The Martin Marietta Orlando Aerospace capital facility used to conduct research and development in infrared radar technology is described. The methodology for choosing the multifunction sensor parameters in light of addressing both applications and technology are discussed. Special consideration is given to the important issue of choosing the radar modulation format for a particular mission application. Several unique signal processing approaches to obtaining range, pulse width and velocity imagery are also presented and a prediction of the LIRS performance is given. Finally, several measured parameters of the system under construction are presented.
FM - CW CO2 laser radar can provide high resolution information useful in a number of applications. Velocity and range data can be obtained from a properly configured system. These data are useful in such diverse applications as wind velocity measurements, navigation, battlefield obscurant mapping and range imaging. Range images obtained from laser radar have a number of tactical applications. Techniques for terminal homing and target recognition based on range information are outlined. The general features of an FM - CW range imaging sensor using a commercial waveguide CO2 laser are described. The basic principles of FM - CW radar are reviewed, and the results of laboratory measurements of the frequency stability and range resolution obtained with diffuse targets presented. Intensity and range images of laboratory objects obtained at short range with the system are also presented.
A coherent, imaging CO2 laser radar has been built and tested in the field. This laser radar uses a single-waveguide CO2 laser and heterodyne detection. Two acousto-optic frequency shifters generate the IF frequency. An acousto-optic standing-wave device provides the 15 MHz intensity modulation used for ranging. The sensor includes a two-axis, dual-aperture galvonometer scanner with selectable field of view and depression angles. The optical system fits on an easily transportable 3 ft by 4 ft optical bench. Both reflectance and range images are produced. The range imagery analysis shows a range resolution of approximately one foot. Statistical analysis of the reflectance data from a military truck, the dirt ground, and an asphalt road shows that they are Rayleigh distributed. The reflectivities of these objects are determined to be around two percent through comparison with laboratory reflectometer measurements.
The experimental Infrared Ranging and Tracking System (IRATE) described herein has been developed for evaluating the capabilities of TEA-0O2 laser radars with heterodyne detection for the ranging and tracking of air targets. This system has been used in field measurements for investigating the return-signal fluctuations resulting from atmospheric turbulence and target induced speckle. Results are presented for measurements on single glint and diffuse reference targets with the system operating in both the heterodyne and direct modes of detection. The feasibility of angle tracking has been demonstrated but additional effort is required for determining system capabilities. The long-range performance of a TEA-CO2 laser radar with heterodyne detection was demonstrated by detecting returns with a high signal-to-noise ratio from mountains at 24.4 km.
This paper describes the design, calibration and performance of a Cpl coherent laser radar testbed. For calibration purposes, a very precise movable target, resembling the travelling Michelson interferometer commonly adopted in laser wavelength measuring apparatus, has been designed. Different types of targets, e.g. mirrors and diffuse targets have been used to characterize the optical efficiency of the laser radar and the signal fluctuations induced by target surface or turbulence. A number of laser radar images are presented and the target detection problem is discussed in conjunction with these images. By an example using measured atmospheric attenuation statistics, we conclude that a CO2 laser radar can obtain acceptable performance out to 5 - 6 km range during moderate turbulence conditions.
The science justification and the feasibility of aircraft based CO2 Doppler Lidar measurements of transport between the free troposphere and the stratosphere or the planetary boundary layer are discussed for a wide range of seasonal and geographic conditions. Ground based coherent CO2 Lidar aerosol scattering experiments using a stable ring resonator ~ 50 mJ/pulse CO2 laser with external injection locking are reported. Comparative studies of injection locked CO2 laser unstable resonators and Master Oscillator Power Amplifiers are reported for future CO2 Lidar missions with respect to requirements of pulse energy, duration/shape, frequency chirp, efficiency for heterodyne detection, and combined Doppler Lidar and DIAL missions.
A coherent laser radar system using a grating tunable injection-locked TEA-CO2 transmitter is being used to measure the altitude dependence of atmospheric aerosol backscatter and attenuation at a variety of CO2 laser wavelengths in the 9-11 μm region. Injection control of the TEA-CO2 laser allows one to obtain single-longitudinal-mode (SLM) pulses which will follow the frequency of the injected radiation if the TEA laser cavity length is adjusted so that a cavity resonance is in proximity with the injected signal frequency, and if various additional conditions are satisfied. Requirements for generation of SLM pulses in this manner from a TEA CO2 laser with an unstable resonator cavity will be discussed. Procedures used for quantitative range-gated measurments of aerosol backscatter and attenuation will also be discussed.
