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This is the first of four articles describing a comprehensive series of radar maps of the entire visible lunar hemisphere carried out at wavelengths of 3.8 and 70 cm and analyzing the echoes in both orthogonal senses of circular polarization. In this paper, the basic techniques of delay-Doppler mapping by radar are developed, and the particular steps employed in mapping the moon are outlined. Succeeding articles present the results obtained and discuss the way in which these results relate to other, nonradar measurements as well as to the actual lunar surface properties.

The entire earth-facing lunar surface has been mapped at a resolution of 2 km using the 3.8-cm radar of Haystack Observatory. The observations yield the distribution of relative radar backscattering efficiency with an accuracy of about 10% for both the polarized (primarily quasispecular or coherent) and depolarized (diffuse or incoherent) scattered components. The results show a variety of discrete radar features, many of which are correlated with craters or other features of optical photographs. Particular interest, however, attaches to those features with substantially different radio and optical contrasts. An anomaly near 63° is noted in the mean angular scattering law obtained from a summary of the radar data.

An improved technique for obtaining stereo radar image pairs is described. The technique uses a unique combination of two different radar beam pattern characteristics. The resulting stereo radar images are illuminated from very nearly the same aircraft position which results in radar shadow and backscatter characteristics which are very similar and in improved image registration possibilities. The reduction in illumination angle and aircraft position differences is shown to be nearly an order of magnitude or more when compared with a previously proposed single flight technique which used two vertical fan beam patterns at different azimuth angles. As a related sidelight it is shown that this previous technique requlres the two fan beam patterns to generate parallax on the images and thus can not be implemented with synthetic arrays squinted at two different squint angles.

High angular resolution is a desirable property of mapping and reconnaissance radar systems. The synthetic-aperture principle reduced the beamwidth of airborne radar by as much as two orders of magnitude. Notwithstanding the long interest in the subject, a quantitative relationship is lacking between beamwidth and resolution. This paper is an attempt to overcome this deficiency. Among its several results, it is shown that the angular resolution of synthetic-aperture radar can be considerably poorer than expected. It is also shown that a modest tapering of the synthetic aperture in those cases offers significant improvement.

Recent developments in airborne Doppler and ground mapping navigation radars and ground and satellite based radio systems are described. Simultaneous lobing and slope tracking techniques can remove the well-known Doppler sea bias error in fast and slowly moving vehicles. Doppler velocity information can be extracted from coherent forward-looking mapping radars, and high position fixing accuracy can be achieved by synthetic aperture radars. In radio navigation systems, such as Loran, Omega, and satellite systems, direct-ranging and differential techniques greatly reduce the geometric dilution and propagation effects which have plagued conventional radio navigation systems. The advantages gained by mixing of the data from these and other navigation sensors in a digital multisensor system are discussed and approaches for processing these data are suggested.

The objectives of the Apollo 17 Lunar Sounder Experiment (ALSE) were to detect subsurface geologic structures, to generate a continuous lunar profile, and to image the moon a radar wavelengths. The first objective is generally impossible on Earth, but is possible on the moon because of the very low EM attenuation found in lunar rocks. A three-wavelength synthetic-aperture radar (SAR) operating at 60 m, 20 m, and 2 m (5, 15, and 150 MHz) was designed to attain these objectives. The design choices reflected a balance of scientific requirements versus Apollo mission and hard-ware constraints. The most difficult choice was that of wavelength, since longer wavelengths have the potential for deeper subsurface penetration at the expense of depth resolution. Shorter wavelengths enhance the ability to generate surface images and accurate profiles. The sounding requirement led to a system requirement for large dynamic range and precise sidelobe control. The radar data from the lunar mission were recorded on photographic film in a conventional SAR format, and were returned to Earth for processing. A combination of optical and digital processing and exploitation techniques was applied to the scientific interpretation of the data. Some preliminary results from the lunar mission have been obtained. The system design and data exploitation techniques developed in support of the ALSE experiment show considerable promise for eventual application to earth-resource survey radar systems.

