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Raindrop spectra are measured with an electromechanical raindrop spectrometer, responding to drops >0.3 mm in diameter. The radar reflectivity profile is measured simultaneously with a 5-cm vertically pointing radar. Measured raindrop spectra are described by two parameters, N0 and A, the intercept and slope respectively of an exponential distribution having the same water content and radar reflectivity factor as the observed sample. Sudden variations in the raindrop spectra and in the reflectivity profile during widespread rain situations were observed. Due to the parameterization used, these sudden variations of the spectra can be recognized easily as N0 jumps. It seems that the N0 jumps indicate the transition from one mesoscale area within the precipitation field to another. The characteristics of the N0 jumps and the related cloud physics are discussed. An empirical model is proposed for the relation between the type of raindrop spectra and the convective activity of the precipitating air mass.

In this letter, we present the results of a method that uses two Doppler radars to observe the three-dimensional wind field during light snowfall. These radars are designed to scan in a series of tilted planes, passing through the two radar positions. From the mean Doppler shifts in the frequency spectra we can determine average radial velocities within the pulse volumes. Since the two radars do not observe the same volume increments at the same time, spatial interpolation is needed before the radial velocities can be combined into a two-dimensional velocity field at a set of common coordinates within each plane. The estimated terminal fall velocity of the snow is subtracted from the particle velocity to obtain the true air motion in the plane. The air motion component normal to the plane is obtained by integrating the equation of continuity in cylindrical coordinates starting with the surface boundary condition at the plane of zero elevation. The velocity field in the common planes scanned by the radars is then transformed into a rectangular coordinate system with one axis oriented parallel to the average wind direction. After the mean wind field is removed from the results, a well organized “eddy” wind field is evident.

An instrument that has been developed for measuring the deformations in shape that result from the movement of a radio telescope reflector is described. A radar technique is used to measure distances from near the focal point of the reflector to selected points on the reflector surface. The short term accuracy of the instrument is ±0.003 in) and when used on the 140-ft telescope in Green Bank, W.Va., good agreement was found between calculated and measured deformations.

We compute the linear parametric convective amplification of electron plasma waves by an O mode pump propagating vertically into a vertically stratified ionosphere. The upshifted plasma line compares favorably with Arecibo observations, center frequency, spectral width, and possibly amplitude when the radar is aimed in a northerly direction. The dependence of the computed enhanced plasma line shape upon density scale length, electron-to-ion temperature ratio, pump power, density gradient scale length, and backscatter radar incidence angle is investigated.

A 32-element HF-phased array has been used to study the angular spectrum of spread F returns from an ionosphere modified by the powerful radio frequency heater transmitter at Platteville, Colorado. The diagnostic radar is a coherent pulse system that provides an angular resolution of 2.7° at the midband frequency of 7.5 MHz, a range resolution of 30 km, and a Doppler resolution of 0.08 Hz. The results are presented in the form of sky maps (brightness versus azimuth and elevation) of the returns as a function of group delay, Doppler shift, polarization, and sounding frequency. The principal result is that the returns that suffer the greatest group delay at a given sounding frequency come from a direction looking up the earth's magnetic field and exhibit negative Doppler shift. Ray-tracing simulation of the observed dependence of the range and the Doppler on the angle of arrival of the returns supports the view that the heater-generated spread F is caused by field-aligned ducts produced within the heated volume. A simulation run for Arecibo, Puerto Rico, in comparison with that for Platteville, reveals that the group delay spread is about the same and the angular spread is approximately in the same ratio as the heater beam widths at the two locations.

An X-band radar designed and built by the Naval Research Laboratory and capable of continuously variable transmitted pulse durations down to 1 nsec is being flown on a NASA Wallops Flight Center C-54 aircraft to investigate the interaction of radar altimeter signals with the sea surface. There is good agreement between the observed radar altimeter leading edge rise time for a 1-nsec transmitted pulse and the rise time predicted from the radar parameters and the sea state determined by a laser profilometer. Using a threshold tracker on 29-pulse averages for a transmitted pulse of 24 nsec, we found that the range noise varied linearly with the power level of the threshold over a considerable range. The altimeter range quantization was 9.35 cm and a minimum range noise of 3.9 cm was obtained. A comparison of pulse-to-pulse correlation observed in the radar data with that predicted by a Monte Carlo simulation is used to obtain a measure of sea surface scatterer motion.

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.

