Abstract-Laser communications systems offer numerous advantages over radio-based systems as a result of the small beam divergence angles and large antenna gains. These advantages are especially pronounced in space-to-space applications where compact hardware can deliver high bandwidth communications spanning large distances with small power. However, the extreme beam directivity makes the design of the lasercom acquisition, pointing and tracking (APT) system especially burdensome. This paper demonstrates how the APT system in turn impacts the topology of a multiple satellite laser communications network. Advanced concepts are proposed that promise enhanced connectivity (and consequently yield improved communications performance and survivability) while allowing cost-effective adaptive sharing of the lasercom resources.

The performance of high data rate communications links can suffer degradation in scintillation due to intersymbol interference and antenna scattering loss. Adaptive equalization has proven to be very effective at mitigating the effects of intersymbol interference on high data rate links when sufficient signal power is available. The performance of an adaptive equalizer is limited, however, under severe scintillation conditions by the reduction in the received signal power due to scattering loss of the receiving antenna. Large antennas are typically utilized in high data rate communications links in order to achieve sufficient signal-to-noise ratios for reliable data demodulation. In scintillation, such large antennas suffer large losses as signal energy is scattered out of the narrow main beam of the antenna. Equivalently, this scattering loss results when the received signal is decorrelated across the face of the antenna aperture. This paper discusses the use of multiple small antennas to collect signal energy and an adaptive equalizer to coherently combine that energy. Such an equalized receiver can then utilize the spatial diversity of the scintillation to enhance performance. If the multiple antennas are physically separated, then spatial diversity can be utilized even in slow, flat fading conditions.

In a communications network consisting of multiple nodes and links located in a hostile environment, optimum shortest path routing presents a challanging problem. Of interest is the robustness of the routing algorithm to node or link failures. Since this type of environment suggests the use of distributed or non-centralized algorithms, the convergence or recovery of the network to the shortest path routing tables is examined. Modeling of the network is accomplished with a typical cost function and flow rate distribution along with fixed link capacities. Results are obtained by computer simulation for two popular routing algorithms applied to a simple network and a network typical of orbital satellite topologies.

The average probability of error for a frequency hopped M-ary frequency shift keyed (MFSK) system is analyzed in the presence of partial-band jamming, multiple user interference, and Rayleigh statistics for each jamming tone and multiple user interferer considered. The uncoded system performance is compared with previously published results. Forward error correction coding is then added to the system to study performance gains under various channel scenarios.

A general scheme for the code synchronization of Frequency-Hop Spread-Spectrum (FHSS) is presented. Synchronization performance is observed in the presence of channel dynamics and is characterized by mean synchronization time and mean time to loss of lock. A fixed-threshold multiple-dwell synchro-nization scheme is shown to adequately mitigate the effects of random hop jamming.

Air defense enhancements under consideration due to increasing air traffic and the cruise missile threat will result in the introduction of a number of low earth orbit satellite constellations that will need to communicate data quickly and reliably. Current air defense communications makes use of commercial satellites in geostationary orbit. The Seek Igloo radars in Alaska communicate to Elmendorf AFB via the Aurora Satellite, as part of the AUTOVON system. Satellite communications are also planned for the North Warning System (NWS) as a primary communications medium. The upcoming satellite constellations, deployed in low earth orbit, will require new networking concepts that provide robust operation and connectivity. This paper addresses the communication network issues of low earth orbit constellations and provides a quantitative measure of the time varying connectivity of space networks.

Deep-space optical communication systems generally require the use of the Sun- lit Earth as the pointing reference. For simple single-frame spatial acquisition processors, the uncertainties in Earth albedo can result in an irreducible error in acquiring the spatial location of the receiver. Multiple frame processing algorithms can be used to estimate the Earth albedo. The prior knowledge of the image shape and orientation can then be used to derive the location of the receiver.

