A review is made of the present state of laser communications. The basic characteristics of lasercom are discussed, and the advantages and shortcomings are delineated. Potential application scenarios are given. A functional block diagram showing the major blocks is discussed, and each is described in terms of present performance parameters. Performance numbers are given in terms of range and bit error rate achieved. Discussion of the various available laser transmitters is given including the advantage and disadvantage of each. CO2 being a gas discharge device has potential lifetime problems and requires heterodyne detection - an added complexity. Nd:YAG requires either discharge lamp pumping or diode pumping. Diode pumping involves matching the diode emission line to a fairly narrow Nd:YAG pump band. There is an unresolved question of diode emission line shifting with age. A summary of the various shortcomings in the technology include, for example, the required heavy optical bench, lack of demonstration of 10-year required life, solid state detector response time, and photomultiplier tube lifetime.
Optical communications can be an attractive alternative to microwave technology in commercial and military space communications, particularly when the data rate required is high. System analysis at the optical frequencies often differs significantly from that at lower frequencies due to the vastly different technology of sources, modulators and receivers and also due to the important role that nonclassical (quantum) noise plays in determining system performance. In this paper important optical communication system architectures and critical system and technology issues that affect system designs will be examined. Coherent (heterodyne or homodyne) systems will be compared to incoherent (direct detection) systems in the context of space communications. Examples will be drawn from current state-of-the-art technologies and projected future technologies.
A study of the possible utilization of optical communications for a deep space link via an Earth-orbiting relay satellite is presented. The optical link is used primarily for high rate data transmission from a deep space vehicle to the relay, while RF links are envisioned for the relay to Earth link. This type of hybrid system combines the advantages of optical frequencies for the free space channel to the relay with the advantages of RF links for atmospheric transmission. Auxiliary optical beacon and low rate RF links may be included for aiding pointing, tracking and timing operations.* A preliminary link analysis is presented for initial sizing of optical components and power levels, in terms of achievable data rates at various distances. The potential advantage of an optical link over present RF deep space link capabilities is shown. The problems of pointing with narrow optical beams are discussed, and the performance degradation due to pointing errors is evaluated. Since the modulation formats of the optical link to the relay and the RF link from the relay may be significantly different, an interface problem may arise in interconnecting the two links at the relay. This requires reformatting the detected optical signals to conform to the retransmitted RF link. The interface problem is discussed and alternative implementations are examined.
Techniques for combining individual semiconductor lasers for use as a laser communication transmitter are discussed. The geometries, aging characteristics, radiation effects, and lifetimes of semiconductor lasers are reviewed, and the resulting effects on the laser communications transmitter are assessed.
Propagation through turbulence can impose severe limitations on the performance of atmospheric optical communication links. Previous studies have established quantitative results for turbulence-induced beam spread, angular spread, and scintillation. This paper develops communication-theory results for single-bit and message transmission through turbulence. Programmable calculator algorithms for evaluating these results are given, and used to examine system performance in some realistic scenarios. These algorithms make it possible for the uninitiated communication engineer to rapidly assess the effects of turbulence on an atmospheric optical communication link.
This paper examines the use of a low power GaAs laser as the beacon source for establishing a high data rate laser communication link. A link signal-to-noise parametric analysis is presented, as well as identification and analysis of noise sources. The GaAs signal characteristics (power and lifetime) and system optical considerations are also discussed.
Results of a parametric study to assess the effects of system tradeoffs and design considerations on the operational capabilities of optical communication links using laser sources and semiconductor detectors are presented. System performance was analyzed using an interactive computer program to determine parameter values required for a given bit error rate. This computer code is applicable to a wide variety of links, laser wavelengths, and background scenarios. Examples of the application of this analysis to several representative space-based laser communication systems are presented, and conclusions are reached regarding whether these systems can achieve design goals set by mission objectives.
The Faint Object Camera is one of five scientific instruments being developed for the Space Telescope to be launched in 1984-1985. This paper outlines how the scientific goals of the Space Telescope of spatial resolution and detection capability are achieved through the performance of the Faint Object Camera. The optical configuration of the Faint Object Camera is presented and it is shown how a range of focal ratios of f/48, f/96 and f/288 are achieved within the standard scientific instrument envelope of the Space Telescope. It is demonstrated how the high spatial resolution of the Space Telescope leads to a high limiting detection efficiency, better than any ground based telescope. The design aspects which allow the high spatial resolution to be maintained over exposures of up to 10 hours are outlined.