The quality of coherent radar images is affected by several factors inherent in the target scattering phenomenon and in the radar system. Certain of these factors, such as resolution, image fluctuation, assumed amplitude cell statistics, noncoherent smoothing, background noise, and the film response, are investigated in detail. Simple statistical models of an extend target are constructed in order to simulate images and to show how these effects are related. It is shown that resolution is the dominant factor that affects image quality. The relative importance of all other factors is highly dependent on resolution.

A maneuvering synthetic aperture radar in squint mode, during a loosely piloted maneuver, is simulated with presence of various navigation system errors. The error sources investigated place emphasis on short-term effects, involving platform servo transients, noise and quantization in accelerometers, interaction of angle pickoff uncertainty with the displacement from platform to radar antenna, and uncertainty in this displacement itself. Simulation results are accompained by interpretive discussion, and followed by suggested areas for further study.

This paper describes some of the early developments in the synthetic aperture technique for radar application. The basic principle and later extensions to the theory are described. The results of the first experimental verification at the University of Illinois are given as well as the results of subsequent experiments. The paper also includes a section comparing some of the important features of real and synthetic aperture systems.

The number of transmitted pulses associated with the Doppler histories of a side-looking radar may greatly exceed the desired azimuth compression ratio of the system. This discrepancy is taxing if the storage required for the azimuth processing is provided by cores, magnetic drums, and the like. Thus, as a practical matter, one considers presumming of the data prior to correlation in an attempt to achieve the desired performance with a minimum amount of digital storage. In this paper, the optimum (in terms of resolution) presummer is derived, along with the optimum apportionment of the available storage capacity between the presumming and correlation operations. Under the condition (or generally pessimistic approximation) that the illumination pattern of the antenna uniformly illuminates a Doppler bandwidth equal to the PRF of the radar, the optimum presumming coefficients are the first Np Fourier coefficients of a function which is one of the Doppler bandwidth to be correlated and zero on the remainder of the PRF bandwidth, where Np is the number of transmitted radar pulses over which presumming is provided. Increasing Np reduces the degradation due to presumming, but may leave inadequate storage for correlation. Hence, we optimize the apportionment between the two operations and present the obtainable resolution as a function of total storage and the number of transmitted pulses in the received Doppler history.

Staff members of the Cornell University Center for Radiophysics and Space Research have designed an ionospheric radar probe to be located near Arecibo, Puerto Rico. The radar will have the following general specifications: 1) Antenna reflector, 1000-foot-diameter spherical bowl, illuminated by a 430-Mc dual-polarized feed. 2) Transmitter of 2.5 Mw peak, 150 kw average power, or 100 kw CW power. 3) Dual-channel receiver, capable of measuring total power, polarization and received spectrum. The radar will initially be used to measure the variation of electron density with height, the fluctuations of electron density at fixed heights and electron temperatures and magnetic field strengths at various heights. Ionospheric drifts may also be measured. The radar will also he able to obtain echoes from planets, information of the moon's surface and possibly echoes from the sun. Hydromagnetic shocks may also be detected and a study of cislunar ionization can be made. The passive system with the large antenna may be used as an instrument in radio astronomy to observe radio emission from planets and from true stars, and to make a survey of radio sources. With additional facilities, many radio astronomy measurements can be made taking advantage of the large antenna aperture and resulting high resolving power.

The general theory of side-looking synthetic aperture radar systems is developed. A simple circuit-theory model is developed; the geometry of the system determines the nature of the prefilter and the receiver (or processor) is the postfilter. The complex distributed reflectivity density appears as the input, and receiver noise is first considered as the interference which limits performance. Analysis and optimization are carried out for three performance criteria (resolution, signal-to-noise ratio, and least squares estimation of the target field). The optimum synthetic aperture length is derived in terms of the noise level and average transmitted power. Range-Doppler ambiguity limitations and optical processing are discussed briefly. The synthetic aperture concept for rotating target fields is described. It is observed that, for a physical aperture, a side-looking radar, and a rotating target field, the azimuth resolution is ?/? where ? is the change in aspect angle over which the target field is viewed, The effects of phase errors on azimuth resolution are derived in terms of the power density spectrum of the derivative of the phase errors and the performance in the absence of phase errors.