This paper discusses typical manned Mars exploration needs for telecommunications, including preliminary navigation support functions. It is a brief progress report on an ongoing study program within the current NASA Jet Propulsion Laboratory Deep Space Network (DSN) activities. In support of NASA Office of Exploration mission requirements, system performance & design options - including DSN architecture, and technology needs to support these exploration opportunities over the next 25 years are outlined. A typical Mars exploration case is defined, and support approaches comparing microwave and optical frequency performance for both local in-situ and Mars-Earth links are described. An objective of this paper is also to identify optical telecommunication and navigation technology development opportunities in a Mars exploration program. A local Mars system telecommunication relay and navigation capability for service support of all Mars missions has been proposed as part of an overall Solar System communications network. The effects of light-time delay and occultations on real-time mission decision-making is discussed; the availability of increased local mass data storage may be more important than increasing peak data rates to Earth. The long term frequency use plan will most likely include a mix of microwave, millimeterwave and optical link capabilities to meet a variety of deep space mission needs.

Communications via optical cross-link between two satellites require reciprocal spatial beam tracking. For intersatellite links (ISL) operating at narrow beamwidths, the line-of-sight pointing error at both satellite stations will have a significant impact on the system communication performance. Large pointing error on either end of the system can combine to impose higher power penalty at the transmitter, and can severely degrade the receiver performance. Tradeoffs in the required optical power and the size of the receiver aperture can be made while keeping the communication bit error rate constant. An approach to the selection of the optimal system parameters is presented. Given the tracking error statistics at each end of the link, and the desired probability of bit error rate (PBE), the combination of optical apertures are found that minimizes the required laser power. A tight error upper bound for direct-detection PPM system were developed for the bit error rate calculation. The spatial angle tracking error statistics modeled for each station are identical Rician distribution with static pointing errors. The product of optical apertures and the RMS tracking error, and the ratio of the required laser power and the RMS tracking error, are found to be constants, which are independent of the actual RMS tracking error for fixed PBE and static pointing error. Practical ISL parameters are used in the selection of the pair of optical apertures. It will be shown that in order to meet the size and weight constraint of the optical package and the requirement of the transmitting laser, suboptimal designs may be necessary for ISL applications. Sets of curves illustrating the impact of tracking error on the design parameters and the associated laser power penalty for different aperture gains and power requirement will be presented.

The importance of pointing and tracking is demonstrated with current deep-space optical communications system concepts. Maximum Likelihood (ML), Minimum Square Counting Error (MSCE), and Maximum Product (MP) estimation algorithms (or decision rules) are derived to estimate the location of the receiving station to sub-pixel resolution. Comparisons of the above algorithms are made, via Monte Carlo computer simulation, in terms of estimator's bias and variance. Optical communication link analyses are made for a typical Earth-Mars scenario, to gain engineering insights. It is observed that both the ML rule and the MSCE rule perform better than the MP rule.

In this paper the NASA Goddard Space Flight Center (GSFC) laser communication pointing, acquisition, and tracking laboratory demonstration program is discussed. This program entails the construction of a flexible brass-board simulation tool, and a supporting computer simulation effort. The result of this program will be a viable test bed to support the design and specification of space-based laser communication terminals and, additionally, to address critical performance issues such as tracking robustness in the presence of background (e.g., stellar or earth) noise and/or internally generated thermal noise.

The utility of employing lasers to perform certain air-to-air communications functions is often questioned on both technological and physical grounds. An obvious question regarding air-to-air laser communications concerns the advantages of using lasers versus more common radio frequency (RF) techniques to satisfy communications requirements. Simply put, what are the advantages? Obviously, when the desire is to control the amount and direction of emitted radiation from a source, lasers are attractive by virtue of their narrow beamwidths. When omni-directional communications are desired and/or the control of emitted radiation is not required, then conventional RF techniques at lower frequencies are better suited to the task. Answers to the more intuitive questions addressing the state of technological maturation and the lack of an all-weather capability have eluded the user community, and stunted the developmental growth of this promising technology. The Avionics Laboratory of the Air Force Wright Aeronautical Laboratories has been actively involved in the development of air-to-air laser communications technologies since 1981. Through both in-house and contracted work, a unique perspective regarding the issues hindering air-to-air laser communications systems development has evolved. Before a significant system development program is undertaken, satisfactory answers to basic questions which deal with operational utility and performance limitations need to be formulated and disseminated to the users in a comprehensive fashion that will allow them to decide if this technology is appropriate to their mission. Also, the identification and pursuit by the R&D community of well balanced initiatives that reduce development risk of certain component areas will provide a greater impetus for future system level development efforts. This paper will present and discuss some of the development issues that need resolution before air-to-air laser communications can be successfully applied to airborne communications problems.

