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Measurements of the velocity of propagation of pulses of radio waves through the atmosphere using techniques developed for accurate radio aids to navigation are described. The time of travel of pulses between two ground stations is accurately determined in terms of calibration marks generated by an accurate crystal, account being taken of the measured delays in the receiving equipment. The mean value obtained for the velocity of propagation in the atmosphere at ground level is 299 695 ± 50 km/sec. This value does not differ significantly from that calculated from the best optical values for the velocity of light in free space by allowing for the refractive index of the atmosphere.

The field of modern radio astronomy is reviewed and particular mention is made of those techniques which have led to advances in other fields of science. Radio telescopes and interferornetry are described. Cosmic radio sources are discussed and some suggestions made regarding their nature. Radar echoes from the Moon and planets are mentioned and the paper concludes by discussing radio astronomical techniques in space vehicles.

Resonant cavities have been used to measure the frequency and check the performance of radar transmitters. The object of this paper is to describe the factors influencing the design of such cavities to cover frequency bands near 3000 Mc/s, and to estimate their accuracy in the measurement of both the mean frequency of the transmitter output and its spectral distribution about the mean. The principles described are of general application to the design and use of cavity wavemeters of high discrimination.

A general theory of the operation of frequency-modulation systems has been developed which can be applied to either Asdic or Radar, and the information obtained from such a system is compared with that from a conventional pulse system and a multipulse system. It is shown that the frequency-modulation system lends itself to more variations in the design parameters (such as range resolution, bandwidth of transmission, rate of information at the display) than the pulse system; but the system using the greatest bandwidth will give the best echo/background ratio. Where the echo/background ratio is not the all important factor, the frequency-modulation system can be rate than a pulse system. designed to provide a higher information

The influence of the ionosphere on ultra-short-wave radar operation is briefly discussed and an account is given of recent investigations on galactic radio noise and solar radio noise. Unwanted effects from both the ionosphere and extra-terrestrial noise sources are found to be less appreciable as the wavelength decreases.

An apparatus is described with which weak ionospheric echoes can be detected and their heights measured; it makes use of frequency modulation and coherent integration. With a transmitted power of 250 watts and an output bandwidth of 1 c/s the apparatus makes possible the detection of echoes corresponding to a reflection coefficient of 3×10−5 (attenuation of 10 nepers). It is pointed out that the principle might be of use in the design of sensitive radar equipment.

Based on the ideas of Rayleigh and Thomson, incoherent-scatter radar was conceived primarily to measure electron-density profiles. However, completely unexpected results revealed the much greater potentialities of the technique.

One of the major limitations to the efficacy of radar is the presence of responses from land, sea or rain-storms in the vicinity of a target, such responses being generally known as "clutter." If the clutter is of amplitude greater than the target-echo, detection of the echo is very difficult; in many cases, however, the clutter is of amplitude smaller than the target-echo, but the latter is lost owing to saturation of the receiver or display equipment by the clutter. It is shown, both theoretically and practically, that such loss can be prevented in the cases of sea and rain clutter by the use of a receiver, the amplification characteristic of which is such that the amplitude of an output signal is proportional to the logarithm of the amplitude of the corresponding input signal; in the case of land clutter, loss is not necessarily prevented, but the probability of such loss is reduced. A logarithmic receiver compresses input signals of any strength less than 100 db above the mean noise power, into output signals of voltage less than four times the output-noise deviation voltage, with negligible change in the detectability of small signals compared with the use of a linear receiver. In addition, with such a receiver, the difference in voltage of two output signals depends only on the ratio of the corresponding input signals, and not on their absolute amplitudes. These properties make a logarithmic receiver superior to a linear receiver for a number of subsidiary applications, some examples being given in the paper.

The propagation factor, introduced into the radar equation to account for forward scattering from the Earth, depends on polarisation. Equations are given that express the propagation factors for circular polarisations in terms of those for linear polarisations.