Phased-array radar systems using wide-band linear-FM pulses suffer from array dispersion. This causes a loss in signal-to-noise (S/N) ratio and in radar range resolution (a broadening of the compressed-pulse width). Depending on the choice of the receiver matched filter (MF) and weighting filter (WF) one or the other of these effects may be minimized. The effects of dispersion in a square planar array with either a parallel feed or a center-fed series feed are studied. Loss in S/N ratio and in range resolution are determined for typical cases, and general curves are given for arrays with arbitrary size, scan range, and signal bandwidth. It is shown that there is an optimum or maximum-useful signal bandwidth for each array, for which the minimum S/N-loss MF-WF design is the same as the ideal-compressed-pulse-shape MF-WF design. For pulses having greater signal bandwidths, time delay steering should be used in the array.

An experiment is described by which the ephemeris of a near-synchronous satellite was determined from passive range observations. The data consist of the measured times of reception at ground tracking stations of electromagnetic signals which are radiated from the satellite at regular intervals. A comparison of the ephemeris to one obtained from Doppler tracking indicates an accuracy of better than 4.9 mrad rms.

Radio scattering measurements have been made of a region of the ionosphere above the Institute for Telecommunication Sciences' ionospheric heating facility at Platteville, Colorado. Strong scattering was observed at frequencies in the HF and VHF bands. The scattering has been shown to originate from electron density fluctuations that are highly elongated in the direction of the geomagnetic field. Radar scattering cross sections of 70 to 80 dbsm are measured in the HF and low VHF bands. The scattering cross section decreases at a rate of 20 to 30 db per octave at the high end of the VHF band. The scattering irregularities are produced by operation of the heating facility with o-mode polarization at frequencies that reflect within the ionosphere. The strength of the scattered energy decreases at a rate of at least 10 db per degree for observation geometries that do not match the geometry for specular reflections from long cylinders aligned with the geomagnetic field. The spatial distribution of irregularities has been estimated for both E- and F-region heating. The temporal properties of the scattering have also been determined including the frequency spectrum of the scattered signals. The processes by which ionospheric heating produces these irregularities are not yet fully explained. Several potentially useful applications of radio scattering from such irregularities are discussed.

The paper presents a model for radio wave scattering from the ionospheric irregularities generated by the intense radio wave transmissions from the Platteville heater. The model is based upon the HF-UHF radar backscatter observations for a physical description of the scattering medium and upon the theory of weak scattering (Born approximation) from anisotropic irregularities for mathematical development. According to the model, the electron density fluctuations responsible for radio wave scattering having an rms value of 1 to 1.5% are aligned along the earth's magnetic field and are distributed in a diffuse pancake-shaped volume of 15 km gaussian thickness vertically and of the size of the heater beam which is 100 km in gaussian diameter horizontally. At radio frequencies the irregularities within the disturbed volume are highly aspect sensitive and consequently only those scatterers which lie on the surface of specularity where a radar views transverse to the magnetic field count in contributing to the received signal. Model computations have been made of the scattering cross sections for monostatic as well as bistatic radar configurations and they are found to be in good agreement with the observations. The radar cross section at a given frequency for bistatic geometry is substantially greater than that for monostatic geometry, all else being equal. Finally, it is shown that the width of the scatter-illuminated bands on the ground is set primarily by the finite dimensions of the scattering volume rather than by the finite aspect sensitivity of the individual scatterers.

A new linked mode of operation has been used to obtain virtually simultaneous measurements by a fully instrumented aircraft and a ground-based high power radar within a train of large amplitude Kelvin-Helmholtz billows in the optically clear atmosphere. Both sets of data were analysed to show the detailed distribution of air motion and turbulence within the billows. The resulting pattern was consistent with a train of vortices each of which was characterized by a vertical wind shear of 6 m s−1 over 100 m, with downward motion of about 1 m s−1 on the downshear side and upward motion of about 1 m s−1 on the upshear side. Between the vortices the vertical shear decreased to almost half the maximum value. The most intense radar echoes occurred within inclined layers connecting the top of one region of maximum vorticity to the bottom of the next. The aircraft, which flew through the upper parts of the billows, encountered turbulence of up to moderate intensity as it penetrated the crests of the billows. Between successive penetrations of the billow crests the aircraft flew close to but just above the region of intense radar echo and encountered smooth air with a slowly varying vertical component of motion.

A coordinated program between satellite, VHF radar, and optical auroral observations was carried out in Alaska during the spring of 1972. On 18 days in February, March, and April the positions of the >130-keV isotropic and cutoff trapping boundaries in the evening sector of the oval were determined on satellite 1971-89a. On 9 of these days, simultaneous VHF 50-MHz auroral radar backscatter echoes were observed to lie equatorward of the cutoff trapping boundary regardless of the phase of the auroral substorm. The analysis of these data with respect to substorm phase, Dst, and position of the aurora shows that the motion of the trapping boundary is a dynamic feature of the substorm that can be monitored and approximately located by the VHF radar echoes.