An architectural design of a ground-based antenna (telescope) for receiving optical communications from deep space is presented. A channel capacity of 100 kbits/s from Saturn or 5 Mbits/s from Mars requires a 30-cm-diameter transmitter and a 10-m-diameter reception antenna. The f/0.5 primary mirror will be hexagonally-segmented, and will have a surface roughness tolerance of 2 μm rms to permit a substantial cost savings. The antenna will receive communications even when the deep-space laser source appears to be located within a small angle of the Sun (small solar elongation). Instead of a long, unwieldy, conventional sunshade, a sunshade consisting of hexagonal tubes will be mounted in precise alignment with the primary mirror segmentation. The ends of the tubes will be trimmed so that both the sunshade and the antenna will fit within a more-than-hemispherical dome whose diameter clears a sphere only 6/5ths the diameter of the primary reflector. This sunshade is useful when solar elongations are as small as 12°. Additional vanes may be inserted in the hexagonal tubes to permit operation at 6° or 3°. The frequency-doubled output of the Nd:YAG source laser will be tuned dynamically to lie within a Fraunhofer line (a spectral interval of reduced solar emission) to carry the signal with reduced interference from sunlight. The source laser and the Fraunhofer filter (a narrow-band predetection optical filter) will be tuned to match the Doppler shifts of the source and background. Typical Doppler shifts are less than 0.05 nm or 53 GHz. A typical Saturn-to-Earth data link can reduce its source power requirement from 8.2 W to 2 W of laser output by employing a Fraunhofer filter instead of a conventional multilayer dielectric filter.

To reduce power requirements for a high data rate optical satellite communications network it is advantageous to broadcast a signal with a very small angular divergence. If the receiver also has a narrow field of view, background light (from the daylit earth, etc.) can be sufficiently blocked with conventional passive filters. A narrow communications transceiver field of view, however, requires an accurate mutual tracking capability. Initial acquisition and subsequent tracking will require a tracking receiver with a much larger field of view than what is necessary for communications. To minimize weight and absolute pointing requirements on the telescope steering mechanism, this field of view should be as large as possible subject to the constraint of keeping collected background light levels below an acceptable level, and maintaining a tracking accuracy better than the communications laser divergence angle. Background light can be minimized and tracking accuracy can be maintained by using a narrow bandwidth, wide field of view imaging atomic line filter which matches the wavelength of a semiconductor diode laser. The operational principles of such an active image preserving atomic line filter are explained, especially as related to satellite tracking for a communications network. Preliminary results showing image preservation at a spatial resolution of 0.5 mm by a cesium atomic line filter at 852 nm with an acceptance bandwidth of 0.002 nm are presented. Expected limiting resolution, conversion efficiency, time response and laser power requirements for image preserving alkali atomic line filters are also discussed.

This paper describes theoretical background and experimental results of an acquisition scheme for free-space laser communications. Mutual acquisition between two optical intersatellite terminals was simulated in a laboratory environment, and statistical results obtained on subsystem performance. Results show that the selected algorithm provides fast (several seconds in a flight system) acquisition with a high probability, and analysis shows the algorithm to be capable of such performance in the presence of a sunlit-Earth background.

We evaluated candidate Quadrant Avalanche Photodiodes as optical communication receivers and optical acquisition/tracking receivers in a testbed which exercises both functions simultaneously.

