal
in the 100 Mbit/s class (see Fig. 2). stability, or receiver frequency tracking that increase
MULHOLLAND & CADOGAN: INTERSATELLITE LASER CROSSLINKS 1013

4. the complexity of the alternative heterodyne-detection
approach. For most parameters, the resonant receiver
requirements lie between those for direct detection
and heterodyne detection. The resonant receiver
approach attacks those key areas that have been major
drivers on system reliability, performance, and cost by
offering a balanced design approach to long-distance
high-data-rate laser communications.
For digital traffic, the full bandwidth, direct,
and heterodyne-detection GaAlAs systems entail
comparable mass, power, and volume. However,
for analog traffic, the GaAlAs heterodyne-detection
system is superior because it uses far less mass
and volume. The major reason that the GaAlAs
heterodyne-detection system is so successful for analog
traffic is that it efficiently accommodates the three
multiplexed communication transponders with a
direct-optical-carrier-frequency modulation technique Fig. 3. Acquisition time.
[7]. The LITE engineering model at M.I.T. utilizes a
semiconductor coherent (heterodyne) detection which
and communications. Initially, there is a large ratio
allows for nearly quantum-limited performance with
between the initial angular uncertainty and the narrow
sensitivity better than that of direct detection at all
beam divergences in the tracking and communication
but the very lowest data rates. Heterodyne detection
links to conserve the limited laser power.
also allows operation with a bright object, such as the
The Laser Crosslink Subsystem (LCS) of
sun, in the FOV; whereas, direct detection systems are
McDonnell Douglas Electronic Systems Company
significantly degraded.
uses the direct pulse detection technique, and
therefore their acquisition algorithm is different
D. Acquisition and Tracking from one using coherent (heterodyne) detection.
Acquisition refers to the process in which the A 100¹ rad acquisition beam is initially scanned
receiving satellite determines where the incoming over the region of uncertainty. The pulse rate of the
beam sent by the transmitting satellite is located. laser is reduced during acquisition to provide higher
Bridging a 42,000 km link with the very narrow peak pulse power required to compensate for the
beamwidth of a laser poses a serious design problem, expanded beam divergence. When each LCS terminal
however, multiple sequential methods of acquisition detects illumination from the opposite terminal, the
are discussed. pointing converges and scan fields are reduced in
One goal for laser communication is the reduction order to increase scan frequency. This continues
in acquisition time and the improvement of acquisition until pointing accuracies are sufficient to support
techniques. The relation between the beamwidth of communications. The scans are then suspended and
the transmitted beam, the receiver’s FOV, and the each LCS transitions to 10¹ rad communications beam
maximum time it takes for acquisition is well displayed pointing and data transmission [2].
in Fig. 3. This figure plots curves of maximum
acquisition time against azimuth uncertainty angle (for E. Tracking and Maintaining Links
constant elevation uncertainty) for a number of beam
size and FOV combinations. The curve on the far left Tracking refers to the process in which the
indicates that the acquisition time may be more than satellites maintain their communication links. In the
five min for a 0.5 deg initiator beam and a one deg LITE system the high bandwidth steering mirror
responder FOV. An examination of the curves towards (HBSM) also correctly points the transmitted beam
the right of Fig. 6 indicates that short search times to the other terminal as it keeps the incoming
can be implemented over much larger volumes of beacon signal centered on the tracking detector.
uncertainty if the FOV of the detector and the beam This allows the compensation of pointing variations
divergence of the initiator are large enough. It should caused by spacecraft motion and vibration. Once
be noted that for wide initiator beam divergences, high the laser transmitter is set up and stabilized, and the
power lasers must be employed in order to close the beam-steering system has completed the bore-sighting
link. procedure (alignment of transmit and receive beams),
For mutual acquisition to occur, each satellite must LITE is ready to acquire and track the incoming signal.
reduce its initial knowledge of the opposing satellite’s Once the signal is acquired the beam is narrowed
location to values compatible with fine tracking which increases its power. When the other terminal
1014 IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS VOL. 32, NO. 3 JULY 1996