Rocket borne Langmuir probe measurements of electron temperature in the E-region are examined in relation to recent laboratory investigations of surface drift effects which can lead to erroneously high and time-dependent electron temperature measurements. The rocket data is consistent with the laboratory expectations thus supporting the suggested importance of surface effects in rocket measurements and in relation to the E-region discrepancy with simultaneous incoherent radar scatter measurements.

Total electron contents obtained by the differential Faraday method during 26 near-overhead passes of the Beacon-B satellite at Aberystwyth are compared with the results of integrating vertical N(h) profiles observed simultaneously by the incoherent scatter radar at Malvern, 117 km to the east. In 19 cases agreement within the experimental errors was found with a standard method of analysis of the satellite data, using a constant value of the magnetic field function M ; in three cases agreement could be reached only by allowing for an unsuspected ambiguity of π radians in the measured differential rotation angle; in two cases when the F2-peak was abnormally high it was necessary to adjust M ; in one case the presence of strong waves made the comparison invalid, and in one case there was a significant unexplained discrepancy. Computation of the mean Faraday height, hF, for the 26 incoherent scatter profiles showed that hF is roughly proportional to the F2 peak height, hmax, and that (hF—hmax) has a mean value of about 80 km. There is some indication that the use of the normal first order expression for the total Faraday rotation leads to overestimation of electron content: this problem can be dealt with by an empirical modification of the standard analysis.

This paper discusses the scattering of waves by an irregular boundary. Classical single scattering theory and Kirchoff diffraction theory cannot handle waves at near-grazing angles of incidence nor account satisfactorily for the intensity of the field scattered into near-grazing directions. Problems of this type are of increasing scientific and technological importance in such fields as radio astronomy, terrestrial radio communication, the design of radar and sonar devices, as well as in many geophysical situations involving large-scale wave motions. In this paper a theory of multiple surface scattering is proposed, and described, in terms of a simple model problem of the scattering of sound from a randomly irregular, hard surface. Multiple scattering becomes significant at grazing directions of incidence and scattering, and the theory is expected to give an accurate description of the scattering processes in such circumstances. The analysis is strictly valid only in the case of small surface roughness, and in this limit numerical results are presented which illustrate the considerable divergence between predictions based on the multiple scattering theory and those of single scattering and Kirchoff diffraction theory.

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 use of Doppler ultrasonics provides one of the few, if not the only, methods of noninvasive measurement of blood flow and velocity in the body. In terms of overall accuracy the pulsed ultrasonic Doppler flow meter, operating in a radar-like range-gated manner, currently offers the best solution. However, under certain circumstances this device suffers from an inherent lack of spatial resolution, resulting in the distortion of the measured velocity profiles. The device is modeled as a convolution integral, incorporating the physical characteristics of the flow meter, and those of the velocity field, as established in previous work. The distortion of the measured velocity profile is corrected by solving the integral equation, or “deconvolving” using the discrete Fourier transforms implemented on a digital computer. The methods discussed for obtaining the “true” velocity profile require only a knowledge of the flow meter characteristics and its output. No assumptions as to the nature of the true profile are necessary.

Several features of the maintenance of breaking gravity waves and turbulence in a marine inversion are examined. A formulation is proposed for a critical Richardson number based on a mutual response of the mean and turbulent states to a wave-like disturbance. The energy balance, based on averaged aircraft soundings, is examined to ascertain the order of magnitude of the component terms in the vicinity of a contemporaneous radar echo. Some physical mechanisms are discussed which may explain some aspects of the sustained existence of the echo layers.

Enhancements of various features of the incoherent scatter spectrum are observed when the ionosphere is illuminated with powerful, high frequency radio waves. The radio waves excite plasma instabilities producing lines or more complex spectral features near the local plasma frequency, at the local ion acoustic frequency, near the local gyrofrequency, and near twice the gyrofrequency. The enhancements occur in a thin slab as observed by the incoherent scatter radar and at both upshifted and downshifted frequencies with respect to the probing radar frequency. The enhancements are observed to vary with time when the excitation is held constant and is turned on or off. The high power radio waves are produced by a 160 kw transmitter feeding a log-periodic pair of curtains mounted at the focus of the 1000-ft reflector and covering the frequency range from 5 to 12 MHz. The effects are observed with the incoherent scatter radar using the same reflector and with ionosondes and photometers. The frequencies of the enhanced plasma line and the ion line and their relation to the pump (high frequency radio wave) frequency are predictable from available parametric instability theory. Other spectral features are being explained as the theory develops with the help of the observations. There remain some discrepancies, in particular the asymmetries in intensity, width, and fluctuations of the upshifted compared to the downshifted plasma lines.