We present profiles of atmospheric aerosol backscatter coefficient, β(π) at 10.6 μm versus altitude at four different geographical locations. The profiles show the general decrease in backscatter expected at higher altitudes.
During 1982 we used the NOAA pulsed Doppler lidar as part of experimental programs to measure precisely lidar system performance and extend the instrument's demonstrated atmospheric monitoring capabilities. Key system characteristics such as pulse shape, chirp, alignment stability, telescope efficiency, and pulse-to-pulse variability were studied and their effect on measurement accuracy quantified. The field experiments also demonstrated the system's capabilities and limitations in monitoring winds, backscatter, turbulence, and moisture. By scanning the wind field at low scan elevation angles, we have observed such small-scale meteorological events as thunderstorm gust fronts, downbursts, and cold front passage. Scans at higher elevation angles enable us to monitor upper level winds, such as those in the vicinity of the polar-front jet. We also monitor backscatter coefficient (β) over both the short and long term. Daily observations show a noticeable decrease in tropospheric β during the winter months. The stratospheric aerosol layer resulting from the El Chichon volcano eruption was easily observed during fall 1982.
The theory of differential absorption lidar (DIAL) systems relevant to range-resolved area monitoring of pollutants is reviewed. A numerical example based on a hydrazine and ethylene monitoring concept is presented and used to show the superiority of high-PRF, low pulse energy heterodyne detection DIAL systems over low-PRF, high pulse energy direct detection DIAL systems in the area monitoring application. Size, complexity, and measurement timeline trade-offs on choice of system architecture are discussed.
The requirements of range resolved DIAL in the infrared are briefly reviewed. It is argued that a system might advantageously be constructed with a single highly tunable laser source that can operate simultaneously in more than one mode provided the mode spacing available is of order the width of the molecular absorption line being used. An experimental facility is described that enables different excitation, gas flow and optical cavity configurations to be tested in the context of a multiatmospheric pressure CO2 laser.
This paper will review experimental work which has demonstrated that it is feasible to obtain continuous frequency tuning between line centers with a pulsed rf-excited high pressure CO2 waveguide laser. Using 7 kW input power at 40 MHz, a 300-GHz-wide tuning range from the R(12) to the R(26) line in the 10.4-μm band has been obtained at 10-atm gas pressure. The laser has been operated at up to 1-kHz pulse repetition frequency with 300-ns-long output pulses showing good amplitude stability.
Several catalyst materials have been tested for efficiency of converting CO and 02 to CO2 for use in a high energy CO2 laser. The composition of the gas mixtures was monitored by mass spectrometry and gas chromatography. A copper/copper oxide catalyst and a platinum/ tin oxide catalyst were used for closed cycle operation of a CO2 laser (0.7 joules/pulse), operating at 10 pulses per second.
Infrared tunable diode lasers were developed in the 1960s and have been a valuable radiation source for high resolution laboratory and in situ spectroscopy. Use of Pb-salt Tunable Diode Lasers (TDL) in heterodyne applications impose stringent requirements on the TDL not normally required for laboratory spectroscopy. A review will be made of progress associated with TDLs in such heterodyne applications. Areas addressed will include such items as lifetime, operating temperature, and factors affecting excess noise. The review will emphasize the experience at Langley Research Center, but will include material from other users. The Langley information will include a description and current status of the Laser Heterodyne Spectrometer experiment and atmospheric solar absorption data obtained from a groundbased heterodyne system.
The features of the RSRE chirp-modulated CW CO2 laser rangefinder/velocimeter based on a purpose-built acousto-optic modulator are briefly restated. The significance of acoustic reflections inside the acousto-optic modulator is described, particularly their increased importance when the transition to a single transmit/receive aperture is attempted; the need then arises to provide effective 60 MHz acoustic damping in the modulator block. Experiments with tungsten-loaded epoxy, with chill-cast Wood's metal layers and with electroplated indium layers are described. Acoustic damping effects are assessed quantitatively on acoustical and optical test benches. It is found that indium layers provide practical effective damping and open the way for single-aperture heterodyne CO2 systems of the modulated-CW type. Optical measurements on modulator blocks shaped to reduce acoustic reflections show that indium damping reduces the first unwanted modulated pulse emerging from the modulator due to acoustic reflections down to -55 db in power relative to the initial wanted pulse, compared with -38 db for uncoated block; unwanted pulses emerging later are reduced even more powerfully and long-delayed pulses become unobservably small.
