I. INTRODUCTION                                                                        Intersatellite laser crosslinks (IS...
power [2]. The profitability of communications bysatellite becomes evident when reviewing the keyfeatures of the MDC laser...
Mass, prime power, and volume estimates                                                           for reliable ISL payload...
the complexity of the alternative heterodyne-detectionapproach. For most parameters, the resonant receiverrequirements lie...
senses this, it sends the signal over to its tracking      receive signal in conjunction with envelope detectionreceiver, ...
TABLE I                                    Optical Intersatellite Link System Parametersperformance for communication conf...
TABLE II  Antenna Diameter Requirements for Baseband Digital Transmission (360-Mbit/s Total Throughput With 10-9 BER) Over...
TABLE III          Antenna Diameter Requirements for Transmission of Three 72 MHz QPSK Transponders Over 42,000 km Range N...
REFERENCES                                                              [9]   Feher, K. (1983)                            ...
Sean A. Cadogan was born in Brooklyn, NY in 1968. He received his B.S. in electrical engineering from theMassachusetts Ins...
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Intersatellite laser crosslinks


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Intersatellite laser crosslinks

  1. 1. I. INTRODUCTION Intersatellite laser crosslinks (ISLs) provide aIntersatellite 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 inJOHN E. MULHOLLAND, Senior Member, IEEE transmitting optical data takes advantage of its smallVillanova 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 prepareof communication that has significantly increased the data for communication. The tracking subsystem mustthroughput that can be managed over typical RF communication maintain the link with the stability necessary to allowsystems, and has significant growth potential. Optical for reliable data transmission. The communicationcommunications offer very wide bandwidths which can be subsystem is responsible for encoding and decodingeffectively 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 isgigabit per second range of RF technology. The use of lasers in very low, which results in RF signals slowly losingtransmitting optical data takes advantage of its small wavelength strength in the atmosphere and can therefore traveland low beam divergence to send highly directed signals over long distances, including over the horizon. On thesignificant distances with controlled losses in intensity. The contrary, laser signals are highly directional, permithigh 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 factcommunications between satellites, or between satellites and that they are line-of-sight [1], causes some importantground stations. design problems that must be addressed. Various intersatellite laser optical crosslink systems are Various ISL systems are discussed in order todiscussed including the Massachusetts Institute of Technology’s display the various subsystems which comprise a laserLaser Intersatellite Transmission Experiment (LITE), the crosslink, and their implementations. Discussion onMcDonnell Douglas Electronic Systems Company Laser the strengths of laser communications is provided, andCrosslink 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 systemimplementations. 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 linkparameters 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 insightsmethods of laser communication, as well as presently implemented on where intersatellite laser optical crosslink systemssystems. 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 reduceacquisition time. II. INTERSATELLITE LASER CROSSLINKS A. Why Satellite? McDonnell Douglas Electronic Systems CompanyManuscript received June 18, 1994; revised March 27, 1995. (MDC) was chosen by the U.S. government in 1981IEEE Log No. T-AES/32/3/05872. to bring laser communications into production byAuthors’ addresses: J. E. Mulholland, Dept. of Electrical and developing a satellite-to-satellite crosslink. The systemComputer 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, theand Data Systems, King of Prussia, PA. laser crosslink needed to be a stand-alone, bolt-on package, which provided terminal control, a despun0018-9251/96/$10.00 ° 1996 IEEE c line of sight, and could operate from raw spacecraftIEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS VOL. 32, NO. 3 JULY 1996 1011