Normalised series-expansion coefficients are presented for the numerical design of the surfaces of extremely short-focal-length/wide angle asperical scanning lens antennas with symmetrical off-axis focal points. Based on theoretical techniques previously developed for the problem, the normalised numerical results presented assume an index of refraction of 1.594 and a fixed off-axis angle of 20° for design purposes. Included also are curves of certain related design parameters, and information on lens-antenna performance characteristics in the form of typical scanning diffraction patterns for radiation or reception. Some worked examples are given.

High-frequency scattering of a plane wave by a finite conducting, rectangular cylinder is treated by geometrical diffraction theory. Formulas appropriate for calculating principal-polarisation radar cross-sections as a function of frequency and azimuth aspect angle are presented. Calculations on a short cylinder are in very good agreement with monostatic measurements.

This paper is a mathematical analysis of a new type of phaseswitched radar which uses four antennae, two for transmitting and two for receiving. By phase-switching the pulses of one of the transmitting antennae and by multiplicative detection of the outputs from the two receiving antennae one can obtain a two-way power pattern proportional to the product of the four directivity (field strength) patterns of the system's four antennae. Furthermore, by appropriate phase and amplitude switching of a single array's element currents it is possible to simulate four coincident arrays whose product pattern can be obtained. The greater flexibility of this system (four pattern functions instead of one) is demonstrated byexamples of improved pattern synthesis. Comparison is made with a conventional array of the same size whose pattern beam-width is the same as that of the phase-switched system; the phase-switched pattern has lower side-lobes and four times as many nulls. The effect of uncorrelated element noise and of a second target (either coherent or incoherent) is also considered. It is shown that for any degree of coherence one can synthesize a better pattern with the phase-switched system.

The requirements for frequency and amplitude stabilisation of the transmitter of a c.w. radar are derived, and loop gain and sensitivity values calculated. It is shown that the desired sensitivity characteristics are provided by a microwave discriminator described previously. The principal criteria in the design of a circuit are considered, and the results achieved with this circuit presented. It is shown that, although adequate performance can be achieved in practice, it is inferior to that which would be predicted.

A digital method of extracting ionospheric information from the Doppler broadening of radar signals is presented. 1 bit correlation, used by Weinreb in radioastronomy, is applied to measuring autocorrelation functions of ionospheric incoherent scatter returns. Calculated correlation functions are fitted to the observations to derive electron and ion temperatures.

It is shown that the improved near-field lateral discrimination, already shown to be obtainable in a pulse radar or sonar by the use of multiplicative signal processing, can be obtained in a c.t.f.m. (continuous-transmission frequency-modulated) system.

Scattering of a plane wave by a finite, conducting, right-circular cylinder is treated by geometrical diffraction theory. Formulas appropriate for calculating radar cross-section as a function of frequency, polarisation, aspect angle and bistatic angle are presented. Calculations for two large cylinders are in very good agreement with monostatic measurements against aspect angle for four linear-polarisation combinations. Contributions from individual scattering centres are plotted. The predicted variation of the cylinder's radar cross-section with bistatic angle corresponds to the bistatic-monostatic equivalence approximation.

The demonstration by Bowles of the radar detection of incoherent radio-wave scattering from free electrons in the ionosphere has shown the possibility of measuring from the earth's surface the electron density the upper side of the F-layer. The distribution in volume and the relative motion of the scatterers gives rise to a range-distributed and Doppler-spread echo on a radar. The choice of the best radar system for the detection of such targets is complex.The paper analyses the response of f.m. and pulse radars to volume-distributed targets, both with stationary and with relatively moving scatterers. The equipment parameters necessary to ensure the best signal/noise ratio are found for the two radar systems, and their merits are compared. The relative performance for a given range-resolution and post-detector integration is shown to depend on the ratio of maximum peak to maximum mean power for the radar transmitter.As an example, the relative performance of particular 90Mc/s f.m. and 400Mc/s pulse systems are compared. The signal/noise performance is similar, but the pulse system is shown to have advantages which render it more attractive.

