We are on the brink of major technology and system breakthroughs for high data rate atmospheric and space communications. At present, radio systems operate below 60 GHz and are the traditional workhorses for satellite communications. Legal constraints and the laws of physics limit data rates on these systems. Near infrared light systems provide the promise to meet space transmission requirements of the 21st century, much as present fiber optic systems satisfy bandwidth requirements well into the next century. Gigabit data rates are being achieved on operational systems. The challenge we face is how soon equivalent transmission rates will be made available for space application. The position is taken that if we can overcome three optical system and technology issues, the challenge will be met sooner rather than later. In declining order of importance the issues are: Optical pointing, acquisition and tracking Laser diode optical sources that produce sufficient output power Optical detector technology

Long-range, wideband data-rate, satellite communication is necessary to support the command-and-control architectures evolving for a number of strategic, tactical, and remote sensing systems. High-resolution sensors which must support wide field-of-view coverage require secure high-rate data links which are resilient to jamming and other interference sources. In particular, spaceborne synthetic aperture radar (SAR) systems are attractive for applications ranging from environmental remote sensing to crisis monitoring. SAR systems can produce high-resolution imagery over protected or restricted areas, independent of local cloud cover conditions.

During the remainder of the 1980's and early 1990's, many technology improvements will be made to achieve high data rate satellite-to-satellite communications. Programs such as the Strategic Defense Initiative Organization's SDS (Strategic Defense System) have projected architecture requirements for data transfer rates ranging from hundreds of kilobits per second to hundreds of megabits per second on satellite crosslinks. The advanced technology required to realize these rates is being investigated even now, but a general solution for the communications problem, defined in part by unique operating environ-ments ranging from benign to highly perturbed, is not possible. This paper focuses on one aspect of the overall satellite communications problem. It begins by briefly comparing radio frequency (RF) with direct detection laser communications (lasercom) for application to high data rate SDS low and medium earth orbit crosslinks. This comparison is based on near term technology suitable for early deployment in the SDS. The RF system is a millimeter-wave system operating in the 60 GHz region of the electromagnetic spectrum. The Lasercom system is considered to operate in the near infrared, from 0.8 to 1.1 micrometers. The trades consider operation in benign as well as the very stressing environments postulated for the Strategic Defense System which includes low and high altitude nuclear bursts as well as the threat of spaced based jamming sources and directed energy weapons. Results of the trades are presented to show the advantages and disadvantages of each type of system. The advantages and disadvantages of the RF and Lasercom systems are of sufficient magnitude to consider combining the best features of each to create a truly new communication system with capabilities far exceeding those of either system. For example, maximum survivabilty results from carrying both systems (RF & Lasercom) on the satellite. However, this approach is costly in terms of weight, power, and real estate. A better approach would be to carry the capabilities of both sys-tems. Such an approach is proposed in this paper. The combination of systems, termed the Dual Mode Communications System (DMCS), is the principal thrust of the remainder of the paper.

The Air Force is interested in laser communication systems for a variety of air-to-air applications. Laser data transmission offers significant advantages over RF systems in certain areas including higher data rates with low transmitter power, narrower beam divergence leading to difficulty in interception, narrower field of view coupled with high off-axis energy rejection which makes jamming a very formidable task, and smaller antenna size which creates minimum installation impact on an aircraft. The applications with the greatest near-term potential involve the transfer of data between large aircraft operating in relatively benign dynamic environments normally present at altitudes of about 30,000 feet. Systems performing these strategic data exchange (SDE) functions must operate at ranges of 100 to 200 nautical miles at data rates of 2 to 3 megabits per second and the probability of bit error rates not exceeding 10-6. The paper presents the major communication channel elements of a design for a lasercom system performing SDE roles. The design is established by comparing the advantages of the different approaches. The final design selection is based on the transmitter characteristics required for each system. The characteristics include physical properties, development risk, cost, as well as the flexibility for meeting more stringent system performance specifications without requiring major redesign.

Design and optimization techniques of microwave and millimeter-wave fiber-optic systems are described with emphasis on the system level parameters and the characteristics of related optical devices. Direct modulation vs. external modulation of laser diode trans-mitter and direct detection vs. coherent detection of the optoelectronic receiver are discussed and compared in terms of important design parameters, components and practical performance issues.

Current medium to low data rate low earth orbit to geostationary orbit intersatellite link services are implemented or designed at S band. These services include those provided by TI)RS and planned for Euro-DRS. S band links have a number of undesirable charateristics which make them less than optimum for this function. These include the need for large antennas at both 14E() and CEO, and operation in a high RF1 environment. This paper discusses the results of a study performed by British Aerospace in co-operation with the European Space Agency into the use of optical communications for this service. Projected advances in semiconductor laser diode powers of up to 1W CVV in a diffraction limited beam are considered. The system derived is designed to serve up to eight users simultaneously from GEO with data rates of up to 2Mbps. The payload demonstrates significant mass, accomodation, and operational advantages over existing S band systems, and suggests that optical technology will have applications outside conventional high data rate ISI, payload designs.

The purpose of this paper is to extend the results of a previous paper comparing the LPI performance of optical and millimeter wave satellite systems to include the effects of scattering on optical LPI performance. As before, the LPI figure of merit used to compare the two media is the Circular Equivalent Vulnerability Radius (CEVR) . The CEVR is defined as the radius of a circle which has the same area as the irregularly shaped region within which an airborne interceptor at a fixed altitude can detect the transmitted signal. Typically, this region is projected onto the surface of the earth to provide a degree of normalization with respect to interceptor altitude.