  2. 2. power [2]. The profitability of communications bysatellite becomes evident when reviewing the keyfeatures of the MDC laser crosslink subsystem:1) reduction of the reliance on foreign ground stations,2) survivability, 3) jam resistance, 4) low probability ofdata intercept, and 5) field of view (FOV) limited onlyby gimbal location relative to the sensor. With one centralized U.S. ground station whichtakes inputs from multiple satellites, the dependenceon multiple foreign ground stations is greatly reduced.This alleviates the time delays and increased factors oferror associated with the distributive nature of multipleforeign ground stations. Survivability is especiallyimportant in times of natural disaster, war, or otherevents which can be detrimental to low altitudecommunication devices (i.e., air craft systems) andground-station-to-ground-station communications. Jamresistance and low probability of error, features of the Fig. 1. Synchronous range crosslink aperture.MDC laser crosslink system, are results of the narrowbeamwidth used. The high altitude of the satellitesleads to a much more expansive FOV which is only advantage over the best achievable communicationlimited 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 shortB. 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 offersLincoln 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 makesa 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 ofcomparatively small antenna (telescope) packages visible and nearly infrared energy. Lasercom sidelobesbecause of its very short wavelength. RF technology, are also generally much lower than RF or millimetereven in the upper EHF region over 60 GHz, requires wave sidelobes, resulting in an inherent resistanceantenna apertures on the order of several feet in to interception or jamming [1]. There are manydiameter to support links with capabilities of more constraints which must be taken into account whenthan a few tens of megabits per second. Fig. 1 choosing a laser subsystem. Some of these constraintscompares the package apertures for 40,000 km links are identified in a later section, which discusses thewhich 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 APPROACHbe taken to payload constraints on size and weightadded by the communications subsystem. A. General Parameters Optical communications also offers very There are many parameters which the systemwide 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 thetechnology, on the contrary, does not have data rate relatively low power of the laser diodes, the designergrowth potential beyond a few gigabits per second, must pay particular attention to beam pointingespecially 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 vibrationalC. 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 fullat 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 packageat 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. 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 masssystem 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 communicationssubsystems should be within the range of 200—300 lb. C. ReceiverIt was also noted that state of the art systems transmitat approximately 300 Mbit/s. Using Fig. 1, for a data Many different types of receivers can be utilizedrate on the order of 100 Mbit/s, a 0.1 W laser requiresin the lasercom system. Some of these receiversan aperture diameter of about 0.4 ft, and a 1 W laser are introduced in a general nature. In general, theof only about 0.2 ft. receiver or detector must be able to transform light into electrical signals. Many, but not all, have someB. 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 alsotransmitter. The laser used must not only be powerful application dependent. If direct modulation is used asenough to transmit the necessary beam over a a communications method, a detector and amplifier isspecified distance, but it must pass a screening test needed. If a synchronous detection method (such asdesigned to select lasers with acceptable operating RF links) is employed, a local oscillator (laser) musttemperature, narrow linewidths, acceptable optical be used. This is the heterodyne case. The detectorproperties, reasonable FM responses, and prospects of choice for Nd:YAG and GaAs wavelengths is thefor long life [4]. A laser must qualify for space avalanche photodiode (APD). An APD can be usedusage in a satellite crosslink system. For example in the tracking and communication phase, which isgas 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 forInability 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 receiveras Zn, Hg, Sn, and Pb which have displayed laser design approach is that it enables laser communicationtransition in the visible spectrum [11]. Therefore a link closure for many applications, by using ansolid state or semiconductor laser is the device of available 14 cm aperture and existing compact diodechoice. Semiconductor lasers, particularly the GaAlAs laser sources (for acquisition and high-data-ratefamily, are good candidates for the laser source communication). The problem of having to close thein a heterodyne system. Semiconductor lasers are communication link with a reasonably sized aperturecompact, have high power conversion efficiency (with has now been circumvented. This alleviates theprime-to-optical output power