GB1229786A - - Google Patents

Abstract

1,229,786. Aerials; pulse radar. MARCONI CO. Ltd. 13 May, 1968 [21 Aug., 1967], No. 38407/67. Headings H4A and H4D. In a radar, the elements of a transmitting aerial array are energized with coherent oscillations of different frequencies, so that a directional transmitted beam traces a predetermined path in space at a predetermined rate. Reflected signals from targets are effectively coherently scanned at a higher rate than the said rate at which the transmitted beam traces its path. As described, a planar transmitting array TA, Fig. 1, comprises 24,090 electromagnetic horns, arranged in 110 columns and 219 rows. The nominal mean operating frequency is 3000 Mc/s., and the spacings between the centres of the horns are half-wavelengths. Adjacent horns in each row are connected or coupled by waveguide links at the back of the array so that, when fed at one end, scanning in azimuth is obtained when the frequency is varied. For this purpose, outputs from a stable oscillator SO and from a selector switch SS, which is associated with a multiple frequency source MF, are connected to respective inputs of a mixer MM, which feeds all rows through a frequency multiplier FM, mixers M1 to M219, and gates G1 to G219. The switch SS is actuated by a source SD, and provides 90 outputs such that the output from the multiplier FM is centred on a frequency of 2860 Me/s., and changes in 90 steps of 2 Mc/s. each, which are equally distributed about that frequency, in 2073À6 milli-seconds. Each of the mixers M1 to M219 receives an input from the multiplier FM and another respective input from a coherent multiple frequency source MFS, which is controlled by a master oscillator MO. The source MFS provides frequencies which are centred on 140 Mc/s. and which are stepped progressively on either side thereof in increments of <SP>1</SP>/ 720 Mc/s. Thus, when the switch SS is in its central position, and a frequency of 2860 Mc/s. is being supplied by the multiplier FM, the central row of horns is supplied at 3000 Mc/s. Vertical scanning is effected in sweeps each of duration 720 micro-seconds, and azimuthal scanning is effected in steps each of duration 23À04 milliseconds. The gates G1 to G219 are operated by a unit D controlled by the master oscillator MO and they cut off power supply to the rows of horns during alternate periods of 720 microseconds, i.e., at a frequency of 10<SP>6</SP>/1440= 695 c/s., corresponding to the pulse repetition frequency of the system. The beam dimensions are about 1 degree horizontally and ¢ degree vertically, and the vertical sweep is 60 degrees. As vertical scanning takes 720 micro-seconds, a target is illuminated for a time of about ¢ x <SP>1</SP>/ 60 x 720 = 6 micro-seconds. In an alternative arrangement, azimuthal scanning is obtained by means of ganged phase shifters between the horns in the rows, instead of using a multiple frequency source MF and a selector switch SS. A receiving array RA is provided which is a counterpart of the array TA. In this case, mixers 1M to 219M receive respective first inputs from rows of horns 1R to 219R, and respective second inputs from a multi-frequency coherent local oscillator unit LMF, which is controlled by the master oscillator MO, and which generates frequencies which are spaced from each other by <SP>1</SP>/ 6 Mc/s. Effective vertical scanning is thus effected at a higher angular speed than that of the transmitted beam, and corresponds to the calculated time of illumination of a target, i.e., 6 micro-seconds. Outputs from the mixers, after being passed through band-pass filters 1B to 219B (band-widths=<SP>1</SP>/ 6 Mc/s.), are combined to give a wide spectrum signal which is fed to utilization means U, which may be of computer type producing data representative of target height or elevation, azimuth, range, and velocity. Means are also described for overcoming elevation ambiguity in the receiving equipment arising because of the presence of more than one principal receiving beam. In one arrangement (Fig. 2, not shown) each output from the local oscillator unit both directly and via a delay circuit (having a delay equivalent to the received pulse length) to the two inputs of a gate, which is controlled by the unit D, Fig. 1, and the master oscillator MO. Output from the gate is then taken to the appropriate receiving mixer. In an alternative arrangement (Fig. 3, not shown), two mixers are provided for each row of horns and one is suppled directly from the local oscillator unit, and the other through a delay circuit. The outputs from the two mixers are connected to a gate, which is controlled as in the first arrangement, and provides output to the appropriate band-pass filter.