The analysis addresses system design requirements and the relevant methodology, subsystem design requirements and component requirements of optical communication terminals for uplinks and downlinks. We focused on the aspects of a communication link and terminals which are unique or different from the traditional RF communication link. The three areas of analysis are: 1) System Engineering and Design of Lasercomm terminals - This analysis includes the major system drivers (wavelength, data rate, bit error rate (BER), detection technique), the optical train (telescope type and diameter, antenna gain, optical losses), pointing and tracking requirements, and candidate link designs (power budgets, terminal designs, and optical schematics). 2) Technology Assessment - An assessment of the current state of the art and expected developments in 1996 is reported. This assessment covers lasers, detectors, optical telescopes and lasercomm systems (maturity, output power, weight and electrical power). 3) Environmental Hazards - Atmospheric propagation hazards such as absorption, scatter, turbulence and cloud cover are discussed, along with interference and radiation.

Current activities in the field of pointing, acquisition and tracking (PAT) system design for free space laser communications generally have as their main drivers a bit rate greater than 10 Mb/s and the low laser power now available from suitable laser diodes (typically 50 mW). These have resulted in large terminals with apertures of 20 to 35 cm and telescopes around one metre long. This paper presents the results of a study performed by British Aerospace which looked at a system using much smaller apertures (around 5 cm) and which would result in more compact terminals. Such volume efficiency introduces the possibility of a single 'central node' satellite communicating simultaneously with multiple user satellites in different orbits. Links at 2 Mb/s with up to 8 users was the design case. This paper shows that the PAT system for a multiple access node is not simply a miniaturised version of one for a large aperture single access system..The design features low friction actuators, acquisition and tracking functions combined in one sensor and a robust solution to the problem of host satellite vibration. Acquisition times of 2 seconds are anticipated provided the central node is already tracking at least one user.

An optical transmitter, based on four redundant 30 mW GaAlAs diode lasers operating at 0.86 #m, is being developed for an experiment in space-based heterodyne communication. Key optical, mechanical and electrical components of the transmitter have been fabricated and tested in prototype form. A module which includes the laser, collimator and electronics has been assembled which yields a wavefront quality of better than X/36 over a 24° C temperature range. The laser and its four-element collimator have been shaken independently at 49 g (rms) without degradation in performance. The transmitter will weigh less than 2.3 kg and consume less than 4.2 W during normal operation.

The advent of space-based coherent diode laser communication systems requires development of space-qualified laser diagnostics to make these systems robust in face of aging laser parameters. This paper describes the packaging and performance of a diagnostic system which sets the wavelength, tone spacing and optical power of a directly modulated frequency-shift-keyed GaAlAs laser transmitter.

This paper provides an overview of the opto-mechanical subsystem (OMS) for the Lincoln Laboratory Laser Intersatellite Transmission Experiment. The OMS contains the telescope, relay optics, and beam steering mechanisms. The optical, mechanical and thermal aspects of the OMS design are discussed and the predicted design performance is presented.

Large aperture phased array antennas operating at millimeter wave frequencies are designed for space-based communications and imaging. Array elements are comprised of active transmit/receive (T/R) modules which are linked to the central processing unit through a high-speed fiberoptic network. This paper demonstrates optical control of active modules for satellite communication at 24GHz. An approach called T/R level data mixing, which utilizes fiberoptic transmission of data signal to individual T/R modules to be upconverted by an optically synchronized local oscillator, is demonstrated at 24GHz. In this study free-running HEMT oscillator, used as local oscillator at 24GHz, is synchronized using indirect subharmonic optical injection locking over a locking range of 14MHz. Results of data link performance over 500-1000MHz is also reported in terms of gain-bandwidth, linearity and third order intercept, sensitivity, and dynamic range.

This paper is concerned with recent developments related to an optically controlled phased array antenna (OCPA). It will discuss the most important requirements of such a system, highlighting important results in optically controlled beamforming, and routing via fibers reference and communications signals.

The use of optical fibers as the delay line element in radio frequency memory loops, promise light weight and small size devices with long time delays. This paper presents the experimental results and performance analysis of a 2-6 GHz fiber optic recirculating delay line, using commercially-available components. Novel techniques using hybrid optical and electrical recirculation are also investigated.

Microwave fiberoptic transmission system provides numerous advantages for satellite communications terminals and antenna remoting. This paper described a 1.3 um, 6-15 GHz, 1 Km fiberoptic system design and performance. The microwave characteristics of the laser diode photodetector are discussed in detail. The preliminary measured system signal-to-noise ratio (SNR) are 110-120 dB/Hz in the frequency range of 6-15 GHz. A 20 to 30 dB SNR improvement is achievable when coherent detection and PD impedance matching are employed.

Novel applications of fiber optic delay lines to improve the capability and performance of radar repeater and phase noise test sets are described. Using a 2.25 km long fiber optic link, we demonstrated generation of an ideal target for a radar repeater test set and measurement of phase noise closer than 100 Hz to a 9.6 GHz radar carrier signal using a delay line discriminator phase noise test set. FM and AM signal-to-noise measurements were made to determine the performance capability of the 10 GHz modulated fiber optic links for the radar applications.