The potential use of the extra-vehicular (EV) suited crewperson's role in supporting possible on-orbit maintenance (service) or assembly/construction of large space structures (optical, communications, solar, etc.) has only minimally been considered. The intent of this Paper will be to focus on alignment considerations as they relate to ranges of EVA tasks from component/module changeout through assembly and/or construction of modular built-up spacecraft to large space structures. Included will be sortie (in Orbiter cargo bay) palletized and demonstration elements, and free flyers with particular attention to larger structures. Emphasis will be given to requirements vs design techniques particularly as they relate to alignment of changed out items or alignment of the structure during the assembly phase. EVA crew functions and capabilities will be discussed focusing on alignment attainment. Finally,factors associated with constraints and limitations in designing spacecraft and large space structures as they relate to use of the EV crewperson in alignment/servicing tasks aro discussed.
The authors have previously reported on the absolute distance measuring systems being developed at Lockheed Missiles and Space Company for use in space on large erectable satellites. One, a two-color CO2 laser system, has a precision of 0.05 pm for aligning optical elements. The second, a Helve phase modulated system, has a precision of 25 pm for the measurement and control of large antennae. The status of these systems is updated with emphasis on extended range distance measurements and work aimed at simplifying the systems. The application of real time distance measurement between satellites is discussed. A concept is presented which will measure absolute distance to 25 pm track angular position to 10 Arad and provide 20 MHz bandwidth communication between satellites several kilometers apart.
The use of laser systems for space-based communication applications has drawn considerable attention recently. It is anticipated that laser communication satellites will become a reality in the mid to late 1980s. The fine pointing and jitter control requirements of those systems presents some unprecedented challenges to control engineers in the areas of acquisition, tracking, and fine pointing. This paper addresses the various acquisition methods and critical technological issues of tracking and pointing control for space-based laser communication system applications under their dynamic operating environment and stringent mission requirements. The most stressing design and development parameters and components for various applications are discussed. The structure of a digital computer simulation program which can be utilized for performance verification, design feasibility, and sensitivity study, is also given.
A precision optical encoder is under development to achieve an accuracy of better than 0.1 arc second over 360 degrees of rotation, or 1 part in 107. The optical encoder basically consists of a drum grating, a Displacement Sensor (also known as Vibration Sensorl 1,2), and an Angular Alignment Sensor'. The rotation of the drum is accurately measured by sen-sing the "sliding" motion of the drum grating with respect to the Displacement Sensor3. The focused displacement sensing beam (HeNe Laser at 633 nm) from the Displacement Sensor has an incidence angle 0 to the grating and coincides with a diffracted beam from the grating. The grating is ruled in the direction of the drum axis and completely around the cylindrical surface of the drum. The Displacement Sensor output measures the "sliding" distance of the rotating drum in the direction of the laser beam and can be expressed as the time integra-tion of γ(R(sine), where 'j( is the time derivative of the rotation angle and R is the radius of the drum. This integration becomes 2ηR(sineθ) for 360 degrees rotation, and this rotation angle can be determined accurately to better than 0.001 arc second with the assistance of the Angular Alignment Sensor. Taking the ratio of the time integration of '7R(sine) and the previously measured 27R(sineθ), the instantaneous drum rotation angle y can be determined. The accuracy of this optical encoder (Δγ) is estimated to be 0.1 arc second (Ay=AD/R(sine) = 4.86 x 10-7 radians) for a Displacement Sensor accuracy of 0.02 μm(AD), a drum radius of 6.5 cm (R) and an incidence angle of 39.3 degrees (θ). Laboratory subsystem tests have verified potential system performance. Many precision optical encoder applications exist which include precision angle measurement and calibration in numerous ground and space areas.
A Lasercom Space Measurement Unit (LSMU) is in hardware integration being prepared for the P80-1 satellite. First launch capability is late 1983. A digital processor provides internal control of the system, line-of-sight pointing from the LSMU to the ground terminal and performs data formatting for telemetry. The control software is organized to provide much flexibility in controlling the lasercom experiment. System control functions and their digital implementation is discussed.
Recent advances in Charge-Coupled Device (CCD) imaging detectors and high speed digital signal processors have prompted the investigation of a new class of Ring Laser Gyroscopes (RLG) at The Aerospace Corporation. Because of the demands, such as survivability, reliability, long life, high accuracy and resolution, and wide dynamic range, that will be made by future space systems on inertial reference technology, the RLG will be increasing in importance in future missions. However, thus far, little consideration has been given to sophisticated techniques for the readout and analysis of the RLG's output interference fringe pattern. This paper presents a system concept of a new technique for sensing and analyzing the fringe pattern produced by a RLG. The system hardware utilizes CCD imaging detectors for multiple channel sensing and digital memory for recording the fringe patterns. The system software employs digital filtering algorithm for converting the fringe movement to incremental angles. This signal processing technique greatly enhances the resolution of RLG's output.
