Patent GB-1005379-A (GB)
Title: Improvements in or relating to radar systems
Inventor: MILNE KENNETH
IPC: H01Q3/26
Filed: 1963-02-11
Abstract: 1,005,379. Radar systems; electronically scanned aerials. DECCA Ltd. Feb. 7, 1964 [Feb. 11, 1963], No. 5549/63. Headings H4A and H4D. A radar system, wherein within pulse frequency scanning is combined with received pulse compression, to achieve long - range radar performance together with high values of data rate, range resolution and height resolution, comprises a transmitter feeding a directional aerial with pulses having monotonic frequency variation, the aerial being such that the direction of the radiated beam depends on the frequency of the pulse and changes through at least several beam widths during each pulse, and a receiver including an equalizer having a delay substantially matching the total transmitter and aerial delay at all radiated frequencies so that all received frequencies resulting from the irradiation of a single target by a radiated pulse arrive simultaneously at the output of the equalizer. In the embodiment of Fig. 1 a transmitter 10 generates pulses under the control of modulator 11 having a non-linear frequency modulation and the frequency spectrum shown at (a), Fig. 2, where the frequency deviation is of the order of 200 Mc/s. to 20 Mc/s. These frequency-modulated pulses are fed via a duplexer 12, to a serpentine fed slot array 14 so arranged that the angle of the radiated beam varies with frequency in the manner shown by line 40 at (c), Fig. 2. The radiation pattern of the array varies slightly with elevation and is shown by 41 and 42 at (c), Fig. 2, for two elevation angles # 2  and # 3 . The beam therefore performs a complete scan in elevation during each pulse period. The beam is directed on to a reflector 15 which shapes the beam in the horizontal plane and is rotated together with the array by a motor 19, such that a two-dimensional cylindrical type scan is performed. Alternatively the scanning in azimuth may be effected by providing a plurality of horizontally spaced arrays 14 fed from a common transmission line via electrically controlled phase shifters. A target will produce a plurality of echoes at different frequencies, corresponding to the frequencies of the main lobe of the beam and the side lobes thereof, as they sweep the target. The echo corresponding to the main lobe is delayed from that part of the transmitted pulse which is at the same frequency by an amount proportional to the range of the target, whilst the frequency of the said echo is indicative of the target elevation. The group of echo pulses from one target are fed via a frequency changer 18 to an equalizer 17 which has a variation of delay with frequency shown at ( f ), Fig. 2, which is the inverse of the sum of the transmitter and aerial delays shown at (b) and (d), Fig. 2, respectively. The action of the equalizer is to combine all the pulses due to any one target. The produced pulses may be directly displayed on an " all beam " P.P.I. display 30 or may be fed to data extraction circuits 32 via frequency (i.e. elevation) selective filters 31. One means of utilizing the combined pulses is shown in Fig. 3 wherein a split static beam configuration is synthesized by passing the pulses through frequency band selective filters 53 and 54, the frequency bands of the filters, Fig. 4 (not shown), overlapping to represent overlapping beams. The detected outputs of the filters are passed to conventional scan and difference deriving means to determine the range of the target from the sum signal and the elevation error of the target from the difference signal. The pass bands of the filters may be made variable. A second utilizing means is shown in Fig. 5 whereby ground clutter and high elevation interference may be blanked out. The signals, which are assumed to lie within the band f- B/2 to f + B/2, where B is the width of the band, and f its mean frequency, are equalized in unit 17, Fig. 1, and fed to a subtractive mixer 62. The second input of the mixer 62 is fed with oscillations from a variable frequency oscillator 61, varying in frequency with time from (f-fo+α) down to (f-fo-#) in the manner shown in Fig. 6, the time scale being proportional to range. The beat frequency output of the mixer 62 is fed to a filter 60 having a bandwidth B and a centre frequency fo. The arrangement is such that signals due to near objects at elevations below that represented by the frequency (f - B/2 + α) and those due to distant objects at elevations above that represented by the frequency (f + B/2 - #) are stopped from passing through the filter. Those which do pass through the filter are returned to their original frequencies by a second mixer 63. The band-width and equivalent frequency of the filter may be varied such that responses are only displayed from targets within a given height layer. The band-width demand on the transmitter and the side lobe bevel may be reduced by the insertion of dispersive elements 16 in the feed to the array 14. The elevation scan may be shared between two or more transmitters. For example a first transmitter may feed a low delay aerial to cover elevations from 0 to 5 degrees and a second transmitter in a different frequency band may feed a higher delay aerial to cover elevations above 5 degrees. This arrangement allows linear frequency modulation for both transmitters and linear delay equalizers. The increasing of frequency with time, the increase of beam angle with frequency and the increase of aerial delay with frequency, may all be reversed. The characteristic of the delay equalizer may be reversed in effect by reversing the operation of the local oscillator in the frequency changer of the receiver 18. A suitable delay equalizer is described, Fig. 8, not shown, comprising a combination of parallel and series tuned circuits having the same resonant frequencies. The amplitude of the pulse may be varied during transmission. Specification 1,005,378 is referred to.