The ruby rangefinders currently being developed for the military present a serious eye hazard, even at ranges beyond several kilometers. In an effort to find suitable alternatives outside of the visible spectrum, a number of possible systems have been examined from the point of view of available laser sources, atmospheric transmission, and available detectors. Detailed comparisons are made of systems operating in the visible, ultraviolet, and infrared. Results indicate that a system employing a Q-spoiled Erbium glass laser operating at 1.54 microns and a germanium avalanche diode detector will provide performance more than an order of magnitude superior to a ruby rangefinder using an S-20 photomultiplier and an equivalent laser pump power. Furthermore, operation at this wavelength appears to offer more than adequate protection against the eye hazard problem.
Laser rangefinding requires three optical functions: transmitting (beam divergence reduction), sighting, and radiation receiving. The simplest concept leads to three separate optical subsystems -- one for each function. Such a system is, however, unreliable for field use. A Bausch & Lomb invention yields a two-axis system in which sighting and receiving are partially combined by a unique beam-splitting aperture mirror. A single objective lens is used eliminating one complex sub-assembly. The special mirror uses a glass substrate onto which an elliptical aperture is etched to delineate the receiver field. Alumi-num is deposited over the surface -- except for the inner portion of the aperture. This film serves to reflect light into the sighting telescope and acts as a receiver field stop. The aperture in the film provides an opening through which radiation is transmitted to the photodetector. The entire surface of the mir-ror is overcoated with a multilayer dichroic which transmits laser light but reflects visible. The system is fail-safe since the sight-ing reticle is the delineated area which also is the aperture through which radiation passes to the photodetector. If a range indication is received, the target must be located within this defined field. If malfunction through mis-alignment occurs, no range indication would be received. That, by our definition, produces fail-safe performance.
This paper presents the results of the field evaluation of a CW laser tracking and range measuring system developed for the Test Track facility at Holloman AFB, New Mexico (Contract No. AF29(600)- 4136). A track borne sled, as a coopera-tive target, is automatically tracked by the laser system providing high accuracy range data. For ranging, the system's basic principle of operation utilizes the laser source as a carrier frequency which is modulated at a frequency of 2.048 GHz. The doppler or phase shift measurements introduced on the modulation frequency are detected and recorded for range com-putation. A signal to noise ratio of 10 db was measured at a target range of 10,000 ft. at the Holloman AFB Test Track; providing a range resolution of 8 x 10-' feet for the system bandwidth of 20 KHz. The tracking or beam steering portion of the system utilizes the target image position technique to provide the angular position error for the servo loop. The system has demonstrated its ability to lock-on a sled at its starting range of three miles and automatically track this high speed sled to a passing range of 1000 feet during a routine operational test run.
The development of lasers, new electro-optic light modulation methods, and improved electronic techniques have made possible significant improvements in the range and accuracy of optical distance measurements, thus providing not only improved ranging methods but also useful techniques for the study of other geo-physical, meteorological, and astronomical problems. One of the main limitations, at present, to the accuracy of distance measurements is the uncertainty in the average propagation velocity of the radiation resulting from inhomogeneity of the atmosphere. Accuracies as high as a few parts in ten million now appear feasible, however, through the use of the dispersion method, in which simultaneous measurements of optical path length at two widely separated wavelengths are used to determine the average refractive index over the path and hence the true geometrical distance. The design of a new instrument based on this method, which utilizes wavelengths of 63281 and 3681X and 3-GHz polarization modulation of the light, is presented. Preliminary measurements over a 5.3-km path with this instrument haXe demonstrated a sensitivity of 3 x 107 in detecting changes in optical path length for either wavelength using 1-sec averageing, and a standard deviation of 3 x 10-7 in corrected length. The principal remaining sources of error are summarized.
Recently the Ballistic Research Laboratories conducted a study to determine the distribution of energy in the beam of the newly developed XM-23 Laser Rangefinder. These data were collected as part of a comprehensive evaluation program to determine the potential eye hazard to friendly troops employing the laser rangefinder in tactical situations. Measurements were made for each of three cavities used with this rangefinder and over four pathlengths between 200 and 1500 meters. Cross-section measurements of the beam were made by intersecting the beam with a specially constructed optical receiver having an input diameter of two feet. The two-dimensional spatial intensity distribution across this aperture was imaged to a film plane along with a calibrated step wedge. The measured film densities were converted through the characteristic curve to relative energies. Data from a one-dimensional scan of the beam were plotted and used to derive probability distribution curves showing the frequency of occurrence of the normalized energies. Standard deviations of the optical data were computed from the cumulative distribution curves of the energies. In addition to the optical data, direct measurements of the index structure constant, Cn, were made. This constant relates directly to the strength of the index fluctuations in the turbulent medium and hence to the optical variance. Both theoretical and experimental comparisons are made between the measured Cn's and the optical data.
