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A pulse-Doppler backscatter radar operating in the HF band was used to test the feasibility to measure ocean surface motions in the North Atlantic Ocean under a variety of environmental conditions. Using the ionosphere as a part of the propagation path, data were collected at ranges from 600 to 2200 nmi. Environmental conditions included a relatively small hurricane, a large storm, and a relatively calm ocean. The Doppler data were converted to wind vectors and compared with weather data taken at the same times and places. Preliminary results indicate that there is, in general, good agreement between wind directions as determined from the radar data and this is indicated on the weather chart at all ranges and under all of the environmental conditions encountered. The agreement in wind strengths is fair to good, but at present reliability is mainly limited to the first-hop data because of multiple propagation modes. Improvements in this technique can be expected from refinement in the theoretical models of scattering, and in developing radar operation methods that minimize multipath responses.

The Skylab S-193 altimeter experiment utilizes a 10- and 100-ns pulse length, 13.9-GHz earth-pointed radar system to obtain earth-surface backscatter measurements from the Skylab spacecraft. Objectives of the experiment are to obtain precision measurements of surface profile for uses in geodesy, oceanography, and earth physics, and to measure radar-signal characteristics from an earth-orbit geometry to provide design information for future radar remote-sensors. The technical approach is that of measuring the power impulse response of the scattering surface. The hardware is designed to operate in five modes: waveform or impulse-response measurement and altitude determination; radar cross-section experiment; signal correlation experiment; 10-ns pulse-compression evaluation; and nadir-seeker experiment. This program is the first reported instance in which these quantities have been measured from orbital conditions. The experimental approach utilized and the post-flight data-processing requirements are discussed and preliminary results from the first manned Skylab mission are presented.

The monitoring of bird movements and flight characteristics by radar can contribute to an understanding of radar clutter and bird migration, can help to alleviate the hazard of collisions between birds and aircraft, and may constitute a useful form of environmental monitoring. Most conventional surveillance radars can detect birds readily but are very much limited in their ability to identify birds. The recording of amplitude and Doppler signatures, however, may allow identification as to size and type and has the potential for increasing knowledge concerning bird flight.

The evolution and state of development of the radars used in the Ground-Controlled Approach (GCA) systems of the U.S. Air Force is described. Emphasis is placed on the radar requirements of this application and the evolution made possible by the advances in component technology. We first review the concept of GCA operation and show how this leads to the requirements of the two radars in the system. Next we discuss the early history of GCA and the AN/MPN-1, the first operational system. The evolution of the GCA radars since the AN/MPN-1 is then summarized. Finally, we describe in more detail the AN/TPN-19. This is the most recent GCA system to be developed, and it makes effective use of array technology, frequency diversity, monopulse, pulse compression, and computer control.

This paper surveys the field of automotive radar and represents an expansion of a previous paper, "Automotive Radar-Problems and Promises" [1]. Both the roadside environment in which the car and driver operate and on-board physical environments in which the electronics must function are treated. Applications involving speed sensing, predictive crash sensing, obstacle detection, braking, and station keeping are addressed. Current radar system state of the art is reviewed. Considerations relating to target discrimination, weather penetration, and basic radar system parameters are covered. Modulation techniques for range determination are examined, and radar system configurations are discussed in an effort to identify the advantages and disadvantages of each. It is concluded that although considerable development work remains before radar becomes a practical automotive system, significant progress has been made.

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.

Radar echoes from meteorological targets have existed almost from the time that radar was first used. In this paper, an equation is presented which relates the echo power to a characteristic of the cloud or precipitation particles within the radar pulse volume. Some examples of how radar has been used for operational purposes are given; these include a description of the utility of radar for storm detection, how radar can be used as an aid to weather forecasting for both the public and aviation interests, and also how radar can be applied to help solve some hydrological problems. Significant research results on the nature of atmospheric structure and processes as derived with the help of radar are presented. Doppler radar has played a prominent role in the investigation of storm dynamics, and high-power high-resolution radars have been used for studying the development and nature of clear-air turbulence and convective processes in the clear air. Recent trends in meteorological radars are indicated and suggestions are set forth which relate to the improved use of radar for operations.