The Perkin-Elmer Corporation has designed and built a cryogenically cooled Fourier transform spectrometer for spaceborne applications. In operation, the spectrometer requires mirrors moving at constant velocity in both forward and reverse directions. To maintain efficiency and accuracy, the time taken to reverse direction and the vibration induced due to this reversal must be kept within low limits. This paper deals with the control system design for maintaining a constant velocity during forward and reverse scans and for smooth direction reversals. The systems aspects of the problem are described, and time-domain techniques of modern control theory are applied for optimization of turn-around profile. The analysis leads to a suboptimal design easily implemented by using analog-type components. Test results of satisfactory performance are also included.
The Space Telescope (ST) Pointing Control System (PCS) slews the optical axis from one target star region of the celestial sphere to the next, and maintains precision pointing at the target star. The spacecraft digital computer processes the attitude and rate sensor data to generate torque commands for the reaction wheels. The PCS has four major elements: the command generator, the control system, the attitude reference processing, and momentum management. The ST Pointing Control System (PCS) is designed to meet the fine pointing performance of 0.007 arc-sec stability, maneuver the telescope, and provide the capability for deployment from, and retrieval by, the Space Shuttle. The PCS objectives are met using fine guidance sensors (FGS) for attitude and derived rate information to achieve fine pointing, and rate gyro assemblies (RGA) for rate and attitude information during maneuvers. The emphasis in this paper is on acquisition methodology for pointing control, flexibility effects of the large optics, and simulators for development testing.
Today's large optical experiments, both free-flying and shuttle-borne, are finding an increasing need for a stable platform in a vibration environment. As the resolution of optical systems has improved, conventional techniques for isolation have become less attractive. Magnetic suspension technology is ideally suited to address this problem. In this paper, the advantages of magnetic suspension over conventional techniques will be discussed both with regard to isolation and fine pointing. The generic form of the vibration transfer function across the magnetic gap will be derived, and evaluated for a typical case.
In December 1980, the Air Force Space Division Lasercom office completed field evaluation of 1 Gbps space laser communications system in an air-to-ground flight test program at White Sands Missile Range, New Mexico. A ground-based brassboard receiver terminal performed the functions of acquisition and tracking, beacon beam pointing, optical data reception at 1 Gbps, and optical data transmission at 20 kbps. The ground terminal is described from an acquisition and tracking viewpoint. The design of fast-steering pointing and tracking inner loops coupled with slower gimbal outer loops is discussed. Results of the recent field evaluation in terms of acquisition times and tracking accuracy are presented.
A prototype 10-channel Vibration Sensor has been developed which features (1) a single HeNe laser and a single photodetector for simultaneous, noncontact, and remote sensing of ten independent vibratory targets, (2) frequency response from dc to beyond 60 Hz, (3) 0.08 pm amplitude resolution and refinable to better than 0.02 pm, (4) signal delay of the sensor output less than 2 psec to the actual vibration, (5) digital signal processing and digital output for computational convenience, and (6) expandibility for more channels. Although the maximum sensible amplitude and frequency product is presently limited to 0.1 Hz (or a maximum range rate of 63 cm/sec) for a 4 MHz electronic bandwidth, this sensor is still nearly perfect for measuring the dynamics of structure and sensing vibrations for active control of structures. The sensor employs a 0.7 mW HeNe laser, an avalanche photodiode, and two Bragg cells, one to provide the target sensing beam with an offset frequency relative to the optical local oscillator (which is split off from the laser out-put) and the other for dividing the target sensing beam into multiple beams, electronically. It is possible to simultaneously sample and hence monitor a large number of targets to which corner cubes have been affixed. Optical path variations due to the vibratory motion of each target is measured by comparing zero-crossings of the heterodyne signal from each target with those of each reference signal, identifiable by a specific Bragg frequency. Each zero-crossing count difference corresponds to an optical path variation of one-half of a laser wavelength or 0.32 pm. The resolution of this sensor has been refined by measuring the timing of zero-crossings of the heterodyne signal from each target and a 0.08 pm amplitude resolution has been demonstrated with an optical head 0.03 cubic meters in volume.
An iterative optical processor (10p) that can solve iteratively systems of simultaneous linear algebraic equations is described. Modifications to the system enable it to solve the algebraic Riccati equation and the linear-quadratic-regulator (LQR) problem of optimal control. We describe the resulting electro-optical processor and illustrate how we implement the Richardson and modified Kleinman algorithms to solve the LQR problem.