During the last three years Sandia Laboratory has been developing a mobile laser (lidar) system for use in detecting artificial clouds composed of ablative materials from re-entry vehicles. This talk presents a discussion of the lidar system developed at Sandia Laboratory and the data obtained from evaluation studies in which the laser energy was reflected from stationary, slow velocity, and low-and high-density targets. These targets were atmospheric clouds, terrain, and retroreflectors. A preliminary comparison of the measure of long range atmospheric transmission to the clear Standard Atmospheric attenuation is also provided.
The Smithsonian Astrophysical Observatory (SAO) operates a worldwide network of observing stations. These stations obtain experimental data from which precise satellite orbits are computed on a CDC 6400 computer at the Observatory's headquarters in Cambridge, Massachusetts. In contrast to many other tracking systems, real-time data are not essential. It can take as long as a year to obtain the necessary time corrections, apply these, and compute the orbits. The important requirement for the useful scientific applications of the results is the accuracy of the computed orbits. And, of course, the accuracy of these orbits depends upon that of the data obtained from the tracking system.
The laser has many properties that make it an attractive light source for a variety of photographic applications. However, not all of the laser properties are required or even desirable. It is important to understand the basic properties of laser light and their effect in a variety of photographic situations. Specifically in this review paper we will be concerned with the problem of image formation - the advantages and disadvantages of laser illumination. Ideas for the control of unwanted properties are described.
A lunar laser system is being developed for Air Force Cambridge Research Laboratories. It has been designed to detect, photoelectrically and photographically, ruby laser pulses which are retro-reflected from optical cube corners loca-ted on the moon. From criteria established for range and photography experiments, designed laser instrumentation is combined with selected transmitting and receiving optics and retroreflectors to form the system. Return signals can be photographed at times of favorable combinations of light aberration and local seeing conditions. They can be detected by a receiving photomultiplier on a one return basis for range measurements under most aberration and seeing conditions. For photography the laser operates in its ordinary mode (40 Joules in 0.4 msec); for range the laser operates in a Q-switched mode (10 Joules in 10 nsec). Because the round trip travel time per pulse is about 2.5 sec, a mechanical switch is used so that one telescope transmits and receives the range signal. Except for the uncertainty in the speed of light, each range measurement will have a maximum error of about 1.5 meters. Such precise range measurements have numerous scientific uses. The number of observations acquired depends on the uncertain observational lifetime of retrore-flectors in the lunar environment. The lunar observatory is the only site in the U. S. now being developed primarily to satisfy ranging and photographic criteria. It is the result of a co-operative research effort between AFCRL and the NASA.
A newly developed electro-optical shaft angle encoder is described and the theory developed for a technique using laser interferometry to eliminate error sources and improve the resolution and accuracy of an encoder. The technique described in this paper utilizes a cw laser to illuminate the outer-most high-density tracks on a code disk made up of alternately opaque lines and transparent spaces. By focusing the laser beam to a small spot incident on the center of the track, a diffraction pattern is formed as the light passes through the code disc. By properly combining a given pair of order contributions by means of small plane mirrors, an interference pattern is formed at the input of a detector. As the disc is rotated, the radially formed line pattern on a given track will result in a continuous phase shift between diffracted orders which gives rise to a sine-wave variation in intensity of the interference pattern at the detector input. Counting this signal cyclically at the detector output will result in a direct count of the number of lines on the track that corresponds to a given angle of rotation. With this tech-nique, a 20-bit encoder has been developed on an Air Force Contract AF33(615)-3092, using a 2.5-inch-diameter code disc. The simplicity and basic stability of this approach are described along with the resolving capability of the encoder in relation to the available energy in the diffraction orders.
The laser gyro represents a new generation of attitude sensors with high resolution digital output. Three orthogonally-mounted laser gyros and a direction cosine computer provide a full freedom, high rate strapdown attitude reference system. A dither system is used to compensate for the natural lock-in of the ring laser, and solid quartz construction provides stability and ease of alignment. The unique characteristics of the laser gyro require test equipment and procedures which are different, though simpler, than those for conventional gyros.
A key element in every aircraft or missile testing program is photographic coverage of the test vehicle in motion. The traditional method of photographing such tests is to have an operator steer his camera manually while trying to keep the target in his field-of-view. Manual tracking, however, has certain inherent limitations, the chief one being the relatively slow response time of the operator. Other indeterminate factors such as his alertness and experience degrade the accuracy of the coverage still further. The solution, of course, is to replace the operator by some automatic target tracking scheme. The types of automatic trackers heretofore available, radar, infrared, or television, bring along a whole new set of problems such as ground clutter, target contrast and tracking resolution. All these difficulties can be overcome by a laser tracking system following a cooperative target. Since the laser tracker is im-pervious to anything except the reflection of its own laser beam, it follows the target automatically with excellent accuracy and resolution.