The problems of low-angle tracking over the earth's surface are explored in the light of rough-surface scattering theory. A detailed model of diffuse scattering is constructed which agrees with available test data and which permits the power distributions in radar coordinates to be estimated. Using this model, the potential performance of various techniques proposed for avoidance of multi-path is evaluated, Several are found effective in maintaining tracks of reasonable accuracy down to one-fourth beamwidth above the horizon, but no generally practical solution to height measurement below that angle appears available.

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.

RF pulses are used in radar and sonar to detect and locate targets in extended media. Short RF pulses (and impulses) can be used to find buried objects or voids by echo sounding, or can be used to probe snow fields or the depths of the earth. Although similar to radar in principle, there are important differences in these applications that can lead to significant variations in the design approach. A novel system is described, with a computer-programmed tunable RF source, that tunes rapidly through all the frequencies of a nanosecond pulse spectrum, makes individual "CW" measurements, stores them, and finally computes a synthetic echo. Target signatures can be "recognized" by calculating correlation functions. Experimental results are presented.

Over-the-horizon (OTH) HF radar using sky-wave propagation via refraction by the ionosphere is capable of detecting targets at distances an order of magnitude greater than conventional microwave radar limited by the line of sight. Some of the characteristics, capabilities, and limitations of OTH radar based on the experience of the MADRE radar as developed by the Naval Research Laboratory are described. Also discussed is the application of OTH radar to air-traffic control and to the remote sensing of sea conditions.

Test results and key principles are given for a radar altimeter designed for meteorological balloons. The instrument, which weighs 160 g and consumes 0.7 W, will fill a gap in meteorological, sensing using balloons-an area where pressure altitude was formerly the preveiling reference. The instrument is basically a delay-lock radar utilizing a superregenerative RF stage. Long-term absolute accuracy of ±10 m and short-term stability of better than- 2 m rms were measured at altitudes of 20 km.

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.

The production of topographic maps requires two kinds of information. First, the detail to be placed on the map sheet must be identified. Second, the positions of the various objects and features must be measured in three dimensions. Current airborne radar technology provides the means to satisfy both of these requirements in adverse weather and at any time, day or night. Radar used specifically for this purpose employs synthetic-aperture techniques to obtain fine resolution measurement in two dimensions and interferometry to obtain the third measurement.

The measurement of the radar cross section of full-size targets in flight presents some unique problems. The techniques employed to maintain the dynamic range of the signal, facilitate data handling, and control the target aspect, all contribute to the efficiency with which the requisite data may be gathered and reduced. The approaches taken in solving these problems are discussed in connection with the U. S. Naval Research Laboratory (NRL) dynamic measurement radar. This system is specially designed for simultaneous pulse-by-pulse measurements at L, S, and X bands permitting a high degree of polarization flexibility. Data reduced from film records, when combined with program-derived aspect information, enables a graphical description of radar cross section to be plotted as a function of target aspect. Data from measurements of a C-54 aircraft have been plotted in this manner and show the variation of σVV, σHH at L and S bands, and σVV, σHH, σVH, σHV, σRR, σLL, σRL, σLR at X band. Operating experience with this system indicates the practicability of performing dynamic measurements.

The scattering matrix of a target generalizes the scattering properties to include a description of their dependence upon radar polarization. The matrix may be determined directly through two successive transmissions at orthogonal polarizations, with reception of each at both of the polarizations. In the laboratory, it is possible to eliminate the requirement for phase measurement by an indirect technique using successive measurements at six polarizations (five of which may be linear). In the case where results are to be applied to linearly polarized radars, it is sufficient to determine the "linear restricted matrix" by six linear measurements. Analysis is provided and measurement technology described and evaluated for both classes of indirect measurement.

This paper is intended to serve both as an introduction to radar cross-section measurements and as an introduction to the several measurement papers that follow in this issue. We discuss the contribution that experimental work makes to theoretical studies. The history of cross-section measurements is reviewed. Some terms are defined, and the major measurement problems are discussed. It is pointed out that the various range geometries and measurement techniques in use today were based on the approach used to solve the basic measurement problems. Some details are given of the more popular present-day measurement systems, i.e., CW, pulsed, FM-CW, Doppler, and the standing-wave ratio method.