The design of bulk acoustic wave Bragg cells for radar warning and other signal processing applications is discussed, comparing lithium niobate, paratellurite and gallium phosphide. Acousto-optic interaction efficiency, acoustic loss, transducer design and acoustic matching are considered together with a selection of requirements for device bandwidth, resolution and efficiency. The bandwidth and efficiency figures for anisotropic and isotropic interactions are compared and the available dynamic range assessed. The mechanical and electrical design of the Bragg cell housing and feeds will be presented together with the methods used to fabricate the cells and monitor the production process. Testing of the device performance will be described and results for a number of cells presented showing reliable achievement of dynamic ranges exceeding 50 dB from cells with better than 1% per watt diffraction efficiency and 1 GHz bandwidth.
Coherent laser radars used for 3-D imaging and aerosol/pollutant sensing collect target returns which comprise a range-spread speckle process. This paper addresses a class of waveform design/waveform evaluation problems for such radars. The maximum-likelihood (ML) processors and Cramer-Rao (CR) performance bounds are developed for short pulse, chirped pulse, sinusoidal amplitude-modulated pulse, and sinusoidal frequency-modulated pulse wave-forms. All four of these waveforms are shown to have nearly identical ultimate range accuracies. However, range accuracy is not the only performance criterion of interest; range resolution, ambiguity, and anomaly must also be considered. Analysis and discussion of some of these aspects of waveform selection are included in the paper.
This paper describes a modified distance transform (MDT), which combines the original distance transform (DT) with a new set of selection rules to be defined, and an appropriate linking algorithm to produce a connected skeleton in computing times that are significantly shorter than the medial axis transform (MAT) implementation for large image arrays. The new set of selection rules is applied over an extended neighborhood and makes the skeleton generated more connected, especially for branched images. These selection rules mostly remedy the limitations of the original DT technique but not entirely, and so a linking algorithm is needed. The linking algorithm described operates on a different size of window larger than the DT window for each skeleton element, but the linking process is operated on a smaller subset of the original image area; i.e., the existing skeleton elements indicated by the new selection rules. In order to minimize the processing time for the linking process, a linearity test is introduced that further reduces the application of the linking process to a subset of linear skeleton elements.
A model has been developed for calculating the CO2 ladar heterodyne mixing current for an arbitrary modulation format. Six specific techniques are considered including amplitude (AM/CW), suppressed-carrier amplitude (AM/SC), phase (PM/CW), linear frequency (FM/CW), hybrid linear frequency/suppressed-carrier amplitude (FM/SC), and short pulse (PULSE) modulation. Detailed models of the different modulators and demodulator concepts are included in the analysis. Laser efficiency, signal bandwidth, detection rate, and sensitivity to speckle effects (especially phase jitter) are discussed as they affect range measurement precision and accuracy. Theoretical and preliminary experimental results indicate that a PM/CW ladar has a 3-6 dB advantage in signal-to-noise ratio over the AM-based formats. The results also indicate improved performance for a PULSE ladar compared with CW ladars if sufficient transmitter efficiencies can be achieved, and that the FM/CW and FM/SC formats are relatively unsuited for high-precision or high-accuracy range imaging.
Previous studies have established a mathematical system model for a compact coherent laser radar which incorporates the statistical effects of target speckle and glint, local oscillator shot noise, and propagation through atmospheric turbulence. This paper reports results from a measurement program at the MIT Lincoln Laboratory aimed at verifying the foregoing model. Simultaneous laser radar returns and scintillation sensor measurements were collected over a one kilometer path in various turbulence conditions, with the radar observing either a glint object (retro-reflector) or a speckle object (flame-sprayed aluminum calibration plate). Three modes of laser radar operation were employed: full field-of-view scanning, reduced field-of-view scanning, and staring. The principal conclusions drawn from analyzing these data are as follows. First, beam jitter must be included in the system model. Second, the jitter-corrected retro-reflector returns do show turbulence induced lognormal scintillation. Third, turbulence-induced beam jitter is the cause for staring-mode speckle target decorrelation.
The image tracking performance of a coherent infrared radar is presented. Heuristic derivations are presented for the tracking accuracy versus image signal-to-noise ratio of centroid, balanced gate, correlation, and template correlation trackers. Expressions for the image signal-to-noise ratio versus carrier-to-noise ratio are obtained for intensity images, thresholded intensity images, range images, and velocity images. Centroid trackers using any type of imagery are shown to yield tracking accuracies which degrade as the target gets larger and are therefore inferior to the other correlation-type trackers. Balanced gate, correlation, and template correlation trackers can all provide tracking accuracies much finer than a pixel. Thresholded intensity, range, and velocity images provide better tracking performance than simple intensity images.