Patent GB-1296726-A (GB)
IPC: G01S13/76
Filed: 1970-08-27
Abstract: 1296726 Radio navigation SIERRA RESEARCH CORP 27 Aug 1970 [10 Sept 1969] 41313/70 Heading H4D General.-In a modified VOR/DME, TACAN or VORTAC system in which the time that an aircraft needs to spend interrogating a DME ground station to establish range co-ordinates is reduced and the number of aircraft which can use a ground station is correspondingly increased, each ground station is provided with a time clock 16, all the clocks being synchronized by a master clock 18 whereby marker signals are synchronously transmitted to the aircraft and each aircraft is provided with a local clock 43 which is brought into absolute synchronism with the ground clocks by adjustment of the times of reception of the marker signals by means of a standard DME measurement of the aircraft to ground station range, said range then being measured by a one way, time of marker signal reception with respect to the local time clock-method, the initial DME measurement taking up 10% of the measurement cycle and the one way measurement taking up the remaining 90%. The system initially operates in the DME mode continuously until approximate synchronism is established between ground and local clocks and then proceeds to operate in the 10% DME-90% one way mode. Initial DME mode.-The coded DME interrogation pulse groups are produced by p.r.f. generator and encoder 30 and fed via gate 91, there being no signal at its inhibiting input, to the transmitter. A timing signal is simultaneously sent on line 30b via uninhibited gate 97 to the DME digital ranging circuit 31. The reply from the ground station is decoded at 29 and fed via uninhibited gate 93 to ranging circuit 31. A digital representation of the complement of the range R, i.e. (200-R) is produced on line 31b and fed to counter 54. A "reply lock on" signal is produced on line 31a of the ranging circuit. Local clock oscillator 43 has its 5 MHz output converted to 5.4 MHz by frequency multipliers and dividers 44 and 45 and this frequency is fed over uninhibited gate 62 to further divider 40 to produce pulses at 135 Hz, i.e. the frequency of the auxiliary reference bursts (ARB) by the ground station, these bursts constituting the marker signal of the general discussion. The 135 Hz pulses switch bi-stable 53 on such that pulses from a 16.172 MHz oscillator 41-each oscillator representing .01 mile commence to be counted by the 200 mile counter 54. Counter 54 already has a count (200-R) so that it will be full at a time after the 135 Hz pulse representing R. This fullness is signalled by a stop count and reset pulse on line 54a and a corresponding trigger signal which is fed to one input of comparator 58. If the 135 Hz pulses are absolutely synchronous with the transmissions of the burst signals, then the trigger signal should be synchronous with the reception of these burst signals. The received burst signal thus produce respective pulses from decoder 60 which are fed to the second input of comparator 58. As long as synchronism is not near, no signals appear on comparator output lines 58a, 58b, 58c. Gate 79 is thus uninhibited and passes the output from one shot 68 to the input of divider 40, whereby the 135 Hz is rapidly advanced until approximate synchronism is achieved and a signal appears on line 58c of the comparator. During this initial mode a 10%-90% duty cycle output from multivibrator 50 cannot get through gate 85 to control the duration of the DME measurement. One-way measurement mode.-When synchronism has been achieved, the signal on line 58c enables gate 85 so that the duty cycle output pass to enable, during the 10% DME period, transmission gate 35, reception gate 94, and timing signal gate 98. Previously used gates 91, 93, 97 are now inhibited. If it is assumed that the local clock is slightly late with respect to the ground clock, a signal will appear on line 58b which enables gate 70 and passes an additional pulse to divider 40. When the local clock is early, the pulse on line 58a inhibits gate 62 and stops a pulse from passing to divider 40. Faster or slower commands may also be sent to oscillator 43. During the 90% one way mode, a 135 Hz pulse on line 40a will start a range counter 42 counting the .01 mile pulses from oscillator 41. The output signal from burst decoder 60 stops and resets the counter, so that its count, readout and displayed at 32 represents R. During the one way mode, ranging circuit is fed with the same "start" and "reset" pulses via respective gates 95 and 99 instead of the DME interrogate and reply representing pulses. The system may be used with a collision avoidance system, outputs marked "TO C.A.S." being used.