The design and construction of a small light-weight anechoic chamber for testing the first orbiting rendezvous radar on a launching pad are described. Performance and weight considerations were evaluated and a practical compromise was reached. A chamber capable of measuring L-band radar angular accuracy and weighing only slightly more than 100 pounds was the result. Radar RF go, no-go checks can now be performed on the launching pad or in other locations with a low cost device.

Radar waves propagating through the turbulent troposphere acquire phase fluctuations which appear as aperture phase errors in synthetic-aperture radar systems. The phase error description follows directly from Tatarski's theory of wave propagation through turbulence; for most situations of interest, the phase error is Gaussian with stationary first increments. The structure function of the phase error is a function of the radar parameters and of the vertical profile Cnm(h) of the microwave refractive index structure constant of the turbulence. A closed form expression for best attainable resolution can then be given as a function of these same parameters, via the results of a phase error analysis by Brown and Riordan. Published data on Cnm(h) and its optical-region counterpart are reviewed. The data from a recent microwave propagation experiment by Thompson and Janes are then analyzed and are consistent with the order-of-magnitude estimates of Cnm: Some unsolved problems associated with applying the results to synthetic-aperture systems are discussed.

Radar astronomy aperture synthesis technique for applying fading radar return signals to lunar and planetary mapping problems

An ideal synthetic-aperture mapping radar system is postulated. The response of this system is then computed for 1) a point target, 2) locally homogeneous terrain, and 3) terrain of arbitrary reflectivity. Relationships for achievable resolution, swath width, and final map SNR vs the independent parameters of vehicle altitude, terrain scattering coefficient, carrier wavelength, vehicle velocity, and average radiated power are developed. The ideal system considered in this paper can be approximated in practice, for small real-antenna beamwidths, by using large time-bandwidth chirp waveforms and sin x/ x illumination of the real-antenna aperture.

Field-strength correlation can provide a means for velocity sensing which is particularly well suited to lunar landing missions. An earth-based transmitter directed toward the moon can set up an irregular field-strength distribution which is the sum of the direct wave and of many waves reflected from the lunar surface. This irregular pattern can be sensed by simple receivers with appropriately spaced antennas on a spacecraft. Because the field-strength pattern will be essentially stationary with respect to the lunar surface, the speed that is measured by cross correlating the features of the,pattern will be that of the vehicle with respect to the moon itself. Since this technique is based on tracking the peak of the correlation function, the nature of this function has been investigated both analytically and experimentally by simulation. In general, the results indicate that the correlation functions which would be obtained in the vicinity of the moon are suitable for tracking. A spaceborne velocity sensor employing this principle should be operable anywhere within a region defined by radial extension of the visible half of the moon out to an altitude of about 1000 miles.

An airborne microwave hologram radar system has been developed which is a two-dimensional analog to optical holography. The field of view of the radar is directly below and to either side of the aircraft. Resolution is realized in the along-track direction by utilizing the synthetic aperture technique, and in the cross-track direction by means of a phased receiving array. The theory of operation is summarized, the demonstration system is described, and results for both the normal and contouring modes of operation are presented.

A review is presented of the several methods which are available for the mapping of both reflectivity and topography of planetary surfaces by ground-based radar. Emphasis is placed on observational methods rather than interpretation of results. The various observational schemes are illustrated by recent observational results.

Optimum deployment of deep submersibles to dive sites requires rapid collection of topographic information, intermediate in precision between conventional depth sounding and direct visual observation. The area and depth resolution of conventional depth sounding systems operating from the surface does not provide the detail necessary for submersible operations. High resolution deep-towed side-looking sonar, television, and camera systems require too much pre-dive survey time. The synthetic aperture array technique, originally developed for side-looking airborne radar, provides a potential solution for this problem. Application of this technique to side-looking sonar involves unique problems in the areas of medium stability, vehicle motion compensation, reverberation limitations, and the range-azimuth ambiguities resulting from the relatively low propagation velocity. An investigation of these problems has revealed that the technology required to develop a synthetic aperture side-looking sonar is available. Swath coverage on the order of 10 km with a resolution cell of 10 meters in water depths of 7000 m appears to be feasible, from a surface ship operating at a speed of 10 knots. The synthetic aperture approach appears to offer considerable savings in cost and complexity compared to conventional side-looking sonar arrays of similar performance.

