Patent GB-1401273-A (GB)
Title: Secondary radar receiver system
IPC: G01S3/46
Filed: 1971-06-30
Abstract: 1401273 Radio direction-finders; aerials A C COSSOR Ltd 29 June 1972 [30 June 1971] 30703/71 Headings H4D and H4A A secondary radar receiver system of interferometer type comprises a plurality of pairs of aerials which are arranged along a common baseline, with the spacings between the two aerials of each pair forming a decreasing progression. A phase sensitive circuit is associated with each pair of aerials and provides digital indications showing whether their received signals are in phase, or are out of phase with the one lagging the other, or are out of phase with the other lagging the one. Digital circuitry responsive to the combination of digital signals from all aerial pairs provides an output digital representation of the number of phase difference nulls off broadside of the most widely spaced aerial pair corresponding substantially to the direction of the received signal. One aerial may be common to all pairs, and the pair spacings may form a binary geometric progression. As described, an embodiment with nine pairs of aerials operates at a frequency of 1090 MHz (27À53 cm. wavelength) and covers a range of angle # from broadside of Œ46 degrees, with a quantizing step in # not greater than 0À116 degrees. The spacing S for the widest pair is 643 feet, and the spacings for the other pairs are, respectively, S/ 2 <SP>1</SP>, S/ 2 <SP>2</SP>,...S/ 2 <SP>8</SP>. At each phase difference null of the widest pair there is a particular digital code in respect of the other pairs, i.e. in terms of -1, 0, + 1, where -1 represents a lag, 0 a null, and +1 a lead. The system remains ambiguous as to quadrant but, where necessary, a tenth aerial pair can be added, with a spacing of less than a half wavelength, to effect resolution. Each aerial actually consists of two back-to-back slotted waveguide aerials arrays 12, Fig. 3, to obtain forward and backward coverage, which are supported by a tower 10 on a concrete base 11. The arrays radiate through horns 13 and may be relatively high, e.g. 10 metres, for air traffic control operations. A trench 14 runs the length of the baseline to carry waveguides 15 extending from the aerials to a building 16. A duplicate system may be provided on a second baseline 17 which is perpendicular to the first baseline, whereby full 360 degree coverage may be obtained. The pairs of aerials are split into three groups of three each for signal decoding, a typical group being shown in Fig. 4a. The signals due to each aerial of a pair are phase compared in a detector 21, and if the output thereof is above a predetermined threshold value one of two monostable circuits 22 (+) or 23 (-) is set at an instant determined by an azimuth strobe signal applied on a line 27. If the output from the detector 21 is not above the threshold value, an AND gate 24 is enabled to allow a monostable circuit 25 to be set by a zero correlation pulse applied on a line 26 after the strobe pulse on the line 27. Coded information in respect of the three pairs of the group is thus obtained on a set of nine lines 28. Decoding circuits (Fig. 4b not shown) include AND gates for the various code combinations and staticizing bi-stable circuits operated therefrom. The AND gate for the all-zero code operates one of two bistable circuits, as determined by a signal from an aerial pair in another group. A special purpose computer for processing the digital values obtained is described (Fig. 5, not shown) although a suitably programmed general purpose computer may be used instead. In another embodiment, each pair of aerials is associated with two phase sensitive detectors, to one of which one of the signals due to the aerials is applied with a phase delay of 180 degrees (Fig. 6, not shown). Consequently, virtual interlaced lobes appear in the phase against angle diagram (Fig. 7, not shown) providing a fourcharacter code for enabling unambiguous interpretation. Interrogating means associated with the receiver system may comprise six radiating horns spaced equiangularly in azimuth at the top of a tower, to give overall coverage (Figs. 8a, 8b, not shown). The horns are fed by a transmitter controlled by a computer, and they may be energized in any desired order, according to the azimuths of aircraft under interrogation. The vertical aperture of each element of such an array may be made very large, for example thirty times the wavelength, so that the radiated beams are narrow in elevation, and said beams may be so directed as to eliminate ground interference (Fig. 9, not shown). An omni-directional aerial, such as could be used in a selective address system when only one aircraft is interrogated at a time, may consist of a stack of bi-conical horn radiators (Fig. 10, not shown). A system is described (Figs. 11, 12, not shown) in which an aircraft transponder repeatedly radiates its address (but no other data) for example, once every second. A supplementary interferometric receiver system is adapted to receive the address code thus radiated, to determine the azimuth of the transponder, and to cause the interrogating transmitter system to interrogate the transponder on the correct azimuth. Consequent replies from the transponder are received on a main interferometric receiver system.