This paper deals with the cumulative detection probability of a search radar when it is scanning uniformly. This is the probability that a target, approaching the radar at a constant radial velocity, is detected at least once by the time it reaches a given range, as distinguished from the more common blip-scan ratio (a single-scan detection probability). It is shown that for constant-velocity targets the range for a given cumulative-detection probability varies as the cube root of the power-aperture product, rather than as the fourth root. Curves of cumulative detection probability, as a function of normalized range, are given for three different target scintillation models. Also, curves of optimum (normalized) frame time, are given as a function of the desired cumulative detection probability for each of the three target scintillation characteristics.

This paper describes the design of an extremely high range resolution FM/CW X-band radar and discusses some measurement data taken with this radar. The Delta radar system employs a 35 per cent bandwidth and was built as a research tool for scattering investigations at Cornell Aeronautical Laboratory under Bell Telephone sponsorship.[1] This equipment has demonstrated a range resolution of 3.0 inches as measured at the signal spectrum half-power points. The primary design objective of the radar is to provide the capability to measure radar cross section of scaled models with a range resolution equal to a small fraction of the target's length. Radar cross-section measurements made with this radar have permitted the separation and identification of the individual scattering contributions on several target bodies. Data from three types of radar targets are presented and discussed. Particular theoretical interest centers on the measurement of the sphere-capped cone, for which the measurements bear out simple arguments of electro-magnetic theory to demonstrate that: 1) Discontinuities of geometric surface curvature on a metallic body are pronounced scatterers. 2) There is no scattering from the optical shadow boundary. 3) There is now good evidence of the "creeping wave" contribution to the scattered field.

Experiments have been performed in which the radiation from a CO 2 laser was coherently detected after being scattered from a moving diffuse reflector. This is generally the configuration of an infrared laser radar. The power-spectral-density of the heterodyne signal was measured and its width was shown to agree with the calculated value based on a theoretical model for the process. Expressions are obtained for the ratio of heterodyne signal bandwidth to heterodyne frequency for the cases of focused radiation, unfocused radiation, and for a typical radar configuration. In most cases, the heterodyne signal is found to possess a narrow-band character. The probability density of the signal envelope was also measured for a known scatterer (providing Gaussian scattered radiation) and was found to be Rayleigh distributed, as expected. The power-spectral-density and envelope probability distribution provide information about a scattering medium or target which cannot be obtained from average-value measurements of the heterodyne signal-to-noise ratio. This information is useful for communications applications, infrared radar, and heterodyne spectroscopy experiments. Finally, a simple and direct method of obtaining information about the statistics of an incident radiation field (which does not involve photocounting) is discussed.

The methods now in the literature for calculating the terrain-backscattered radar-altimeter echo utilize the antenna power-gain patterns and the terrain radar cross section per unit area, usually designated σ 0 . In this paper, the polarization characteristics of the antennas and the echo depolarization by the terrain are taken into account for the first time. The formulas for the backscattered return are presented in terms of antenna and terrain parameters that can be measured directly or calculated from other measurements. These parameters are the four transmitting- and receiving-antenna gain patterns for the orthogonal polarization components, the two functions giving transmission- and reception-phase difference between the polarization components, the terrain-depolarization factor that accounts for the average backscattered power cross polarized with respect to the incident wave, and the radar cross section σ 0 per unit area. It is shown that the older formulas used for radar return from the near vertical are correct only when both antennas are linearly polarized and have the same direction of polarization. The errors that result from failure to consider polarization effects are illustrated by examples.

The use of the scattering matrix leads to a formulation of radar reflectivity that is independent of radar polarization, target range, and target rotation about the line of sight. The resulting formulation is applied to the problem of object recognition in free space. Experimental results for right circular cones are presented.

The pack ice in the Arctic Ocean was the subject of a special remote sensing mission conducted jointly by the National Aeronautics and Space Administration, the Navy Oceanographic Office, the U. S. Army Cold Regions Research and Engineering Laboratory, the Arctic Institute of North America, and the University of Kansas. One of the most significant results of the experiment was verification of the ice type identification potential of a 2.25-cm-wavelength radar scatterometer. The results of the radar experiment are presented and the data are analyzed to determine the characteristics of radar backscatter from various Arctic ice types. A quantitative analysis of the data indicates that identifiable radar return "signatures" are obtained for each of several specific sea ice types. A qualitative analysis based on the Kirchhoff-Huygens principle suggests that a categorizing factor, denoted as the surface roughness factor, can be assigned to each of several ice types.

