Electro-optical and radar technologies are being merged in the form of coherent infrared radars. In this paper we discuss the potential military applications of these devices. We begin by reviewing the factors favoring coherent infrared radars over alternatives such as conventional radars or electro-optical sensors. These include increased resolution, adequate weather penetration, and signature stability. Next we outline the military applications of existing sensors and indicate those applications where coherent infrared radars offer the potential of improved performance. Finally, the state-of-the-art in coherent infrared radar technology is indicated by a brief summary of several current development programs.

Efforts in laser radar transmitters, receivers, and componentry appropriate for coherent infrared radar are overviewed. Included will be test evaluations and accomplishments on chemical lasers, chemical transfer lasers, electric transfer lasers, and pulsed electric discharge lasers. Developments in wideband detectors/heterodyne receivers are addressed as well as results in AM/FM modulators and beam deflectors. This paper will summarize the technology results accomplished under sponsorship of the U.S. Army Ballistic Missile Defense Advanced Technology Center (BMDATC) by both private and governmental concerns. Waveform generation and high performance stability achievements are addressed including techniques implemented. Typical achievements include test results on lasers with regard to temporal and spatial stability, gain measurements, waveform generation techniques, actively stabilized CO2 local oscillators, adaptive receiver techniques to correct for laser stability errors, 2.5 GHz bandwidth detector arrays, improved acousto-optic Bragg deflectors, and high PRF modulators. Technology problems facing coherent ladar implementation are considered.

Developments during the past decade have demonstrated coherent CO₂ laser radar technology for a variety of military applications. The feasibility of incorporating heterodying CO, laser radars in airborne scenarios has been demonstrated for wire detection, terrain sensing, and high resolution 3 axis Doppler velocity sensing for navigation and hover. Additional multifunction capabilities for target discrimination and weapon guidance have been investigated and are undergoing further developments. This paper surveys the military applications, hardware configurations, results of flyable prototypes, and the technology status of multifunction CO₂ laser radars.

Since 1977, NOAA has been investigating the feasibility of a global wind-sensing lidar in response to the need for altitude profiles of the wind vector to serve as input to global circulation models. The lidar technique obtains the wind information by assessing the Doppler information contained in CO₂ laser radiation backscattered from the atmospheric aerosol (particulates, droplets, etc.). The results of studies undertaken to define the proposed system concept and hardware for a space-shuttle-borne demonstration flight are presented. Resulting from considerations of wind vector accuracy, area coverage and sampling density, signal detectability and space shuttle compatibility requirements were a continuous conical scan of the telescope with a half cone angle of 62° centered on nadir, telescope diameter (1.25 m), laser pulse energy (10 J), pulse repetition frequency (8 Hz), pulse duration (6 ins) and laser wavelength (9.11 pm). Requirements that the concept imposes on platform stability, navigation and attitude determination, line-of-sight pointing and lag angle compensation, on-board Doppler processing, prime power and thermal dissipation are discussed. The system configuration intended for a hardware design study is presented.

The development of CO₂ Lidar is important for a wide range of remote sensing applications involving trace gas concentrations (using Differential Absorption Lidar) and the effects of atmospheric dynamics on trace gas transport, meteorology, and aviation safety (using Doppler Lidar). The purpose of this paper is to discuss the CO₂ Lidar systems technology and signal processing requirements based on measurement needs and measurement sensitivity studies for ground, aircraft and space platforms.

Since 1967 Raytheon Company has been pursuing the design and development of CO₂ laser radar systems. Coherent detection has been used in both continuous wave and pulsed systems which heterodyne infrared radiation backscattered from atmospheric aerosols. The instruments and techniques reported here have been used for velocity measurements of ground wind, aircraft trailing vortices, wind shear, clear air turbulence, and two-dimensional wind fields.

We describe detailed performance characteristics of NOAA's pulsed, coherent, hybrid TEA, CO₂ Doppler lidar. Included are acoustically generated inter-pulse frequency jitter of the hybrid cavity, on-line center and off-line center plasma-generated intra-pulse chirp, and transmitter-receiver feed-through isolation. Data processing and display are discussed. Data taken in the clear atmosphere into the stratosphere, including VAD scans For mesoscale convergence, are presented, as well as calibration procedures.