A compact, rigid and lightweight diode-laser-pumped Nd:YAG laser module has been custom-designed, built and environmentally tested. This module is power efficient and has no movable mounts for optical alignment. All optical elements have been bonded onto the module using space applicable epoxy. Using two 200 mW diode laser arrays for pump sources, 126 mW of continuous-wave (cw) output has been achieved with about 7% electrical-to-optical conversion efficiency. This laser module was environmentally tested by subjecting it to vibrational and thermal conditions similar to those experienced during launch of the space shuttle. The module performed well after all tests were completed. A set of 21 semiconductor diode laser arrays were also tested under shuttle-launch vibrational and thermal conditions. In addition, a 600-hour unaccelerated life test was subsequently performed on the diode lasers. Upon completion of vibrational and thermal tests most diode lasers showed little or no degradation in output but some damage to the laser front facet was observed. Following the lifetest, significant degradation was observed on those lasers that had poor performance to begin with. Test results along with suggestions to improve the reliability of the Nd:YAG laser are discussed.

In the past several years, much attention has been given to the overall problems of acquisition and tracking for free space optical communications. Accomplishing these functions with minimal investment in hardware size, weight and power is essential to the successful evolution of this type of communication hardware. This paper describes electro-optical beam switching and beam steering devices that are based on the application of ferroelectric chiral smectic C liquid crystal materials. These devices are intended to provide an alternative to the use of electromechanical hardware for beam control, fine tracking and point ahead in optical communications termi-nals. Work to date indicates that small device size, high bandwidth and low drive power can all be achieved.

A coherent array of 10 amplifiers has been demonstrated using an active integrated optics technology based on AlGaAs laser diode material. These arrays have demonstrated power outputs of 300 mW CW with only 2 mW injected power. The fractional coherence between any two amplifiers in the array is greater than 0.8. These arrays have been used to amplify a 10 MHz amplitude-modulated signal, which is the first step in the development of a laser communications system.

Diffraction-limited imaging of an unknown, incoherently illuminated object is demonstrated through strongly aberrated optics with unknown aberrations. No reference source other than the object itself is used to calibrate the aberrations. Two different techniques are discussed and their merits are compared.

A relatively low cost and low technical risk method has been demonstrated for optically abutting mosaic detector arrays to achieve very large staring sensor focal plane assemblies. A recent demonstration of this technology achieved 0.15 pixel registration of two mosaic infrared arrays operating at 50°K. DBA has patented this technology (Patent No. 4634882, trade name RIMSTAR) and has applied it to a proof of principle infrared staring sensor which is designated IRX.

Quarks in atomic nuclei interact with neutrinos and antineutrinos. Within a single nucleus, the quarks are strongly coupled to each other. For single crystals with high Debye temperatures, atomic nuclei may be coupled to each other with sufficient strength to affect total cross sections. A new method for detection of neutrinos and antineutrinos gives total cross sechions larger than earlier methods, for single crystals, by more than 10²⁰. The method is reviewed and directive features discussed.

Scintillating fibers coupled to position sensitive photomultipliers have good angular precision and good energy resolution in detecting gamma-rays. Recently high photon yielding and long attenuation length step index scintillating plastic fibers have been developed. Scintillating fibers of 1 mm diameter made of polystyrene doped with butyl-PBD (λ = 420 nm) and clad with PMMA (poly mptAylmetacrylate), have resulted attenuation lengths over 2 meters. Scintillating fibers stacked up into scintillating fiber planes U, V and W that are rotated by 60° angle relative to each other and coupled to position sensitive photomultipliers can be used as high resolution imaging gamma-ray detectors. With this arrangement the Compton electron or pair production point can be determined by the scintillation photons reaching the photomultipliers. A 3-dimensional conversion point accuracy is expected to be 6 _rms~1 mm. A large variety of Compton and/or pair production gamma-ray telescopes using scintillator blocks coupled to vacuum photomultipUer tubes has been built earlier for space based experiments. In these cases the scintillator block dimensions were large thus limiting the angular accuracies and resolutions. Here we are presenting the design of a large area gamma-ray detector with high angular and energy resolution for space based experiments, using scintillating fibers and recently developed position sensitive photomultiplier tubes. This detector is under development at the UCLA/UTD laboratories.

