Patent GB-1499327-A (GB)
Title: Radio navigation beacon
IPC: H01Q21/06
Filed: 1974-07-30
Abstract: 1499327 Radio navigation equipment STANDARD TELEPHONES & CABLES Ltd 29 July 1975 [30 July 1974] 33574/74 Heading H4D Radio navigation equipment for use in, e.g. Doppler MLS as described in Specification 1,234,541 or in a pulse or CW direction finding arrangement, comprises a plurality of aerial elements arranged in a partially filled planar array of rows and columns and means for cyclically scanning the elements in pairs, the elements of each pair lying in the same row (for azimuth scanning) or column (for elevation scanning), such that the spacing between the elements of each successive pair changes by a fixed incremental amount, successive pairs lying in different rows (or columns). The pairs are pre-selected in random fashion whereby the moving beam so produced is substantially unaffected by obstacles and surfaces which would produce unacceptable multipath interfering signals in prior art systems. In a simple exemplary embodiment, Fig. 1, the partially filled array has 28 elements arranged in positions of an orthogonal lattice comprising 16 horizontal places and 12 vertical places. Fig. 2 (not shown), in tabular form, gives the commutation cycle for a complete scan period. Over the first three steps radiator pairs (x 1 y 5 )(x 16 , y 5 ); then (x 1 , y 12 )(x 15 , y 12 ); then (x 3 , y 1 )(x 16 , y,) and so on are chosen so that the x coordinate difference follows the sequence 15, 14, 13 ..., 0, ... - 15 over a scan period. As embodied in a Doppler MLS beacon, Fig. 3, R.F. source S feeds rotary phase shifter P at frequency F a tapped circular line of length just under one wavelength, rotated back and forth at a speed such that one output is at a frequency (F + f) and the (F-f) where 2f is the displacement frequency required between the two radiations A and B. Commutators CA and CB are programmed to the required energization sequence. The received and detected signal in the airborne receiver comprises the beat frequency between radiations A and B i.e., the offset frequency 2f displaced by a value which is proportional to the cosine of the angle of aircraft beaming with respect to the horizontal line of the antenna array. The above described scan is for azimuth, elevational scanning being performed by choosing pairs of elements having for example constant x-coordinate positions, and one antenna array may function in both elevation and azimuth using FDM or TDM techniques. As embodied as a CW direction finder, Fig. 4, one receiver R 1  samples antennas A 1 , A 2 , A 3  to produce one wavetrain while receiver R2 samples B 1 , B 2 , B 3  &c. where A 1 B 1 , A 2 B 2  &c. represent successive selected pairs of elements. In R1 the I.F. is offset frequency f from the I.F. in R2; and owing to successive phase steps the centre of the spectrum thereof is at frequency f(1 + ksin#) where # is the signal bearing angle from the antenna array boresight axis; k is a constant. In another embodiment (Fig. 5, not shown) a pulse signal direction finder is described wherein each antenna element has its own receiver. It is further stated that the invention may be applied to low angle tracking radar.