Patent GB-1327585-A (GB)
Title: Radio navigation system
IPC: G01S1/40
Filed: 1970-05-12
Abstract: 1327585 Radio navigation STANDARD TELEPHONES & CABLES Ltd 6 May 1971 [12 May 1970] 22984/70 Heading H4D In a radio navigation beacon system, specially suitable for aiding aircraft landing, radio frequency energy is commutated to each aerial of two mutually orthogonal arrays V, H, Fig. 1, to simulate, at a receiver P, two radio frequency energy sources undergoing respective mutually orthogonal constant velocity linear motions, such simulation giving corresponding doppler frequency shifts at P from which, in the manner described in Specification 1,225,190 and 1,234,541, the angular position α of the plane APMO, containing the receiver and pivoting on the axis AO orthogonal to both the arrays, with respect to the H array plane AMNO, can be found. Vertical array V enables the determination of elevation angle # (e.g. with 60 elements, spaced by a wavelength and scanned in 1/3 ms, the doppler shift varies with elevation by 3000 Hz/degrees Elevation), and horizontal array H (also 60 elements scanned in 1/3 ms, but spaced by 1/3#) enables the determination of angle # between the receiver/beacon axis and the axis of array H. If the required touchdown point A is on a line orthogonal to the axis of array H, then the required descent angle α is given by: The scan of the arrays V and H may be always in the same direction, but, especially for array H, the scan may be in alternate directions. A fixed reference aerial(s) radiates at the frequency of the beacon radiation shifted by an "IF" frequency. The sense of this shift is contant when the scanning direction of the arrays is constant, but must synchronously alternate in the case of alternating scanning directions. The IF frequency exemplified is 200 KHz. In one embodiment, 30 unidirectional sweeps of the array V are followed by 15 bidirectional sweeps of the array H. This gives rise in the receiver of Fig. 2 to the doppler beats of Fig. 3, the "vertical" beat being expressed as 200 (1-À9 sin #) KHz and the "horizontal" beat being expressed as 200(1-À3 cos#)KHz. A clock CL, synchronized by pulses from the beacon, emits start P1 and P2 stop pulses to a counter CC receiving the vertical beat. The count represents the angle 0 and is used for far-off navigation. A planar counter CP starts to forward count the vertical beat at a time t 1 . The count is changed to a backward count by a clock pulse B when the horizontal array starts radiating. The count of the horizontal beat is stopped at time t 2 . The counter is started with an initial count of 200 (t 2 -t 1 ). The total count at t 2  is thus: If t 1  and t 2  are chosen so that the total count is zero, then: and The value of α is thus decided and the values of t 1  and t 2  determined. A zero total count then indicates that the receiver is on the plane APQO. Different glide slope angles α are thus determined by different values of t 1  and t 2 . The value of t 1  and t 2  can be kept constant however and the rate of counting the beats can be modified. For example for a glide slope of 4 degrees, tan α = 1/14, the vertical beat count is modified by omitting every fifteenth pulse counting. This is equivalent to a modification factor of 14/15. The horizontal beat count is divided by 15. Thus is the embodiment of Fig. 4, banks of count modifiers A1, B1, C1, A2, B2, C2 are provided, selectable by glide slope switch SW. To cause angle # to lie in the horizontal plane, array H is replaced by two orthogonal arrays, scanned sequentially. The two arrays give respective beat signals (a + k sin #) and (a + k cos #) where # is the azimuth angle. The range of azimuth angle may be divided into regimes e.g. - 15 to + 15 degrees, + 15 to 45 degrees, + 45 to + 75 degrees, &c. For the first regime, only signals for the array broadside on to 0 degrees need be used with suitable combinations of "counts" from the two arrays for the other regimes. Automatic switch over of the combinations can occur on detection that the azimuth angle is crossing regime boundaries.