The distinguishing of point-like man made gamma ray sources from point like astrophysical or cosmological sources can be achieved by analyzing the line structure of the gamma ray spectrum in the 0.5 - 10 MeV energy range. A special computer program has been developed for this analysis and tested in Monte Carlo simulations. An application of this method has been made for burster manifestations of superconducting cosmic strings using the High Resolution Gamma Ray Telescope developed by the UTD/UCLA collaboration.

This report considers the use of millimeter to submillimeter wavelength space based radars for strategic defense and considers the prospect of developing rf sources to power them. The main application envisioned here is midcourse decoy discrimination for which the radar would have the capability of producing, with modest power requirements, a series of images in real time, at strategic range. These images have resolution from 2-20 meters, depending on the radar configuration. One advantage of a radar over a passive system is that with advanced radar techniques of monopulse and SAR and ISAR, the diffraction limited resolution for a given antenna size can be exceeded by a large amount. A monopulse system generally scales favorably at longer wavelength, while a SAR has more favorable scaling at short wavelength. Wavelengths from 300 μm to 1 mm are good compromises and work well for both radar configurations. One interesting scaling law is that for a SAR at short wavelength, the energy per pixel of image is independent of wavelength. This then gives a tremendous advantage to the most efficient rf source, and fast wave, electron beam driven sources could have significant advantages over CO₂ lasers in this respect. The state of fast wave technology is then reviewed, and the prospect of developing a tube to power such a space based radar is evaluated.

The feasibility of developing a practical amplifier in the 'Optical' Klystron configuration at a wave-length of 2 mm has been investigated. The effect of beam velocity spread on the efficiency and gain of the 'Optical' Klystron was calculated. Greater sensitivity to beam velocity spread was found for three cavities than for two cavities. The efficiency was reduced to one-third of its cold beam value when axial energy spread Δ_γ/_γ0 = 5.5 x 10^-4 for three cavities, 6.7 x 10^-4 for two cavities, and 2.8 x 10^-3 for a single-cavity free electron laser. However, it is estimated that even for the case of three cavities, electron beams of suitable quality might be achievable.

At M.I.T. a submillimeter gyrotron is being operated using a waveguide cavity with an iris at its output end. This experiment operated at second harmonic for 1-2 μsec pulses with a 75 kV, 10 A electron beam. Second harmonic emission has been observed at 13 frequencies from 301 GHz to 503 GHz with output powers of 1-22 kW and a 15 MHz frequency bandwidth. The highest output power was 22 kW with an efficiency of 3.5 % at 467 GHz and an output power of 15 kW with 6% efficiency was obtained at 417 Ghz. A variety of diagnostics that were used to discriminate between fundamental and second harmonic emission will be discussed.

The Innovative Science and Technology Office is in the planning stages of a sounding rocket experiment. The primary purpose of the proposed experiment will be to passively measure the optical radiation, produced during the boost phase, in the aerodynamically heated bow shock. An assortment of onboard spectrometers and radiometers will gather data from observations made looking forward, along the direction of flight. Spectral and radiometric measurements will be made for altitudes between approximately 40-100 km, and for rocket velocities in the range of 3-4 km/sec. This velocity and altitude profile is different than those studied previously. In earlier work, experimental data has been obtained only under reentry conditions, which correspond to higher velocities. As a preliminary to the design of the instrumentation for this experiment we have been involved in a series of flow field computations in order to make signal strength predictions. The results of these computations are discussed in this paper. These results are then used to develop some of the parameters that will be needed for designing the instrumentation package proposed for this experiment. The reasoning behind some of the specific systems choices will also be elucidated.