The scant two-decade existence of radar astronomy has seen important contributions by this new technique to positional astronomy, to the metrics of the solar system, and to the study of lunar and planetary surfaces. The role of millimeter wavelengths in this new astronomy is discussed and the problems unique to these short wavelengths defined. Brief mention is made of the importance of correlation of active and passive measurements and important new millimeter wave component requirements are listed. The implementation of an eight-millimeter radar capable of detecting the moon is described. At the eight-millimeter wavelength, the moon is found to be generally rough and has a radar cross section of about seven percent of its projected geometrical area. Methods of improving these measurements and their feasibility are outlined. Future equipment configurations capable of extending millimeter wave measurements can be implemented, given critical component development success.

The application of Thomson (or incoherent) Scatter observations to the study of the earth's ionosphere is described. Those aspects of theory of Thomson Scatter that have been put to practical use in ionospheric investigations are reviewed briefly and the type of radar equipment constructed for these investigations is discussed. Methods of measuring electron density, electron and ion temperatures, and ionic composition are then reviewed. Other applications of the technique--to the study of the neutral density and temperature of the upper atmosphere, drift motions, the flux density of fast photoelectrons, and the orientation of the earth's magnetic field--are also described.

Two approximate methods have been developed for calculating the radar cross sections of nonconducting bodies. The superposition approximation used to obtain the radar cross sections of dielectric clad bodies requires that the scattering properties of the component parts be known. The modified geometrical optics method, which yields the radar cross sections of dielectric bodies, requires no such initial information. These methods can be applied to some bodies with shapes that cannot be treated by means of the classical boundary-value solutions. Furthermore, the computations required are usually considerably simpler than for the boundary-value solutions even when the latter are obtainable.

The 120-foot diameter Haystack antenna operates at X-band and is used for satellite communications, radio astronomy, and radar astronomy. Haystack is used to study stars, planets, the sun, the moon, and earth satellites. In order to point the antenna to within 1/10 beamwidth (22 seconds of arc) and to provide flexible control and data processing for diverse users, a digital computer is tightly integrated into the control system. The parameters of an experiment and computer control are arranged via simple operator discourse through a keyboard/printer. The computer system can simultaneously direct the antenna, process receiver signal data, drive convenient operator displays, and permit operator discourse. Thus, processed signal data can be observed in real time, and experiment modifications rapidly implemented. The computer control system is in full operation.

Certain sequential tests of statistical hypotheses known as sequential probability ratio tests (SPRT) possess an optimum property that they can accelerate, on the average, the termination of the test or alternately execute the test in the same time with less error, or with the same error but on less distinguishable hypotheses than a fixed sample-size test. These properties are of practical importance when applied to the radar problem, but in the application of sequential methods to radar surveillance some modification of an SPRT is frequently necessary. In particular, design problems relating to phased array radars are considered. This paper reviews the problem of angle search constraints, existence of multiple resolution elements in range and velocity, possibility of a flexible allocation of pulses, and fluctuations of radar cross sections of targets. Representative numerical results are included to indicate what performance can be expected with the use of sequential methods. An attempt is made to present an integrated theoretical and practical picture. Key references are listed.

The rapid variabilty of pulsar signals and the dispersive nature of the interstellar medium introduce a number of problems which are not inherent in other radio-astronomical observations. These problems are discussed and some of the techniques currently being used to cope with them are described.

The beacon subsystem is one of many sources providing information to the air traffic control system. In automatic systems, various types of target information are derived from the beacon replies in a special purpose computer known generally as a processor. An important aspect of processor performance is its ability to detect targets in a noisy environment and to accurately determine the location of these targets. Among the important measures of target detector performance are the false alarm rate, the probability of detection, and azimuth accuracy. The intimate relationship between these three characteristics is described in this paper and specific examples are given using a sequential observer target detector. It is shown that increasing the range resolution of the system also benefits target false alarm rate and azimuth accuracy. Thus, as advances in technology permit improved range resolution, greater overall system performance can be expected. Nevertheless, with current technology, the azimuth accuracy which can be achieved with a statistical detector is better than the azimuth equivalent of one pulse repetition period even in a noisy environment.