conversion efficiency problem that previously existed with the traditionalbetween 20% and 50%), architectural simplicity, direct-detection approach to laser communicationsand utilize single-frequency operation. As a part of with a reliable, high-power high-beam-quality (Strehlthe 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 receivercommunications links based on commercially available design approach also has immunity to large optical30 mW GaAlAs lasers with wavelengths between 0.83 background interference, while not overloadingand 0.86 ¹m. These lasers were adequate for crosslinks requirements on transmitter frequency, thermalin the 100 Mbit/s class (see Fig. 2). stability, or receiver frequency tracking that increaseMULHOLLAND & CADOGAN: INTERSATELLITE LASER CROSSLINKS 1013
  4. 4. the complexity of the alternative heterodyne-detectionapproach. For most parameters, the resonant receiverrequirements lie between those for direct detectionand heterodyne detection. The resonant receiverapproach attacks those key areas that have been majordrivers on system reliability, performance, and cost byoffering a balanced design approach to long-distancehigh-data-rate laser communications. For digital traffic, the full bandwidth, direct,and heterodyne-detection GaAlAs systems entailcomparable mass, power, and volume. However,for analog traffic, the GaAlAs heterodyne-detectionsystem is superior because it uses far less massand volume. The major reason that the GaAlAsheterodyne-detection system is so successful for analogtraffic is that it efficiently accommodates the threemultiplexed communication transponders with adirect-optical-carrier-frequency modulation technique Fig. 3. Acquisition time.[7]. The LITE engineering model at M.I.T. utilizes asemiconductor coherent (heterodyne) detection which and communications. Initially, there is a large ratioallows for nearly quantum-limited performance with between the initial angular uncertainty and the narrowsensitivity better than that of direct detection at all beam divergences in the tracking and communicationbut 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) ofsun, in the FOV; whereas, direct detection systems are McDonnell Douglas Electronic Systems Companysignificantly degraded. uses the direct pulse detection technique, and therefore their acquisition algorithm is differentD. Acquisition and Tracking from one using coherent (heterodyne) detection. Acquisition refers to the process in which the A 100¹ rad acquisition beam is initially scannedreceiving satellite determines where the incoming over the region of uncertainty. The pulse rate of thebeam sent by the transmitting satellite is located. laser is reduced during acquisition to provide higherBridging a 42,000 km link with the very narrow peak pulse power required to compensate for thebeamwidth of a laser poses a serious design problem, expanded beam divergence. When each LCS terminalhowever, multiple sequential methods of acquisition detects illumination from the opposite terminal, theare discussed. pointing converges and scan fields are reduced in One goal for laser communication is the reduction order to increase scan frequency. This continuesin acquisition time and the improvement of acquisition until pointing accuracies are sufficient to supporttechniques. The relation between the beamwidth of communications. The scans are then suspended andthe transmitted beam, the receiver’s FOV, and the each LCS transitions to 10¹ rad communications beammaximum time it takes for acquisition is well displayed pointing and data transmission [2].in Fig. 3. This figure plots curves of maximumacquisition time against azimuth uncertainty angle (for E. Tracking and Maintaining Linksconstant elevation uncertainty) for a number of beamsize and FOV combinations. The curve on the far left Tracking refers to the process in which theindicates that the acquisition time may be more than satellites maintain their communication links. In thefive min for a 0.5 deg initiator beam and a one deg LITE system the high bandwidth steering mirrorresponder FOV. An examination of the curves towards (HBSM) also correctly points the transmitted beamthe right of Fig. 6 indicates that short search times to the other terminal as it keeps the incomingcan 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 variationsdivergence of the initiator are large enough. It should caused by spacecraft motion and vibration. Oncebe noted that for wide initiator beam divergences, high the laser transmitter is set up and stabilized, and thepower lasers must be employed in order to close the beam-steering system has completed the bore-sightinglink. 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 narrowedlocation to values compatible with fine tracking which increases its power. When the other terminal1014 IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS VOL. 32, NO. 3 JULY 1996
