Patent GB-1301128-A (GB)
Title: Improvements in or relating to air traffic control systems
Inventor: STRATTON ANDREW
IPC: G01S11/08
Filed: 1970-05-20
Abstract: 1301128 Radio navigation system DEFENCE SECRETARY OF STATE FOR 19 May 1971 [20 May 1970] 24413/70 Heading H4D Radio navigation system at an aircaft landing station and on co-operating aircraft enables the aircraft to adjust their spacing or headway possibly in dependence on their speed, such that use of air traffic controllers is reduced and the maximum safe air traffic density is allowed. In the transmitter at the landing station Fig. 1 a stable master oscillator 1 and counter circuits 2 produce a clock pulse train (a) Figs. 2, 3 and cause modulation signal generator 3 to modulate transmitter 4 such that a step frequenting modulated signal (b) is transmitted, each frequencing step lasting s/c where s is the desired aircraft spacing and the modulation pattern repeating nominally at clock pulse intervals. The nearest aircraft to the landing station is tuned to receive and detect the f 0  step; the next nearest is tuned to receive and detect the f 1  step and soon. In the receiver in the nearest aircraft range R 1 , the signal (b) is received as signal (c) after delay td 1  = R 1 /C. A stable oscillator absolutely synchronized with the landing station oscillator 1 (see Journal of the Institute of Navigation, vol. 22, p. 381) produces the clock pulse train (a), the pulse t = 0 of which sets a bi-stable 17 to enable AND gate 19 to pass pulses from oscillator 7 to the forward input of reversible counter 21. The rate of these pulses is such that tr are counted in the step period s/c. The count in the counter starts at tr/2 and increases until the f 0  frequency step is detected (a), whereat a pulse at time td 1  = R 1 /C from 9 resets bi-stable 17 so that gate 19 is disabled and AND gate 20 is enabled so that the oscillator pulses pass to the backward input of the counter. After (td 1  - (S/2c)),  the count reaches tr and a pulse passes through AND gate 22 to set bi-stable 23 and start the reference pulse (d). After [td 1 + (S/2c)) the count reaches zero and bi-stable 23 is reset, terminating pulse (d) and resetting counter 21 to tr/2. If the aircraft is at the correct distance according to the approach programme, the middle of reference pulse (d) is coincident with the f 1 /f 2  transition. An error in its position gives a displacement e 1  of the reference pulse (e 1  = (R 1 /c)-(ks/c)). To detect e 1 , the reference pulse is used to energize an oscillator 11 mixing at 12 with the f 1  and f 2  steps and the two difference pulses (having widths equalling the respective overlaps of the reference pulse and the f 1  and f 1  steps) are filtered out at 13 and 14 and width compared at 15. The resultant valve of e 1  is fed to an indicator and/or aircraft speed control. It is assumed that the aircraft are all travelling at V 0  so that when the next cycle of steps occur, they are nearer by V 0 # and the cycle is retarded by V 0 /c whereby they should be received at the aircraft at the correct time, i.e. if for aircraft 1, the error e 1  has been used to correct its speed, then the next reference pulse should exhibit no positional error e 1 . The speed is assumed to be V # V 0  and so an error e 2  exists and is compensated for. By each aircraft locking on to a respective frequency step-i.e. the second nearest to f 2  and the nth nearest to fn, all the aircraft will eventually acquire the programmed spacing and speed. The widths of the frequency steps can differ if the aircraft are to have different spacing and speeds. The doppler frequency component in the received signals may be determined to give the aircrafts true speed.