The use of 3D (three dimensional) matched filtering has been shown to be a powerful procedure for detecting weak targets of a known velocity imbedded in a strong background. The concept is based on electronic digital signal processing following imaging of the target scene on a CCD array. The array produces a sequence of data frames and the 3D algorithms produce filtered images in which the targets emerge as observable tracks. These algorithms have been implemented and verified via both simulation and real data processing. The system is constrained however in that full benefit of the processing requires a 2D transform to be executed each frame time, and therefore is the ultimate limit in scene size and processing speed. Recent study at USC has been devoted to investigating the feasibility of an all-optical system to perform the equivalent 3D processing. The required scene transforms are obtained optically, and the 3D algorithm implemented via optical beam combining, storage, and read-out. This paper presents the results of the current study, indicating suggested implementations of the optics and their required hardware development. Devices needed to succcessfully implement the algorithms are discussed, and basic shortfalls and anomalies are considered. Key elements appear to be the spatial light modulators and stobed read-outs of holograms or storage plates.

Plasmas have recently received considerable attention as potentially efficient long wavelength (infrared to microwave) phase conjugators using degenerate four-wave mixing (DFWM). In such processes, two electromagnetic (EM) pump waves (`forward' and 'backward') plus a 'probe' electromagnetic wave interact through the nonlinear low frequency susceptibility of the medium to produce a fourth 'signal' or 'conjugate' electromagnetic wave. The process may also be viewed as an Bragg scattering of an EM wave from a low frequency density grating. The density grating is generated by the coupling of one pump wave to the probe wave via pondermotive or thermal pressure forces. In this paper, the possibility of using ionization nonlinearities in weakly ionized plasmas for wave mixing applications is discussed. By locally heating the weakly ionized plasma, one may selectively ionized portions of the plasma to produce a low frequency density grating. The applicable wavelength regimes, plasma parameters, and response times for ionization nonlinearities will be discussed.

The engagment of penaided, nuclear armed Ballistic Missile Re-entry Vehicles (RVs) by a Theatre Missile Defence (TMD) system requires the use of a robust and adaptive discriminsation system to identify warheads from accompanying decoys and other penetration aids. TMD systems will be characterised by their electronic countermeasure environments, and short flight times of the ballistic missile threat. In such environments time is of the essence for TMD commanders to make effective decisions about the allocation of defence weapon systems. The identification and classification, i.e the discrimination, of warheads in a theatre environment is therefore especially stressing requiring detailed analysis and quantification. The discrimination system must be capable of processing object-attribute data derived by sensor systems throughout the four major flight regimes (boost, post boost, mid course, terminal) of ballistic missiles, in many different measurement domains, i.e in multi-feature space. Historically discrimination system designs have been based on a deterministic approach where the defence assumes that signature differences will eventually appear between the warheads and the penetration aids. The classic example is the use of the K factor, where Gaussian distributions - with different mean values for the warheads and penetration aids - are assumed for specific feature space. These classification methodologies process the multi-spectral and multi-spatial data using algorithms designed to separate features within these domains. This allows, in the first instance, the identification of different groups of threat objects. This is followed by their classification into either warheads or one of a number of penetration aids - perhaps through some form of population analysis. These methods rely upon identifiable differences appearing between warheads and penetration aids and are therefore known to be vulnerable to robust countermeasure designs. This has lead to a penaid matching philosophy based on minimisation of the Bhattacharyya Distance. The approach adopted by the research described within this paper is to add inferencing algorithms to the discrimination process - in combination with a database of a-priori information on likely countermeasure options, and their associated signatures. The discrimination system is then able to adapt to the measurements made during the course of the raid, and reason about the types and numbers of countermeasure employed by the offence; through this process postulating the configuration of the penetration aids on the missile systems. In this way specific features of interest can rapidly be identified allowing the discrimination system to concentrate on those areas in preference to those where no worthwhile data is available; thus optimising the use of sensors' viewing time in the course of battle. The paper reviews the principles behind the use of an Artificial Intelligence (AI) based approach to discrimination, and describes the design of an evaluation facility established to quantify the utility of this methodology.