  5. 5. senses this, it sends the signal over to its tracking receive signal in conjunction with envelope detectionreceiver, narrows its beacon and points it toward as in RF systems [8]. The key components are transmitLITE. LITE senses this increased power and increases and local oscillator lasers, optical input couplers, andthe tracking bandwidth to 1 KHz to improve tracking high bandwidth electronics.performance. After all acquisition is complete, thecommunications session can begin [4]. IV. LINK BUDGET Fine tracking allows the use of narrowcommunication beams for high-data-rate transfer A. SNR and Data Errorsbetween the two satellites, simultaneously maintainingpoint-induced burst communication errors at The final performance of the system dependsacceptably low levels. A burst error rate of 10¡5 to on the signal-to-noise ratio (SNR). Noise in an RF10¡6 is acceptable and is achievable with a 1 ¾ tracking system is usually thermal noise, or device noise. Inaccuracy of about one-twentieth of the null-to-null link budget calculations for an RF communicationtransmit beamwidth. The spectral content of the link the carrier is an electromagnetic wave. Noise in ansatellite platform disturbance errors determines the optical system consists of thermal, as well as quantumtrack 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 analysisin the beam. In the beginning of lasercom, direct of optical links is 10¡9. From this BER, a SNR ismodulation was the only option available. Information determined. These two factors, in conjunction withwas sent via the duration of pulses of laser power. the range of transmission, are utilized in choosingNow the lasers can be modulated like RF carriers (i.e., the transmitter and receiver. If the digital trafficfrequency 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) trafficfive major parts in the Ball Aerospace design; the and 17 dB for baseband digital traffic, including alaser/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 theprocessing 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, asthe 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. Theon—off keying (OOK) systems are usually implemented sensitivity of the receiver is measured as number ofusing scramblers, or forward error correction coding in detected photons per bit (at peak power) necessary toorder to improve the correlation properties of the data achieve a BER of 10¡9. Once the receiver sensitivitysignal [8]. In order to recover the signal and regenerate is known, the amount of power needed from thethe information sent, PPM maximum likelihood transmitter must be determined. There are variousreceivers require a symbol clock recovery circuit. other noise-inducing factors, such as differences inFor OOK systems the amplitudes are regenerated by temperature throughout the atmosphere significant tothreshold decision. The synchronization requires a cause a perceptible change in the index of refractionphase-locked loop triggered by data transitions [11]. presented to a laser beam as it passes through. ThisA quadrant detector composed of four APDs split can result in beam broadening, tearing and steeringat a focal plane by a pyramid, light pipes, or fibers of portions of the beam, causing fades and surges inmay also be used as a communications detector (3 ns the optical beam as a result of variations in powerrise and fall) in addition to a track detector, if the density. The probability of bit error is thereforequadrant outputs are summed. Duchmann and Planche dramatically increased. Atmospheric turbulence, andindicate that in their communications system, the pointing inaccuracies are other factors which canreceive function consists of a low noise APD-based introduce bit errors and degrade the performance ofdirect detection of the incoming signal followed by a communications link.a non-return-to-zero (NRZ) regeneration of thebaseband 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 thator differentially encoded phase-shift keying (PSK) proves link closure. It is used to determine whethermodulation is used. The detection principle consists the SNR is high enough for data to be successfullyof the active mixing of a local oscillator signal and the transferred. Link budget calculations determine systemMULHOLLAND & CADOGAN: INTERSATELLITE LASER CROSSLINKS 1015