The types of ultrasonic delay lines in common use are described, their limitations are noted, and important design considerations are outlined. Particular attention is given to recently evolved dispersive lines suitable for radar pulse compression systems.

The advent of round-trip radar measurements has permitted the determination of the ranges to the nearby planets with greater precision than was previously possible. When the distances to the planets are known with high precision, the propagation delay for electromagnetic waves reflected by the planets may be calculated and used to synchronize remotely located clocks. Details basic to the operation of a lunar radar indicate a capability for clock synchronization to ±20 µs. One of the design goals for this system was to achieve a simple semiautomatic receiver for remotely located tracking stations. The lunar radar system is in operational use for deep space tracking at Jet Propulsion Laboratory and synchronizes five world-wide tracking stations with a master clock at Goldstone, Calif. Computers are programmed to correct the Goldstone transmissions for transit time delay and Doppler shifts so as to be received on time at the tracking stations; this dictates that only one station can be synchronized at a given time period and that the moon must be simultaneously visible to both the transmitter and receiver for a minimum time of 10 min. Both advantages and limitations of the system are given. Finally, an experiment is described which has detected the effects of lunar topography and libration on radar results; a monthly cyclic effect in time synchronization of about ± 6 µs is shown.

Limitations in the state-of-the-art of remote sensing instrumentation and restriction of experimental ground truth, imposed by the inaccessibility of the oceanographic environment, have inhibited the application of remote sensing techniques to oceanographic research and the implementation of operational environmental monitoring programs. However, preliminary experiments are demonstrating the application of infrared sensors; surface mapping radars, radar scatterometers, surface profiling lasers, passive microwave radiometers, pulsed laser depth-finding systems, and optical scanners, cameras, and television systems to the problems of sea surface temperature measurement, sea state determination, sea ice surveillance, fisheries research, and subsurface reef detection. Data obtained from recent experiments are presented herein. It is concluded that particular attention should be paid to the development of microwave sensors to obtain an all-weather capability. Intensive research programs should be established to develop techniques for interpretation of subsurface phenomena based on surface measurements. This would enable construction of three-dimensional models describing the dynamic characteristics of the ocean.

In recent literature radiation emitted or reflected from a body or surface has been modeled as equivalent radiation from a collection of individual discrete radiators. This model has application to the analysis of forward and backscatter from rough surfaces, clutter and chaff models for radar, and cavity emissions at optical frequencies. In this paper we investigate radiation using a generalization of the above radiator ensemble. We allow some degree of coupling to exist between individual radiators, and we assume each radiator emits random energy bursts as a Poisson process. Analysis is confined to power spectral densities and first-order statistics of the resulting scalar radiation fieid. The results indicate that the pulsed radiator model can account for many properties of radiation. A by-product of the analysis is a filter model for dispersive channels, applicable to radar design problems. Theoretical results are compared with previously reported experimental results wherever possible. Some consideration is given to a relativistic interpretation of the radiator ensemble.

Three distinct methods of making high-range resolution radar cross-section (RCS) measurements are discussed. Comparisons are drawn for nanosecond pulse, FM/CW, and frequency-stepping methods of measuring location and magnitude of discrete scattering sources. The effects of using wide bandwidth transmissions on RCS amplitude level and fine structure are quantitatively estimated, and analytical results are presented. The three techniques are effective and yield the desired information about the magnitude and location of discrete scattering sources. Minor differences in data format and convenience of operation are noted, and it is concluded that a need for additional investigation of wide-band scattering phenomena is necessary.

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.

Addition of a radar to a scanning microwave radiometer is shown to be simple and to consume little power. Since both the radar and the radiometer receive signals having the statistical properties of noise, both receivers may use the Dicke synchronous detection system, provided the radar receives enough independent samples. Application of the combined instrument to spacecraft measurement both of oceanic winds and waves and of precipitation has more promise than the use of either instrument alone, since they can, in part, calibrate each other. An aircraft imaging radar radiometer has potential for applications where superposition of the thermal radiometer image on a relatively static radar image will aid in interpretation.