This paper presents a discussion of the background requirements for, and the design and calibration of a coherent Doppler lidar which is oriented specifically toward the measurement of low values of atmospheric backscatter, β (π), at a wavelength of 10.6μm. The lidar to be described is a compact, continuous wave system which can operate in two modes--multiple particle scattering and single particle scattering. In the multiple scattering mode, the lidar employs an extended focal volume and utilizes a technique similar to Dicke switching to achieve measurement of the volume backscatter. In the single scattering mode, the focal volume is reduced to ensure a high probability of single particle scattering. Measure-ment of the single particle backscatter as a function of time leads to the formation of signal histogram from which the volume backscatter can be inferred. In addition to providing the atmospheric backscatter value, appropriate data processing algorithms and focal volume calibration allow the single particle mode to yield information on the atmospheric aerosol scattering cross-section distribution. The system hardware and signal processing are described in this paper along with the algorithms used to calculate the backscatter, (7). Calibration techniques described include the use of known targets such as spinning disks and wires.

The heterodyne detection efficiency of a large-aperture, coherent laser radar, defined as the actual signal-to-noise ratio (SNR) divided by the ideal SNR, is derived for detected phase-front distortions resulting from atmospheric turbulence or diffuse target scattering. Specific expressions are developed for a general target reflectivity distribution and a receiver having a finite-detection aperture, local-oscillator aperture, and local-oscillator beam. Numerical calculations of the heterodyne efficiency (using the derived expressions) show the reduction in heterodyne detection-efficiency resulting from atmospheric turbulence, near-field effects, and target speckle.

Coherent laser radars offer new technical options for a variety of target detection and imaging scenarios. Such systems will, of necessity, be subject to the vagaries of atmospheric optical propagation, viz., turbulence, absorption, and scattering. This paper presents a mathematical system model for a compact heterodyne-reception laser radar which incorporates the statistical effects of target speckle and glint, local-oscillator shot noise, and propagation through either turbulent or turbid atmospheric conditions. Using this model, results are developed for the imace signal-to-noise ratio and target resolution capability of the radar. Clear-weather propagation through the turbulent atmosphere is shown to affect the compact laser radar primarily through scintillation. Low-visibility weather propagation is shown to degrade the resolution of the radar. Sample performance calculations for a realistic infrared radar are Included.

This paper will describe measurements of target signatures performed with a CO₂-laboratory laser radar of a homodyne doppler type. In the exneriments performed so far, we have studied receiver sensitivity, dependence on aperture and target signatures. Some studies of imaging and ranging out to 2 km have also been done. The measurements of target signatures refer to signal amplitude spectra, noise and rms signals. Based on these parameters the detection probabilities at various false alarm rates were calculated. Several types of targets were used. Sandblasted Al-plates were typical speckle targets and a corner cube was used to simulate glint targets. Military vehicles were used as well as objects of interest for obstacle avoiding like wires and frame work. Comnarison with theoretical results for speckle and glint targets was made.

Based on the extended Huygens-Fresnel principle, universal curves representing signals reflected from a diffuse target, which may contain multiple glints, are calculated for uniform transmitted and local oscillator beams. For a far-field diffuse target, substantial gain on the signal-to-noise ratio may be obtained by increasing the receiver aperture up to three times the transmitter aperture. The receiver heterodyne efficiency for the two beams with slight misalignment is also presented in this paper.

The image signal-to-noise ratio and statistical variance of image centroid estimates are derived for coherent heterodyne and homodyne laser radar images of extended diffuse targets. Both intensity and binary target images are studied. Binary image is shown generally to offer a more reliable estimate of the target centroid.

The relative merits of coherent and noncoherent laser radars are reviewed. An information theoretic approach is used to investigate the tradeoffs between degrees of freedom (resolution) and signal-to-noise ratio in maximizing information in radar images. Similar tradeoffs between carrier-to-noise ratio and degrees of freedom are obtained for maximizing the signal-to-noise ratio in detection and estimation problems. It is shown that under some conditions noncoherent laser radars provide better performance than coherent radars, while under other conditions, the reverse is true.

A technique is described which will provide both magnitude and direction of the transverse component of velocity of a remote target. A laser beam is transmitted toward a remote diffuse target and the backscattered light is collected and mixed with a suitable local oscillator reference field. The method is based on the general spatio-temporal correlation function of the signal currents from two heterodyne detectors. The time derivative of this function, evaluated at zero time delay, is directly proportional to the component of target velocity parallel to the separation of the detector elements. A two-element by two-element detector array can therefore be used to measure the two orthogonal transverse velocity components. Each component is found from the correlation properties of the signals from diagonally opposed pairs of detector elements. Since the radial velocity component can be found from conventional laser Doppler techniques, all three components of the velocity vector can be measured simultaneously. Practical considerations and experimental results will be discussed.