  6. 6. TABLE I Optical Intersatellite Link System Parametersperformance for communication configurations bytrading off such parameters as aperture, transmitterpower, and data rate. This section shows the designand definition of the communications link. Opticalpower budget calculations are performed in six systemsto determine antenna diameter requirements as afunction of average transmitter power. The systemsare: 1) carbon dioxide laser system with heterodynedetection, 2) neodymium-doped laser system withdirect detection, as in the LCS laser of McDonnellDouglas, which utilizes solid state GaAs diodes topump a Nd : YAG rod, 3) In GaAsP laser systemwith direct detection, 4) GaAlAs laser system withdirect detection, 5) GaAlAsP laser system withwavelength division multiplexing and direct detection,and 6) GaAlAs laser system with heterodyne detection.These systems have been previously introduced inearlier sections which discuss transmitter and receiveroptions. Typical parameter values are used throughoutthis discussion in order to determine the antennarequirement for each of the six systems as a functionof optical transmitter power. Table I gives the system Fig. 4. Antenna diameter requirements for CO2 system.parameters used in the link budget calculationsfor the different optical intersatellite link systems.These calculations were performed for each of the The Nd-doped system was evaluated uner mode-lockedmodulation formats discussed, and a link margin of conditions. Results for the other systems are similar5 dB was assumed in all cases. The results for the CO2 to the GaAlAs System. It must also be noted thatand AgAlAs systems are displayed in Figs. 4 and 5. the actual average transmitter power for the analog1016 IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS VOL. 32, NO. 3 JULY 1996
  7. 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 powersformats 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 severalwere estimated to compare the six systems, and the methods which can be incorporated into thecorresponding antenna diameters were then obtained acquisition and tracking phase, as well as thefor 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 systemlevels, and calculated diameters of these baseband enhancements.signals. Table III provides a similar comparison for thetransmission of three QPSK transponder signals. The A. Acquisition and Tracking EnhancementsCO2 and Nd systems require the smallest diametersfor transmission of baseband digital signals but use The narrow beamwidth of the transmitted opticalmore complicated modulation techniques and less beam presents design difficulties in the actualefficient transmitters than systems based on GaAlAs acquisition of the signal. To broaden the beam andheterodyne detection system, although difficult to send it out from the laser calls for much more powerMULHOLLAND & CADOGAN: INTERSATELLITE LASER CROSSLINKS 1017
  8. 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 toamount of intensity is to be sent over 42,000 km links. process the different incoming beams, which wouldThe authors propose using the fact that in the far linearly increase payload size and weight, or a singlefield (Fraunhouffer) light sent through an aperture processor can take multiple inputs and process themwill 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 suchbe 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 receivingsinx=x, and a sinx=x will result in a square pulse. The satellite without concern for their data becomingresulting far field pattern should be chosen so that its available to other users (particularly important insymmetry will facilitate finding the “center” where the personal communications and proprietary or secureactual beam will be present. It should have an area communications). Due to the short wavelength ofof increased spatial area, or of spatial area significant optical systems, it has been noted that there is aenough to make it useful to place the initial field high degree of directivity. Careful attention must bethrough an aperture. If the signal is broader, it will paid so that once the acquisition signal is receivedbe easier to find. It has been discussed that a 2-phase and the system switches to communication beamsacquisition phase can be used to save energy while the that the beam divergence is not wide enough toreceiver satellite is trying to locate the transmitting allow interference between the various incomingsatellite. Another suggestion by the author is to use communication signals.a three-phase approach. Trades should be made to The Optical Communications Technology groupdetermine if a very large, very powerful pulse or at Lincoln Laboratory believes that the technologypulse sequence as an initial phase will cut down the is available for deployment of operational laserreceivers initial field of uncertainty or FOV enough to communication systems in the several hundredsignificantly decrease acquisition time. megabits per second range, with near term technology to be able to support multipke gigabits per secondB. Communications Enhancements links in small and reliable packages [3]. It has been noted in certain systems (such as the C. Receiver EnhancementsLITE system), that redundant laser diodes are presentbut are used solely as backup when other diodes fail. In recently developed low-effective k silicon APDsThey may also be used to provide the necessary power (k = 0:002 to 0.005, depending on wavelength), ain the case of weaker lasers. The authors suggest sensitivity of 68 photons per bit at a BER of 10¡9that data throughput be increased by simultaneous has been measured on a direct-detection receiveroperation of multiple lasers in the transmitter section, developed using a lser diode (¸ = 810 nm) with anto 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