The ground stations used in the West Ford experiment, one in California and one in Massachusetts, can transmit and receive CW X-band communications signals simultaneously. In addition, each station can be changed within minutes to a radar capable of tracking and measuring the characteristics of the dipole belt and other satellites. The high performance of these stations also permits other scientific experiments. New radio and radar techniques and components such as high-power X-band klystrons, low-noise masers, and high-performance waveguide devices are used at these stations. The operation of the stations and the microwave components and techniques are described in this paper.

The evaluation of radar and active sonar matched filter ambiguity functions is critical to waveform choice, the proper selection of reference signals, and the ability to estimate system performance. In the radar case the evaluation is facilitated by use of the narrow-band approximation wherein Doppler distortion of the transmitted modulation function is neglected. However, modern-day sonar signals are usually broad-band with respect to the carrier frequency and long in duration so that the narrow-band approximation cannot be employed. This creates difficulties in both analytic and computer ambiguity function evaluations. Analytic difficulties arise from the inability to solve the appropriate integrals; computer difficulties, based on direct cross correlation, arise from the long computation time required. This paper shows how evaluation of the ambiguity functions of FM sonar signals having high time-bandwidth products can be greatly facilitated by asymptotic approximations derived on the basis of the principle of stationary phase. Application of the theory allows many seemingly difficult ambiguity evaluation problems involving highly complicated integrals to be solved rapidly using algebraic techniques. As examples of the theory, the cases of linear FM under constant target velocity conditions and hyperbolic (Doppler invariant) FM under constant target acceleration conditions are worked out. In addition, some interesting adjustable Doppler tolerant characteristics of a special modulation technique (parabolic FM) are described.

The conditions for radar cross-section measurement are given in terms of permissible variations in the amplitude and phase of the incident field at the target. A number of minimum range conditions are listed and discussed. The minimum range associated with the uniform square aperture antenna is analyzed and its relationship to the sensitivity of measurement is determined. Measurements of small targets in the near zones of focused and unfocused apertures are examined. The substantial range reduction possible with the use of a suitable microwave lens, when measuring large and medium size targets, is indicated.

Observations of radar echoes from the clear atmosphere are presented and the scattering mechanisms responsible for the two basic types of clear-air echoes are discussed. The commonly observed dot echo originates from a point in space and usually shows little variation in echo intensity over periods of about 0.1 second. The results of the most recent investigations of these clear-air dot targets are consistent with the conclusion that most, if not all, of the dot echoes are caused by insects or birds. The second type of clear-air radar echo appears diffuse in space, and signal intensities vary considerably over periods of less than 0.1 second. The echoes often occur in thin horizontal layers or as boundaries of convective activity; these are characterized by sharp gradients of refractive index. The reflectivity-wavelength dependence of these echoes is consistent with the theory of scattering by fluctuations in refractive index, and the signal intensities can be accounted for by the spectral characteristics of refractive-index variations observed directly. Some features of clear-air atmospheric structures as observed with radar are presented. These structures include thin stable inversions, convective thermals, Benard convection cells, breaking gravity waves, and high tropospheric layers which are sufficiently turbulent to affect aircraft.

Electromagnetic open-guiding structures, or surface waveguides, for continuous-access guided communication (CAGC) and obstacle detection ("guided radar"), are described for use in ground transportation, such as railways, highways, and more advanced guided systems. The experimental and theoretical work at Queen's University on surface-wave devices and their application to obstacle detection are reviewed in detail. It is concluded that there is considerable promise in these techniques and that obstacle detection, in particular, deserves much more attention than it appears to have received.

This bibliography consists of 1420 references from over 275 journals or other sources in the open literature, and represents contributions from over 1225 authors. Material has been selected from that now being compiled at the Georgia Institute of Technology for a comprehensive collection of abstracts and references entitled "A Bibliography of Radar Reflection Characteristics." Coverage is extensive but not exhaustive, is international in scope (with heavy representation from the Soviet literature), and is restricted to the time period 1957-1964. Subject coverage includes. scattering and diffraction theory, simple geometrical shapes, geophysical targets (aurorae, ionosphere, meteor trails, sea surface, terrain, and angels and related atmospheric phenomena); planets and space targets; and plasmas. Other topics include: absorbers, aircraft, missiles, chaff and dipoles, acoustic scattering, laser radar, reflectors, and measurement techniques and equipment.