It is shown that when a distant object such as the Moon or a planet is observed by a continuous wave (c. w.) radar employing a single aerial, components of the two-dimensional Fourier transform of the target’s radar brightness distribution may be obtained by computing the autocorrelation function of the complex amplitude of the returned signals. When the motion of the radar relative to the target is suitable, sufficient components may be measured to allow the two-dimensional radar brightness distribution to be recovered. This process has been carried out for the Moon. Observations have been made with a radar operating at a wavelength of 73 cm transmitting circularly polarized signals and receiving the orthogonal polarization. Reduction of the data has enabled an unambiguous two-dimensional map of the lunar radar brightness distribution to be constructed with an angular resolution of 3'.

A simple criterion is defined which allows a quick estimate of a radar's capability to resolve two targets with differential motion. The criterion is applicable to general types of target motion, and hence will yield nominal resolution performance for arbitrary systems that utilize relative motion, such as synthetic aperture radar or inverse synthetic aperture radar, Doppler filters, systems that make use of target spin, and so forth. The criterion is considered both for weighted and unweighted signals.

In this paper some recent measurements of the radio echo power scattered by the lunar surface are reported. These measurements were made at a radio frequency of 440 Mcps using a parabolic reflector of 84 ft diameter and a peak transmitted power of approximately 2·5 MW. The received echo power was processed by a high-speed digital computer which is an integral part of the measuring system.

Research during the past five years has enabled us to define certain potentials and problems in the use of radar imaging systems for mapping and resource evaluation. This review article concentrates on the applications and difficulties encountered with a multiple polarization K-band real aperture radar system and touches very briefly on possible applications of synthetic aperture, multi-frequency, panchromatic, and other systems. Application studies have shown that imaging radar may be useful adjunct to aerial photography and in some cases an acceptable substitute for photography in the study of broad scale geologic structures, and in deriving stream network parameters in regions where mapping is inadequate. A review is also given of studies on crop discrimination with radar, in the mapping of natural vegetation, and in certain types of regional land classifications.

A joint agency controlled oil slick experiment, sponsored by the United States Coast Guard, was conducted in the Pacific Ocean in the Fall of 1970. The Naval Research Laboratory's synthetic aperature radar was used to detect and monitor the slicks at frequencies of 428, 1228, 4455, and 8910 MHz during the low sea state conditions encountered. At frequencies of 1228 MHz and higher, the slicks were depicted with sharp boundaries. At 428 MHz, the boundaries were indistinct. Approximately 400 liters of oil was detected as it was being discharged from the generating ship and larger spills were mapped from the initial thickness to equilibrium thicknesses of 1 micron or less. Thin streamers of oil and wind blown films were also imaged. Area growth rates were obtained for 2500 liter spills of API 26.1 crude oil and API 9.7 fuel oil on a calm sea. The respective rates, from approximately 1 to 4 hours after the spills, were 134 m2/s and 16 m2/s.

This paper develops the expressions for computing the footprint on Earth (i.e., power density contours) of a satellite antenna. The expression for determining the antenna pattern coordinate as a function of the Earth coordinates is derived. The antenna pattern operator which converts the pattern function into the available power density is obtained. In this case it is assumed that the Earth-based antenna is directed towards the satellite. The radar operator is also derived; this operator yields the power density intercepted by the Earth.

Properties of a side-looking airborne radar (SLAR) designed for geoscience rather than military use are presented. The speckled nature of usual single-frequency radar images may be reduced by averaging in azimuth and by averaging in range or transmitting excess range bandwidth. With synthetic aperture systems, averaging is possible in both range and azimuth, but only range averaging (excess bandwidth) is reasonable with most real-aperture systems. Multispectral sensing appears to offer advantages for radar comparable with those for photography, on the basis of experimental spectral response curves. Matching the transfer characteristics of receiver, recorder, and film is necessary to achieve quality images. Special techniques may be necessary to permit discriminating targets of similar radar return while at the same time allowing for the very wide total range of returns. The calibration necessary to achieve repeatable results and permit use of automatic data processing may be obtained by inserting a stepped sample of transmitter signal into the receiver to place calibration blocks on the image. Both multiple look-angle coverage with a single flight line and stereo can be achieved by use of fore-and-aft squinted antennas for alternate transmissions.