  9. 9. REFERENCES [9] Feher, K. (1983) Baseband transmission systems and power efficient[1] Casey, W. L., Doughty, G. R., Marston, R. K., and Muhonen, J. modulation techniques for linear and nonlinear satellite (1990) channels. Design considerations for air-to-air laser communications. Digital Communications: Satellite/Earth Station In SPIE Proceedings, 1417, Los Angeles, CA, 21-2, 1990. Engineering.[2] Deadrick, R. B., and Deckelman, W. F. (1992) Englewood Cliffs, NJ: Prentice-Hall, 1983. Laser crosslink subsystem–An overview. [10] Marshalek, R. G., and Koepf, G. A. (1988) SPIE, Vol. 1635, Los Angeles, CA, Jan. 23—24, 1992. Comparison of optical technologies for intersatellite links[3] Boroson, D. M. in a global telecommunications network. An overview of Lincoln Laboratory development of Optical Engineering, 27, 1 (Aug. 8, 1988). lasercom technologies for space. [11] McIntyre, R. J. (1991) MIT Lincoln Laboratory. Comments on performance of coherent optical receivers.[4] Marshalek, R. G., and Paul, D. K. (1990) Proceedings of the IEEE, 79, 7 (July 1991), 1080—1082. Mass, prime power, and volume estimates for reliable [12] Boroson, D. M. (1993) optical intersatellite link payloads. LITE engineering model–I: Operation and performance In SPIE Proceedings, 1218, Los Angeles, CA, Jan. 15—17, of the communications and bean-control subsystem. 1990. In SPIE Proceedings, 1866, Los Angeles, CA, Jan. 1993.[5] Marshalek, R. G., Smith, R. J., and Begley, D. L. (1992) [13] Hect, E. (1987) System impact of the resonant laser receiver for free-space Optics (2nd ed.). laser communications. Reading, MA: Addison-Wesley, 1987. SPIE Proceedings, 1635, Los Angeles, CA, Jan. 23—24, [14] Pillsbury, A. D., Taylor, J. A. (1990) 1992. Optomechanical design of a space-based diode laser[6] Borner, S., and Heicher, J. (1989) transmitter assembly. 4-PPM modulator/demodulator with fully digital signal In SPIE Proceedings, 1218, Los Angeles, CA, Jan. 15—17, regeneration. 1990. In SPIE Proceedings, 1131 (1989), 195. [15] Verdeyen, J. T. (1989)[7] Noldeke, C. (1992) Laser Electronics (2nd ed.). Survey of optical communication system technology for Englewood Cliffs, NJ: Prentice-Hall, 1989. free-space transmission. [16] Ross, M. (1975) In SPIE Proceedings, 1635, Los Angeles, CA, Jan. 23—24, Direct photodetection space laser communications. 1992. In Convention Record: Electronics and Aerospace Systems[8] Duchmann, O., and Planche, G. (1991) Conb., 1975, 174-I—174-H. How to meet intersatellite links mission requirements by [17] Chan, V. W. (1983) an adequate optical terminal design. Heterodyne lasercom systems using GaAs lasers for ISL In SPIE Proceedings, 1417, Los Angeles, CA, Jan. 21—22, applications. 1991. In Conference Record: International Conference on Communications, 1983, E1.5.1—1.5.7.MULHOLLAND & CADOGAN: INTERSATELLITE LASER CROSSLINKS 1019
  10. 10. Sean A. Cadogan was born in Brooklyn, NY in 1968. He received his B.S. in electrical engineering from theMassachusetts Institute of Technology, Cambridge, in 1990, and an M.S. in electrical engineering from VillanovaUniversity, Villanova, PA, in 1993. From 1990 to 1992, he worked at General Electric Aerospace as an Edison Engineering Program memberholding positions in the Systems Integration, Systems Analysis, and Verification and Test Engineering groupsin Management and Data Systems. While in the Sensor Systems Engineering groups in Management and DataSystems. While in the Sensor Systems Engineering group he was the project leader on a study that quantifiedthe 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 MartinMarietta 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 VillanovaUniversity to develop the microwave engineering technology area for both education and research. Before joiningVillanova University, he was employed at the General Electric Space Division as Manager of the CommunicationEquipment and Antenna Engineering Laboratories. His assignments have included the development ofmicrowave filter analytical techniques and the design of waveguide and directional filters and the Ku and Xfrequency 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 withthe microwave transmission segment of a major military satellite data communications system. Prior to joiningGeneral Electric, he provided consultation in radar clutter, multipath, propagation effects and radiation hazardsat the RCA Missile and Surface Radar Division. He also provided analytical support for the AN SPY-1 radar inthe areas of antenna matching, random materials, monopulse tracking collimation and alignment, and sidelobeblanking. Dr. Mulholland is a registered Professional Engineer in Pennsylvania, past Chairman of the AntennaPropagation/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