5. senses this, it sends the signal over to its tracking receive signal in conjunction with envelope detection
receiver, narrows its beacon and points it toward as in RF systems [8]. The key components are transmit
LITE. LITE senses this increased power and increases and local oscillator lasers, optical input couplers, and
the tracking bandwidth to 1 KHz to improve tracking high bandwidth electronics.
performance. After all acquisition is complete, the
communications session can begin [4]. IV. LINK BUDGET
Fine tracking allows the use of narrow
communication beams for high-data-rate transfer A. SNR and Data Errors
between the two satellites, simultaneously maintaining
point-induced burst communication errors at The final performance of the system depends
acceptably low levels. A burst error rate of 10¡5 to on the signal-to-noise ratio (SNR). Noise in an RF
10¡6 is acceptable and is achievable with a 1 ¾ tracking system is usually thermal noise, or device noise. In
accuracy of about one-twentieth of the null-to-null link budget calculations for an RF communication
transmit beamwidth. The spectral content of the link the carrier is an electromagnetic wave. Noise in an
satellite platform disturbance errors determines the optical system consists of thermal, as well as quantum
track detector update rate (bandwidth). noise generated by asynchronous photon plinking in
signals, because for an optical communication system,
the carrier is a photon.
F. Communications
In free-space laser optic communications, the link
There are different methods of modulation of the budget is defined by an allowable bit error rate (BER).
laser beam which can be used to send information The acceptable BER commonly used in analysis
in the beam. In the beginning of lasercom, direct of optical links is 10¡9. From this BER, a SNR is
modulation was the only option available. Information determined. These two factors, in conjunction with
was sent via the duration of pulses of laser power. the range of transmission, are utilized in choosing
Now the lasers can be modulated like RF carriers (i.e., the transmitter and receiver. If the digital traffic
frequency or even phase modulation, see [5]). is received with a 10¡9 BER it corresponds to a
The communications subsystem is composed of SNR of 16.2 dB for quadrature PSK (QPSK) traffic
five major parts in the Ball Aerospace design; the and 17 dB for baseband digital traffic, including a
laser/modulator, detector/local oscillator, LO laser 1.4 dB modem implementation margin in both cases
(heterodyne system), communication electronics, [10]. The analog traffic requires a 17 dB SNR at the
processing electronics, and passive optics. receiver output. For the receiver, when determining
The data format has to be constructed according to the receiver specifications, the modulation format, as
the nature of the data. For pulse-position modulation well as the detection scheme must be considered. The
(PPM), digital modulators are required [7], while necessary SNR must also be taken into account. The
on—off keying (OOK) systems are usually implemented sensitivity of the receiver is measured as number of
using scramblers, or forward error correction coding in detected photons per bit (at peak power) necessary to
order to improve the correlation properties of the data achieve a BER of 10¡9. Once the receiver sensitivity
signal [8]. In order to recover the signal and regenerate is known, the amount of power needed from the
the information sent, PPM maximum likelihood transmitter must be determined. There are various
receivers require a symbol clock recovery circuit. other noise-inducing factors, such as differences in
For OOK systems the amplitudes are regenerated by temperature throughout the atmosphere significant to
threshold decision. The synchronization requires a cause a perceptible change in the index of refraction
phase-locked loop triggered by data transitions [11]. presented to a laser beam as it passes through. This
A quadrant detector composed of four APDs split can result in beam broadening, tearing and steering
at a focal plane by a pyramid, light pipes, or fibers of portions of the beam, causing fades and surges in
may also be used as a communications detector (3 ns the optical beam as a result of variations in power
rise and fall) in addition to a track detector, if the density. The probability of bit error is therefore
quadrant outputs are summed. Duchmann and Planche dramatically increased. Atmospheric turbulence, and
indicate that in their communications system, the pointing inaccuracies are other factors which can
receive function consists of a low noise APD-based introduce bit errors and degrade the performance of
direct detection of the incoming signal followed by a communications link.
a non-return-to-zero (NRZ) regeneration of the
baseband electrical signal [9]. B. Link Budget Calculations
In fiber-based state of the art heterodyne receivers,
continuous phase frequency-shift keying (FSK), The link budget is a numerical calculation that
or differentially encoded phase-shift keying (PSK) proves link closure. It is used to determine whether
modulation is used. The detection principle consists the SNR is high enough for data to be successfully
of the active mixing of a local oscillator signal and the transferred. Link budget calculations determine system
MULHOLLAND & CADOGAN: INTERSATELLITE LASER CROSSLINKS 1015