Several single mode TEA CO2 lasers have been constructed and evaluated with the aim of providing frequency stabilized sources for various coherent infrared radar systems. Methods including injection locking, intra-cavity selective amplification, unstable resonators, short cavities and pulse amplification have been combined with various stabilization techniques to produce frequency stabilized output pulse energies from millijoules to several joules with pulse lengths ranging from fifty nanoseconds to several microseconds. Pulse to pulse frequency instabilities of 50 kHz have been obtained over long time periods for a hybrid system and pulse repetition rates approaching 1000 per second are anticipated for the oscillator amplifier system. Intra pulse frequency characteristics have been measured (chirp) and a theoretical model has been developed to explain the observed behaviour.

An electronically, scanned laser radar has been designed and is currently being developed which has unprecedented agility and speed. The agile scanning of both the transmitter and receiver is controlled by the variable reflectance of a V0₂ film on a cathode ray tube (CRT) faceplate. Electron beam heating is used to create reflecting spots on the VO₂ film which, in turn, control the transmitter laser beam direction. Components for the laser radar are currently being developed and tested. Experimental results are in complete agreement with predicted analytical results.

Low power CO₂ laser radars having nominal output powers of 10 watts are currently being developed for a broad spectrum of operational and tactical applications including terrain following, terrain and obstacle avoidance, Doppler navigation, target identification and weapon delivery. No single modulation waveform can perform all of the required sensor functions, but recent advances in modulated waveguide laser technology have made it possible to obtain from a single programmable transmitter such diverse modulation formats as cw output, Q-switched, cavity dumped and RF amplitude modulated waveforms. The intracavity electrooptic modulation techniques used to generate these waveforms in a coherent, frequency stable transmitter will be discussed and both theoretical and experimental modulation formats will be presented. The utility of the various waveforms for particular sensor applications will be considered, along with the impact of laser dynamics on the efficiency of the modulated laser.

Recently developed wide-bandwidth HgCdTe photodiodes and photoconductors have shown near-ideal heterodyne performance and have led to greatly increased CO2 laser heterodyne system capabilities. For example, NEPs as low as 4.3 x 10^-20 W/Hz at 1 GHz have been achieved with 77K photodiodes and photodiode arrays, and an NEP of 1.8 x 10^-19 W/Hz at 40 MHz has been realized with a p-type HgCdTe photoconductor operating at 195K. In this paper, design considerations (including limitations due to various HgCdTe material parameters), performance predictions and the current technological status of wide-bandwidth HgCdTe photodiodes are presented. In addition, the use of p-type HgCdTe photoconductors as elevated-temperature photomixers are discussed and some recent results are presented.

Laser systems using coherent optical heterodyne detection of signals have recently become interesting for a variety of applications including imaging, communications, spectroradiometry, and optical/infrared radar. In a number of these applications it is advantageous to use arrays of detectors to increase angular, snatial, or spectral coverage. Arrays permit this coverage to be increased Tqhile maintaining or reducing requirements on scanner systems, laser pulse rates, and individual detector bandwidths. This paper describes a neviirliolor-,raphic technique for performing the beam shaping in amplitude and phase which is required for high signal-to-noise and high resolution array performance. In this paper we review the holographic beam shaping concept as annlied to heterodyne arrays and its relation to laser radar systems. Experimental results are presented for holographic beam shaping devices using both linear and binary recording techniques.

A digital correlator/spectrum analyzer has been developed for the analysis of signals of 0-5 MHz bandwidth. The simple architecture includes two analog input channels (allowing crosscorrelation and cross-spectral analysis), 8-bit quantization, correlation calculation with up to 128 lags, and DFT output circuitry. Correlation functions and spectra are produced at intervals of 1 ms or longer, in both analog and digital form. Pipeline architecture allows unity duty cycle operation, with no loss of data between spectra. Extensive use of parallelism and LSI arithmetic units makes possible computing power of over 600 million operations per second in an 8" high package which requires about 200 watts.

This paper analyzes the detection process for targets in laser speckle noise using results from classical detection and estimation theory. The analysis is applied to the real problem of an airborne laser radar system looking down at targets on the ground. An optimum strategy for adaptive signal processing is described and graphical results are presented to facilitate laser radar system design.

The detailed operational performance of coherent optical radars can often be dominated by speckle related phenomena. This paper reviews the basic aspects of speckle and describes a coherent laser radar that automatically maintains a speckle maximum in the receive aperture of a common transmit/receive telescope.

Radar cross sections (RCS) of power line cables were calculated and measured at four mm-wave frequencies (18, 34, 56, and 94 GHz) and two laser wavelengths (10.6 and 1.06 pm) as a function of aspect angle. Three polarizations were considered: horizontal (HH), vertical (VV), and cross polarization (HV). Dry and wet cable conditions were simulated, and differing roughnesses were considered. The calculated and measured results are presented in this paper.