Describes a simple mechanical way of moving a model target during laboratory measurements of radar cross section to simulate the precession motion of a satellite or reentry vehicle. The value of the simulation lies in the speed with which it can be accomplished and in its use as a diagnostic tool to aid the analysis of typical signature data

A description is given of a corner-sphere, an aluminum sphere with an excised octant, which serves as a dual-standard echoing body. Its measured and calculated echo patterns are shown. An optical derivation is given for the echo area of the circular corner.

Pattern recognition and its application to ocean surveillance and the near-real-time screening of tactical reconnaissance imagery presented to the pilot were chosen as a typical adaptive systems project in the Navy. The performance of a pattern-recognition device is compared with that of a human operator. It is concluded that the human operator will make the final decision, but he will be aided by a pattern-recognition device. The interpretation of unfamiliar information such as radar imagery or radar range profiles poses considerable problems for the human operator, and therefore pattern recognition is needed the most in this area. The basic pattern recognition process is briefly described and an example is given of processing techniques for pattern recognition which were implemented. The requirements and the need for a pattern-recognition device for ocean surveillance and near-real-time automatic screening of tactical resonnaissance information are outlined.

The continuing evolution of new components is resulting in corresponding advances in radar systems. The paper commences with a brief review of some of the basic radar parameters which have been influenced in the recent past by improvements in component technology and an indication is given of the ranges of these parameters which are now available to the designer. The principal part of the paper is a survey of the recent advances made in radar sub-systems and systems and in the understanding of the radar characteristics of targets and clutter. The review of sub-systems covers electronic beam steering, pulse compression, signal and data processors, constant false alarm rate circuits and displays. System principles discussed include within-pulse scanning, low-angle tracking techniques and synthetic aperture radar.

A difficult problem with ground moving target radar (GMTI) detection is how to consistently track targets moving through non-homogeneous regions of clutter such as forest and urban boundaries. Although attempts have been made to mitigate this detection problem using terrain mapping data, such data does not give current clutter information due to change in vegetation, roads, buildings, and seasonal variation. We propose to use synthetic aperture radar (SAR) imagery to enhance the detection performance of GMTI radar. We use a multiresolution Markov model to represent both target and background clutter. This multiresolution structure allows us to accurately match GMTI clutter with the geographically registered SAR imagery for consistent moving target detection through clutter boundary areas.

A series of coordinated experiments has been performed, consisting of simultaneous radar echo and auroral particle flux measurements over Alaska. The radar data were obtained by Stanford Research Institute, based at Homer, Alaska; the particle flux data were obtained from the Lockheed group of instruments aboard the polar-orbiting satellite OV1-18, during the spring and fall of 1969. Both radar and satellite data are of good to excellent quality, and general agreement is observed between location and intensity of radar echoes and the precipitating particles producing the aurora. Detailed analysis (not a part of this work) appears worthwhile to determine a characteristic signature for radar returns from aurora, as distinguished from solid body returns.

In this paper we describe a new method for creating three-dimensional images using pairs of synthetic aperture radar (SAR) images obtained from a unique collection geometry. This collection mode involves synthetic apertures that have a common center. In this sense the illumination directions for the two SAR images are the same, while the slant planes are at different spatial orientations. The slant plane orientations give rise to cross-range layover (fore-shortening) components in the two images that are of equal magnitude but opposite directions. This differential cross-range layover is therefore proportional to the elevation of a given target, which is completely analogous to the situation in stereo optical imaging, wherein two film planes (corresponding to the two slant planes) result in elevation-dependent parallax. Because the two SAR collections are coherent in this particular collection mode, the images have the same speckle patterns throughout. As a result, the images may be placed into stereo correspondence via calculation of correlations between micro-patches of the complex image data. The resulting computed digital stereo elevation map can be quite accurate. Alternatively, an analog anaglyph can be displayed for 3-D viewing, avoiding the necessity of the stereo correspondence calculation.

