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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 new concept in synthetic aperture radar, called SASAR, which uses a segmented aperture, is described. Use of the segmented aperture allows appreciable extra receiving antenna gain to be realized. Each subarray of the receive antenna is equal in length to the transmit antenna; the system performance is increased approximately by a factor equal to the number of subarrays. To allow array combination of the subarray signal outputs requires a phase-shift factor (varying with azimuth) to be applied to each subarray signal. A digital implementation of this preprocessor is sketched out; it uses a push-down storage stack to store the range histories for a synthetic aperture from each subarray. Appropriate phase shifts are added to the stacks and a sum of stack values then provides the combined output range history sequence. Possibilities of using analog delay lines for preprocessing are also discussed. Pattern errors due to subarray size and receive array near field are examined and constraints are given.

A system for identifying moving objects such as vehicles, trucks, buses, containers, etc., in which a passive label having a number of dipoles in accordance with a code is attached to the object, is described. A synthetic aperture radar consisting of a Gunn diode mounted within a resonant cavity and connected to a horn is used to illuminate the label. The Gunn diode operates as a homodyne so that signals from the label are also detected by the diode as a number of overlapping chirp signals, which are sent to a computer for processing. The computer output is a pulse train with pulses corresponding to dipoles in the label. A number of tests have been made indicating that labels can be read regardless of range and vehicle speed. A number of horns may be strategically located in a city or along a highway, and these horns can be multiplexed into a common computer to provide an inexpensive all weather automatic vehicle identification system.

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.

Long-wavelength radio astronomy observations of high resolution are of considerable scientific interest but are limited from Earth-based telescopes due to the opacity of the ionosphere. For ratio telescopes orbiting above the ionosophere, the viewing window is restricted to about 1 to 5 MHz. To obtain the maximum resolution allowable by interplanetary plasma scintillations in this range requires antenna diameters on the order of tens of kilometers. A concept for obtaining high-resolution, low-frequency observations of time-stationary discrete sources and the background continuum distribution is presented which circumvents the need for physically large antennas by using two small satellites in an interferometric mode. Antennas on each satellite measure relative phase and amplitude of the radio sources, and the combination of satellite relative motion together with ground data processing is used to synthesize an antenna aperture. Complete sky coverage is obtained by computer phasing of the elements of the aperture plus orbital precession of the aperture plane. An example system that utilizes this concept is presented.

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 digital processing approach has been devised for performing motion compensation in a high-resolution airborne synthetic aperture radar in the presence of simultaneous longitudinal (speed change), lateral (turn), and vertical (climb or dive) maneuvers. Both side-look and squint are accommodated in a unified scheme, which is validated by various simulation runs reported herein. Present attention is focused on theoretical verification, irrespective of mechanization or specific parameter values.

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.

Friedman, H., "NAS-SSB Calls for Cluster-Satellite Deployments/' Astronautics & Aeronautics, Vol. 7, No. 1, Jan. 1969, p. 23. 2 French, F. W., Rodman, A. K., and Huguenin, G. R., "A Synthetic Aperture Approach to Space-Based Radio Telescopes," Journal of Spacecraft and Rockets, Vol. 4, No. 12, Dec. 1967, pp. 1649-1656. 3 Hrastar, J. A., "Dynamic Control Study for Two Closely CoOrbiting Satellites," X-732-70-169, 1970, NASA. 4 Bainum, P. M., Stuiver, W., and Harkness, R. E., "Stability and Deployment Analysis of a Tethered Orbiting Interferometer Satellite System," paper presented at the Eighth European Space Symposium, Venice, Italy, May 1968, Applied Physics Lab., John Hopkins Univ. 6 Liebelt, P. B., An Introduction to Optimal Estimation, Addison-Wesley, Reading, Mass., 1967, Chap. 6.

Experimental and theoretical results are given for the one and two-dimensional Covington-Drane optical synthetic apertures and the thin annular aperture. The segmented thin annulus is also discussed as a special case of the thin continuous annulus. Experiments are performed by masking a diffraction-limited lens with the appropriate pupil function and then measuring the imaging characteristics of the aperture. All the synthetic apertures are shown to have the full synthetic MTF's predicted by the theory. The Covington-Drane doubles the resolution of the central aperture and the thin annular apertures synthesize the full lens spatial frequency response. Optical processing of synthetic aperture photography is discussed and examples of inverse spatial filters are given.

This paper considers the effects of slowly moving targets as they appear in the output of an airborne coherent side-looking synthetic aperture imaging radar. The image of a moving reflector is described, and two approaches to airborne moving target indication (AMTI) are summarized. It is shown that the effects of target movement are decreased as the radar scan rate is increased, and are increased as the (Doppler processed) compression ratio is increased.

The influence of random phase errors on the angular resolution of a focused synthetic aperture radar system is treated. The principal measure of performance has been taken as the mean envelope power at the system output. This system output power is evaluated exactly, although not in closed form, based on the following assumptions: 1) the real beam pattern is Gaussian; 2) the random phase error is essentially a geometry-independent ergodic process with a Gaussian amplitude distribution and zero mean; and 3) the random phase error has a Gaussian correlation function. The curves presented in this paper can be used to estimate expected system power response, expected system resolution and effective aperture length beyond which, in the presence of phase error, little gain in resolution is expected. It was found that multiple sources of error with different correlation intervals make explicit solution of the integral equation for system power response practically impossible. In this situation, a reasonable approach is to evaluate the system power response separately for each error. If one of the errors is clearly dominant, it may be regarded as bounding achievable performance.

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.

A simple method is developed for predicting the dynamic range of a data processor (correlator) associated with a synthetic aperture mapping radar system. The primary source of noise in standard optical correlators is the data film base and grain noise. The primary noise in an electronic correlator (using photographic film for data storage) is phosphor noise and data film grain noise. For this study however, only film grain noise was considered. The only inputs required to predict the dynamic range are the radar system parameters, the published film granularity and one other film constant called the scattering coefficient. Although this paper is restricted to the case of photographic film associated with a synthetic aperture radar system, the techniques developed are useful for other types of transparency storage material and many other applications of film transparencies for data storage.

A synthetic aperture radar system is described which is suited to the target classification role and has a minimum data storage requirement. The corresponding data processor can employ the fast Fourier transform technique.

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 simple method for constructing a microwave hologram is described. In this method, a common antenna is employed for both transmitting and receiving, while the rotation of this antenna results in the generation of a synthetic aperture in the angular direction. The behavior of this type of hologram is explained easily by that of a "concave zone plate."

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.

A microwave or acoustic form of holographic interferometry is described in which synthetic aperture procedures are used.