A method of signal processing the output of a linearly frequency-modulated continuous-wave (f.m.c.w.) radar is described. The system performs a spectrum analysis, and it is based on a matched-filtering technique, similar to pulse compression, using a dispersive delay line and an extra associated frequency-swept oscillator. The main advantages over existing f.m.c.w. radar signal-processing techniques are (a) negligible loss in signal/noise ratio (b) the ability to trade output-signal bandwidth with maximum range (c) the fact that the detected signal output consists of serial time pulses, with the amplitudes and time delays representing the signal strength and range of each target, respectively. Time sidelobes are inherent in any f.m.c.w. radar and also in the signal processing proposed; a method is discussed for their reduction, and the effects of nonlinearities on the system performance are considered. The system is most suitable for use in high-resolution very short-range radar, such as that used for observing harbour docking and airfield surface movement.

When acoustic waves are incident upon a rigid body or electromagnetic waves are incident upon a perfectly conducting body, the amplitude of the scattered radiation usually varies in a very complicated manner with direction of incidence, with direction of observation and with frequency. Even if consideration is restricted to back-scattering, the resulting scattering directivity curves appear to offer no clue to the shape of the scattering body. Yet the visual identification of objects by scattered radiation is an everyday experience. This paper endeavours to clarify the mechanism whereby the eye perceives shape and to see how this is applicable to methods of display in sonar and radar equipment. The analysis, which is qualitative, is based upon a recent theory of the mechanism of scattering of short wavelength acoustic radiation by a rigid body immersed in a fluid medium, and its electromagnetic counterpart.

Some aspects of an f.m. c.w. radar with a sawtooth frequency sweep are considered. The exact beat note for a discrete target is calculated and its Fourier transform is obtained. A scheme previously given by Gnanalingam for producing a coherent system is shown to be only approximately valid, and an alternative method is proposed. The effect of Doppler shift on the return is discussed. Range discrimination is examined critically.

The bistatic scattering matrix for a finite conducting right-circular cylinder has been derived using the geometrical theory of diffraction. Initial evaluation of the analysis showed a very good agreement between theoretical predictions and monostatic measurements of radar cross-section as a function of polarisation and aspect angle. This letter now establishes the validity of that analysis in bistatic applications and for calculating scattering phase as well as radar cross-section.

It is sometimes of interest to compare the radar cross-section for transmitting and receiving a given linear polarisation with that for another linear polarisation. Even if only average values are required, such an experiment may be difficult to implement because echoing characteristics might change within a time interval smaller than that required to change transmitted polarisation. Depending on target characteristics, desired results might be obtained by simultaneously receiving the two linear polarisations of interest and transmitting a circular polarisation. Applicable equations are developed and used as an aid in interpreting recently reported sea-echo data.

Equations are given that express scattering for transmitting and receiving circularly polarised waves in terms of the characteristics for linear polarisation. Special cases are discussed, for which it is possible to estimate the radar cross-section for one polarisation based on data for other polarisations. Knowledge of polarisation characteristics is sometimes helpful for acquiring insight into the scattering mechanisms of complex surfaces.

A method is described of measuring small peak phase deviations from the required characteristic in linear frequency-modulated continuous-wave and pulse-compression radar transmissions. The method is particularly useful for measuring nonlinearities of the order of 1% in wide frequency sweeps.

The performances of silicon p-n-p punchthrough oscillators have been studied experimentally at X band in a coaxial cavity. A range of mechanical tuning of more than 3 GHz has been obtained. The oscillator performance at current densities up to about 200 A/cm has been related to the measured diode impedance. The wide-active-band, sensitive-electronic-tuning and low-noise properties of the punchthrough oscillator suggest promising applications as a microwave signal source, as a local oscillator, in f.m.-c.w. radar and in a.f.c.

A method, accurate to second order in small quantities, is described for estimating the Faraday rotation occurring along a radar propagation path, from radar scattering-matrix measurements. The estimation procedure contains a built-in check on its accuracy.

Chirp-radar pulse compression by means of proton-spin echoes has been experimentally investigated. These experiments are easier than those with electrons, because they are feasible at room temperature, and it is possible to change the relaxation time and resonance width continuously. In radar applications, the main difficulties are likely to be found in leakage and attenuation. These difficulties can be reduced by working at frequencies as high as possible and improving the experimental setup.