GB862690A - Radar scanning system - Google Patents

Abstract

862,690. Radar; frequency discriminators. HUGHES AIRCRAFT CO. Feb.11, 1955, No. 4213/55. Classes 40(5) and 40(7) A radar system adapted for use on a moving craft comprises means for generating pulsed high-frequency energy; means for cyclically shifting the frequency of the generated energy in successive steps, each step having a predetermined duration; radiating means coupled to the generator for radiating the energy in a beam having a direction variable over a predetermined scanning sector in response to variations of the frequency of the energy and for receiving echoes of the radiated beam; receiving means coupled to the radiating means and responsive to the echoes; control means coupled to the generator and to the receiving means to vary the direction of the beam in accordance with the steps of frequency variation and for rendering the receiving means responsive at any instant to echoes having the same frequency as that of the radiated beam; and an error signal generator responsive to the deviation of the altitude of the craft from a pre-determined altitude and coupled to the control means for shifting the frequency of the energy fed to the radiating means in a manner to compensate for deviations of the craft from the pre-determined altitude so that the scanning sector is maintained in a fixed direction in space. In one arrangement, Fig. 1 (not shown), there are three pulse radar sets in parallel, each operating at a different frequency and feeding a common aerial through a common multiplexer unit. The sets are operated sequentially by means of a gating circuit so that three different directions are scanned sequentially. In Fig. 2 the frequency of oscillation of R.F. oscillator 204 is controlled by a staircase voltage generator 208 so that it moves sequentially from one frequency to another, for example over three frequencies. A discriminator 205 and feedback circuit 209 provide automatic control of the oscillator to one of the desired frequencies. The output from oscillator 204 is heterodyned with the output from crystal oscillator 210 in unit 206, the output from which is amplified, pulsemodulated, and fed to aerial 26 through a duplexer 33. An output from oscillator 204 is also heterodyned with the output from crystal oscillator 219 in unit 207 the output from which is fed to the receiver mixer 13; thus the I.F. frequency is the difference between the frequencies of the two crystal oscillators 210, 219. A trigger generator 36 triggers the pulse modulator 30 and triggers the oscillators 210, 219 alternately, the oscillator 210 during transmission and the oscillator 219 during reception. The direction of the beam transmitted may be as shown at 45, 46, or 47 according to the frequency of oscillator 204. A unit 510 is provided to control the frequency in accordance with the attitude of a ship or aircraft so that the beam remains pointing in a constant direction in spite of rolling. In Fig. 4 the outputs of three crystal oscillators 306-308 tuned to different frequencies are fed sequentially by a commutator gate 309 to an amplifier 310, the output of which is fed to a transmitter mixer 206, where it is heterodyned with the output of an oscillator 301, and to a local oscillator mixer 207 where it is heterodyned with the output of an oscillator 303 to provide the local oscillations for the receiver mixer 13. Aerials. Fig. 5 shows an aerial suitable for use with the radar systems described above. A screw 402 having a rectangular thread is tightly surrounded by a cylinder 401 so that the thread forms with the cylinder a helical waveguide of rectangular cross-section which is fed by an input waveguide 403. The cylinder is supported at the focus of a parabolic reflector 404 by a bracket 407 and by the bent portion of the input waveguide 403. Slots 405 are provided in the wall of the cylinder facing the reflector; the slots may be of dumb-bell shape, as shown, or may be rectangular or circular. As the frequency of the input is changed the direction of maximum output of the aerial changes in a linear manner, Fig. 13 (not shown). Frequency discriminators. A frequency discriminator for producing an output proportional to the deviation of a signal from one of three predetermined frequencies comprises an input waveguide 250, Fig. 3, which divides into two branches coupled to two other waveguides 255, 256 by two sets of resonant slots, one set tuned just above the predetermined frequencies and the other tuned just below. Rectifiers 257, 258 are arranged in waveguides 255, 256 and are connected with their outputs in opposition to each other, so that the resultant output from terminal 276 is proportional to the deviation of the signal from one of the predetermined frequencies.