6. TABLE I
Optical Intersatellite Link System Parameters
performance for communication configurations by
trading off such parameters as aperture, transmitter
power, and data rate. This section shows the design
and definition of the communications link. Optical
power budget calculations are performed in six systems
to determine antenna diameter requirements as a
function of average transmitter power. The systems
are: 1) carbon dioxide laser system with heterodyne
detection, 2) neodymium-doped laser system with
direct detection, as in the LCS laser of McDonnell
Douglas, which utilizes solid state GaAs diodes to
pump a Nd : YAG rod, 3) In GaAsP laser system
with direct detection, 4) GaAlAs laser system with
direct detection, 5) GaAlAsP laser system with
wavelength division multiplexing and direct detection,
and 6) GaAlAs laser system with heterodyne detection.
These systems have been previously introduced in
earlier sections which discuss transmitter and receiver
options.
Typical parameter values are used throughout
this discussion in order to determine the antenna
requirement for each of the six systems as a function
of optical transmitter power. Table I gives the system Fig. 4. Antenna diameter requirements for CO2 system.
parameters used in the link budget calculations
for the different optical intersatellite link systems.
These calculations were performed for each of the The Nd-doped system was evaluated uner mode-locked
modulation formats discussed, and a link margin of conditions. Results for the other systems are similar
5 dB was assumed in all cases. The results for the CO2 to the GaAlAs System. It must also be noted that
and AgAlAs systems are displayed in Figs. 4 and 5. the actual average transmitter power for the analog
1016 IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS VOL. 32, NO. 3 JULY 1996

7. TABLE II
Antenna Diameter Requirements for Baseband Digital Transmission (360-Mbit/s Total Throughput With 10-9 BER) Over 42,000 km
Range
Note: *Optical power of each transmitter.
engineer, requires a substantial reduction in average
transmitter power than the nonheterodyne case.
The CO2 heterodyne, GaAlAs WDM, and GaAlAs
heterodyne systems require the smallest antennas for
analog transmission. Since the WDM system is very
reliable and simpler to implement, it is preferred in
short-term applications. With technological advances
in GaAlAs heterodyne systems it will become the
preferred choice for analog transmission of three
separate transponder signals [11].
V. CONCLUSIONS
Different methods of laser beam modulation have
been used over the years to send information. In the
early days of lasercom, direct modulation was used
where the laser was turned on and off just as Morse
code signals were used. The speed of modulation
had to be checked as well. Now the lasers can be
modulated like RF carriers (i.e., frequency or even
phase modulation).
The range in data rate from tens of kilobits to
tens of megabits was previously exclusively covered
Fig. 5. Antenna diameter requirements for GaAlAs system. by Nd : YAG lasers modulated with an M-ary PPM
format. Now, new pulsed diode arrays are capable of
operating with PPM modulation with peak powers
formats is 0.75 times the value read from the graph of tens of watts at megabit data rates. State of the art
[11]. systems now transmit at approximately 300 Mbit/s.
Anticipated transmitter power levels in an ISL It is the authors’ opinion that there are several
were estimated to compare the six systems, and the methods which can be incorporated into the
corresponding antenna diameters were then obtained acquisition and tracking phase, as well as the
for transmission of three baseband digital transponder communications phase to improve system performance.
signals. Table II gives the modulation formats, power Sections VA and VB discuss these proposed system
levels, and calculated diameters of these baseband enhancements.
signals. Table III provides a similar comparison for the
transmission of three QPSK transponder signals. The A. Acquisition and Tracking Enhancements
CO2 and Nd systems require the smallest diameters
for transmission of baseband digital signals but use The narrow beamwidth of the transmitted optical
more complicated modulation techniques and less beam presents design difficulties in the actual
efficient transmitters than systems based on GaAlAs acquisition of the signal. To broaden the beam and
heterodyne detection system, although difficult to send it out from the laser calls for much more power
MULHOLLAND & CADOGAN: INTERSATELLITE LASER CROSSLINKS 1017