Space measurements of electric fields have provided instrumentation for measuring atmospheric parameters and a better basis for understanding the electrical coupling between the magnetosphere and the atmosphere. Applications of an incoherent scatter radar (developed for ionospheric electric field research) to the measurement of atmospheric winds and turbulence and of Langmuir double probes (also developed for space research) for measurement of atmospheric electric fields are described. The increased knowledge of magnetospheric electric fields has focused attention on the electrical coupling between the magnetosphere and the atmosphere with conclusions that should considerably modify previous physical concepts in both domains.

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.

Good agreement was found between the velocity spectra obtained from Doppler radar measurements of the variations in the mean wind field and the spectra of velocity fluctuations simultaneously measured by an aircraft-borne hot-wire anemometer. This experiment is the first in which such a comparison was attempted where the radar return was from fine-scale refractivity fluctuations due to turbulent mixing and not from precipitation particles of artificial scatterers.

Auroral zone E-region neutral winds have been derived from simultaneous measurements of ion drift velocities in different altitudes by the incoherent radar facility at Chatanika, Alaska, on a quiet day before and during the great magnetospheric storm of Aug. 3-9, 1972. The neutral wind expected for a day-night pressure asymmetry appears to be strongly opposed by ion drag and local pressure gradients in the auroral oval.

Previous work on the second-order contributions of radar sea echo by Hasselmann and Barrick is generalized to include the effect of capillarity (i.e., surface tension) in the hydrodynamic part of the transfer coefficient, the lossy dielectric properties of the sea, and the angle of incidence and the polarization of the electromagnetic radiation. The present theoretical results apply to the backscattering of electromagnetic radiation from water waves that are gravity or gravity capillary waves. The introduction of surface tension in the analysis allows for second-order resonant interactions of the gravity capillary wave components. To include resonant interactions of gravity waves, the analysis must be extended to higher order. In general, resonant interactions produce higher-order contributions in the Doppler spectrum at the frequency of the first-order Bragg line that represent the nonstationary and non-Gaussian properties of the water surface. Furthermore, because of the different dispersion relation of gravity capillary water waves, the secondary lines of the second-order Doppler spectrum now are shifted toward the Bragg frequency and 2½ times the Bragg frequency. Numerical results are presented from our generalized theory for various radar and sea (water surface) parameters.

Various shortcomings of modulated subcarrier attenuator calibrators are discussed. Very precise attenuation measurements on short lengths of waveguide have been obtained by inserting into the subcarrier channel a tuned reflectometer system which contains a waveguide switch for setting the zero. Using this technique, total spreads of less than 0.0013 dB have been obtained at 4 GHz in 10 completely independent measurements on the same unknown. The full theory of this method is given in the Appendix. The total range of a modulated subcarrier system has been extended to over 100 dB by using a 1-W microwave oscillator and placing in the subcarrier channel a rotary attenuator with very good resettability and a fully matched two-path configuration that contains two waveguide switches and the unknown.

The influence of the oscillator noise on the minimum detectable signal of a Doppler RADAR with a self-mixing IMPATT-diode oscillator is evaluated. For very short-range RADARs, it is the AM noise which limits the signal-to-noise ratio and thus the range.

Expressions for the interplanetary radar time delay in the one-body problem of an arbitrary gravitational theory are deduced. The structure of the theory enters the result through two functions of one variable,f(r) andq(r), always connected by an identity.

Two theoretical models of shear layers in the atmosphere are examined. The conditions for their dynamic stability are found and their predictions of wavelength to layer-thickness ratio are compared with classical models and with available observational data. Although the models are only rigorously applicable to in-compressible fluids, it is suggested that they also represent conditions in the atmosphere, and clear-air returns published by Katz from the high-power pulse radar at Wallops Island are especially emphasized. Model 2 appears to be able to account for the narrow band characteristic of many of the observed events and also to explain better than other models the observed wavelength to layer-thickness ratios.

This paper briefly reviews the theory and practice of four ground-based radio wave methods of sounding the Earth's ionosphere that came into being since the Second World War. Two of the methods, Wave-Interaction (Cross Modulation) and Partial Reflection (Differential Absorption) were devised as means of investigating the D -region of the ionosphere, which is not open to study through conventional HF sounding. The third high-power radar technique discussed is the Moon-Reflection (differential-Faraday rotation) method of measuring the total content of the Earth's ionosphere. This was developed initially to determine the extent of the ionosphere above the F 2-layer peak. The fourth technique discussed is the incoherent scatter method of sounding the ionosphere. This provides measurements of electron density, electron and ion temperature, some parameters of the neutral atmosphere (including winds) and electric fields. Incoherent scatter presently enjoys considerable attention as it rivals and complements in situ sampling with rockets and satellites.