In estimating future trends in radar scattering research and development, the authors take the view that the dynamics of progress and change will be governed largely by user demands. Emerging developments in radar for space applications, reflectivity from plasma and other permeable bodies, object resolution and recognition, and synthesis of radar cross-section characteristics will in the main stimulate and constrain the direction of future scattering research. The authors speculate on the likely contributions to be expected from known theoretical and experimental approaches to scattering problems. The relative merits of quasi-optical approximation, boundary value solutions, and integral equation formulations suitable for high-speed digital computers are examined. The scientific frontier includes the response of targets to partially coherent electromagnetic waves and the resolution and identification of distributed and statistically representable objects by tomorrow's data processing radar sensors. Available techniques needed to control or modify the diffraction and scatter of targets are discussed in terms of future requirements assuming that objects with resonant dimensions and composed of complex materials will be of growing importance. Promising experimental methods are evaluated which will be of aid in attacking such difficult problems as plasma reflectivity, bistatic characteristics of extended targets, and near zone scattering. It is anticipated that the electromagnetic scattering properties of matter in shaped and distributed form will provide the essential information needed for space exploration and many vital military operations.

The use of coherent detection can theoretically allow optical radar systems employing laser transmitters to achieve considerably improved receiver sensitivity, particularly in conditions of high background radiation. However, there are many practical factors that can limit sensitivity in coherent optical detection, which are described. It is shown that to achieve an efficient coherent optical radar, one would generally like a pulse width less than 10 µsec and a spectral line width less than 10 Mc.

A review is given of detection and estimation methods in current use in radio and radar astronomy. Methods of determining position and brightness distribution of radio sources by narrow-beam antennas, interferometry or lunar occultations are discussed. Observational procedures for the estimation of frequency spectra are explained. The fundamental problem in radar astronomy, viz., that of mapping, is reviewed, and several methods for the removal of ambiguities are considered. Schemes for the determination of range and velocity of a planet are also described.

Radar backscatter from the ground was investigated in May, 1966, by aircraft and rocket flights over the Tularosa Basin area in New Mexico. An L-band radar with a peak power of 100 kW was flown to an altitude of 165 km, where echoes in orthogonal polarizations were monitored. The purpose of the experiment was to observe the behavior of the radar echo as a function of altitude and subsequently to relate the echo behavior to the measured electrical characteristics of the surface. Soil samples taken from the target area and measured in the laboratory had an average moisture content of 3.6 percent, dielectric constants between 2.8 and 4.8, and loss tangents between 0.001 and 0.2. The loss tangents computed from penetration measurements were somewhat higher (0.37). In most cases, the echo power amplitude appeared to vary as the inverse altitude squared for both the direct- and cross-polarized vertical incidence echo components. The average values for dielectric constant and loss tangent computed from the echo characteristics were e = 3.4 and tan Δ = 0.25, which are in fair agreement with the soil sample measurements. An unexpected result was the observation of a specular cross-polarized echo component.

As an experimental verification of a novel application of the laser-Raman radar, frequency-shifted Raman components of a Q-switched ruby-laser pulse from atmospheric molecular gases and from chemicals in an oil plume, such as CO 2 , CO, SO 2 , O 2 , and N 2 , were analyzed selectively. These results demonstrate the practical usefulness of this new optical system as a real-time air pollution probe.

Integrated electronic techniques have been developed to the point where they can be applied to many types of military equipment. This paper discusses how these techniques are being used to increase the reliability and reduce the size and weight of three selected types of equipment used in military communications and radar systems. Redundancy is used in the first of these equipments, an ultrareliable UHF transceiver. The two additional equipments, a VLF/LF frequency synthesizer and a radar receiver, are greatly reduced in size and weight by the direct application of microcircuit techniques. The basic problems created by system size reduction are discussed, and the design and fabrication techniques used for the three equipments are described.

This paper outlines the fundamental aspects of the technology of sonar. Particular attention is given to the properties of the ocean that limit the propagation of underwater sound and to the inherent background noises that limit the performance of sonar. The basic choices that are available in design to meet these limitations are discussed and the present state of the art is described in broad terms.