The similarity between side-looking radar records and true holograms, noted by the author at the first U. S. — Japan seminar in 1967, is briefly reviewed. Extension of the side-looking radar concept to stationary (doppler-free) coherent radars is discussed, including both monostatic and bistatic versions. The concept of a passive (cooperative) hologram radar system is put forward.

We analyze the effect of target motion on synthetic aperture radar images and discuss how a multi-aperture radar can detect, focus and relocate moving targets in clutter. AN/APY-6 radar data is used for analyzing the radar imaging of ground moving targets. Some applications of the time-frequency transform to detect and focus moving targets are also discussed.

This paper describes an active synthetic aperture coherent imaging technique which completely fills and doubles the diameter of a sparsely sampled aperture array, thus providing better imaging than would ordinarily be expected from the sparsely sampled aperture alone. The technique applies to any coherent radiation, but is most applicable to microwave (radar) and acoustic (sonar and seismic) imaging.

RADARSAT-2 is an advanced synthetic aperture radar satellite scheduled for launch in 2003. The satellite will be equipped with an active phased array antenna which will be used to provide a wide variety of beams and modes, including all the beams and modes of RADARSAT-1 and additional Ultra-fine resolution, dual and quad-polarimetric modes. The large number and variety of modes presents a major calibration challenge and, because the satellite is intended for operational use, efficient means for achieving and maintaining this calibration are essential. This paper identifies the three principal types of target which are planned for use in achieving radiometric, polarimetric, geo-location, and other types of calibration for RADARSAT-2, and describes how the data will be analyzed to generate the required calibration information.

In 1998 a VHF (80-116 MHz) synthetic aperture radar (BioSAR) mounted on NASA's C-130 aircraft collected data over a series of Smithsonian Tropical Research Institute (STRI) test sites along the Panama Canal Zone in the Republic of Panama. A biomass surface map of the area was generated by interpolating between BioSAR measurement points for a set of parallel flight lines. Accuracy testing showed that there was good agreement (/spl plusmn/10%) between the sensor-derived estimates of above ground biomass and the field data.

The IMP-6 spacecraft was detected at 140 000 km using a 430-MHz radar. Several radar observables were measured.

The usual synthetic aperture radar operates from a moving aircraft and achieves the effect of a very long synthetic antenna with its accompanying high gain and resolution. This letter suggests the application of the same concept to a ground-to-air radar, whereby a small ground-based transmit-receive antenna would effectively be provided with a large synthetic gain against nearby planes flying at uniform speeds.

An experimental demonstration of synthetic aperture radar principles using a CO 2 laser is described. The work described is an extension into the infrared spectrum of a similar experiment by the authors at visible wavelengths reported earlier.

Certain structural geologic features may be more readily recognized on sidelooking airborne radar (SLAR) images than on conventional aerial photographs, other remote sensor imagery, or by ground observations. SLAR systems look obliquely to one or both sides and their images resemble aerial photographs taken at low sun angle with the sun directly behind the camera. They differ from air photos in geometry, resolution, and information content. Radar operates at much lower frequencies than the human eye, camera, or infrared sensors, and thus “sees” differently. The lower frequency enables it to penetrate most clouds and some precipitation, haze, dust, and some vegetation. Radar provides its own illumination, which can be closely controlled in intensity and frequency. It is narrow band, or essentially monochromatic. Low relief and subdued features are accentuated when viewed from the proper direction. Runs over the same area in significantly different directions (more than 45° from each other), show that images taken in one direction may emphasize features that are not emphasized on those taken in the other direction; optimum direction is determined by those features which need to be emphasized for study purposes. Lineaments interpreted as faults stand out on radar imagery of central and western Nevada; folded sedimentary rocks cut by faults can be clearly seen on radar imagery of northern Alabama. In these areas, certain structural and stratigraphic features are more pronounced on radar images than on conventional photographs; thus radar imagery materially aids structural interpretation.