8. TABLE III
Antenna Diameter Requirements for Transmission of Three 72 MHz QPSK Transponders Over 42,000 km Range
Note: *Optical power of each transmitter.
**In CW operations.
than the laser may be able to provide if the same terminal. Either multiple processors can be used to
amount of intensity is to be sent over 42,000 km links. process the different incoming beams, which would
The authors propose using the fact that in the far linearly increase payload size and weight, or a single
field (Fraunhouffer) light sent through an aperture processor can take multiple inputs and process them
will be captured in the focal plane, at the receiver as separately, and multiplex the results accordingly.
the Fourier transform of the signal. This cannot only This approach would allow certain options such
be done in time and frequency but also in space and as multiple users transmitting data simultaneously,
spatial frequency. A rectangular slit will result in a utilizing only one transmitting and one receiving
sinx=x, and a sinx=x will result in a square pulse. The satellite without concern for their data becoming
resulting far field pattern should be chosen so that its available to other users (particularly important in
symmetry will facilitate finding the “center” where the personal communications and proprietary or secure
actual beam will be present. It should have an area communications). Due to the short wavelength of
of increased spatial area, or of spatial area significant optical systems, it has been noted that there is a
enough to make it useful to place the initial field high degree of directivity. Careful attention must be
through an aperture. If the signal is broader, it will paid so that once the acquisition signal is received
be easier to find. It has been discussed that a 2-phase and the system switches to communication beams
acquisition phase can be used to save energy while the that the beam divergence is not wide enough to
receiver satellite is trying to locate the transmitting allow interference between the various incoming
satellite. Another suggestion by the author is to use communication signals.
a three-phase approach. Trades should be made to The Optical Communications Technology group
determine if a very large, very powerful pulse or at Lincoln Laboratory believes that the technology
pulse sequence as an initial phase will cut down the is available for deployment of operational laser
receivers initial field of uncertainty or FOV enough to communication systems in the several hundred
significantly decrease acquisition time. megabits per second range, with near term technology
to be able to support multipke gigabits per second
B. Communications Enhancements links in small and reliable packages [3].
It has been noted in certain systems (such as the C. Receiver Enhancements
LITE system), that redundant laser diodes are present
but are used solely as backup when other diodes fail. In recently developed low-effective k silicon APDs
They may also be used to provide the necessary power (k = 0:002 to 0.005, depending on wavelength), a
in the case of weaker lasers. The authors suggest sensitivity of 68 photons per bit at a BER of 10¡9
that data throughput be increased by simultaneous has been measured on a direct-detection receiver
operation of multiple lasers in the transmitter section, developed using a lser diode (¸ = 810 nm) with an
to be received by an array of receivers at the receiving extinction ratio of 0.02 [13].
1018 IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS VOL. 32, NO. 3 JULY 1996

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MULHOLLAND & CADOGAN: INTERSATELLITE LASER CROSSLINKS 1019

10. Sean A. Cadogan was born in Brooklyn, NY in 1968. He received his B.S. in electrical engineering from the
Massachusetts Institute of Technology, Cambridge, in 1990, and an M.S. in electrical engineering from Villanova
University, Villanova, PA, in 1993.
From 1990 to 1992, he worked at General Electric Aerospace as an Edison Engineering Program member
holding positions in the Systems Integration, Systems Analysis, and Verification and Test Engineering groups
in Management and Data Systems. While in the Sensor Systems Engineering groups in Management and Data
Systems. While in the Sensor Systems Engineering group he was the project leader on a study that quantified
the impacts of bit errors on digital processing, and the implementation of the Bose—Chaudhuri—Hocquenghem
(BCH) coding algorithm to detect and correct bit errors. He is presently a Hardware Systems Engineer at Martin
Marietta Aerospace, formerly GE, in Valley Forge, PA and resides in Norristown, PA.
John E. Mulholland (S’57–M’61–SM’87) received the B.E.E. degree from Villanova University, Villanova,
PA, in 1960, the M.S.E.E. degree from Drexel Institute of Technology, Philadelphia, PA, in 1965, and the Ph.D.
degree in electrical engineering from the University of Pennsylvania, Philadelphia, in 1969.
In 1985, he joined the faculty of the Department of Electrical and Computer Engineering at Villanova
University to develop the microwave engineering technology area for both education and research. Before joining
Villanova University, he was employed at the General Electric Space Division as Manager of the Communication
Equipment and Antenna Engineering Laboratories. His assignments have included the development of
microwave filter analytical techniques and the design of waveguide and directional filters and the Ku and X
frequency bands and the development of automated RF measurement techniques for components and systems.
More recently he has led the development of the interface definition of the command and control segment with
the microwave transmission segment of a major military satellite data communications system. Prior to joining
General Electric, he provided consultation in radar clutter, multipath, propagation effects and radiation hazards
at the RCA Missile and Surface Radar Division. He also provided analytical support for the AN SPY-1 radar in
the areas of antenna matching, random materials, monopulse tracking collimation and alignment, and sidelobe
blanking.
Dr. Mulholland is a registered Professional Engineer in Pennsylvania, past Chairman of the Antenna
Propagation/Microwave Theory and Techniques (AP/MTT) Society, Philadelphia Section of IEEE.
1020 IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS VOL. 32, NO. 3 JULY 1996