Patent GB-1113672-A (GB)
Title: Improvements in or relating to systems and methods for synchronizing electric clocks
Inventor: GRAHAM WALTON
IPC: G01S11/08
Filed: 1965-07-20
Abstract: 1,113,672. Radio navigation. CONTROL DATA CORPORATION. 20 July, 1965, No. 30803/65. Heading HAD. In a system for synchronizing electronic clocks of a plurality of movable and/or fixed stations a transmitter at each station emits a radio signal as a synchronizing signal at a predetermined time related to a timing signal of its clock, each station includes a receiver for detecting the synchronizing signals from other stations, and circuit means at each station responds to its own synchronizing signal and that received from one other station to synchronize the clocks of the pair of stations, the clocks of pairs of stations being synchronized successively without any one station serving as a master station. In the system described an operating interval, assumed to be one second, is divided into 548 transmission positions, the first position being a start pulse B, positions 2, 4, 6 . . . 200 being for ground station information pulses G, positions 3, 5, 7 . . . 199 being for aircraft station information pulses A and positions 201 . . . 548 being for interrogation pulses I for obtaining and maintaining synchronization. No station transmits G or A pulses until it is synchronized and each station transmits a B pulse. During synchronization time each station transmits a reply pulse R in the interval between the position at which it transmitted its own I pulse and the next position, and an R pulse is only transmitted by a station after the first I pulse from another station is received, that R pulse being used by the other station to achieve synchronization. Synchronization between two stations. Fig. 2.- When two stations X, Y are in synchronism the situation is as illustrated in Fig. 2; if they are not in synchronism the interval T RPY  between the transmission of pulse I x  and the reception of pulse R y  at X is not twice the interval between the transmission of pulse I x  and reception of pulse Iy at X and the error is used in an analogue circuit to operate a phase shifter until the error is reduced to zero, Fig. 4 (not shown). Coarse synchronization utilizes preliminary adjustment of time of I pulse transmission from a station based on observation of B pulses from other stations; when a new, unsynchronized station is to become part of a system of synchronized stations it will detect a few B pulses of large amplitude from nearby stations followed by increasing numbers of B pulses of smaller amplitude from further stations. The new station is assumed not to know its distance from the nearest synchronized station (coarse synchronization would otherwise not be necessary) and it sets the phase of its oscillator to transmit its I pulses by synchronizing with the first B pulse it receives, Figs. 6 and 7 (not shown). Synchronization of pairs of stations from manystation field.-In a so-called " green " condition a station receives an R pulse after a single I pulse and is certain that the R and I pulses were transmitted by the same station, and a received R pulse is a response to its own I pulse. If the " green " condition is fulfilled the station is permitted to synchronize with the station which sent the I and R pulses. In a so-called " red " condition a station receives more than one I pulse prior to receiving an R pulse, and no attempt is made to synchronize. Use of pulse pairs.-In the preferred embodiment the I and R pulses are actually pulse pairs, and one pulse pair interval can be assigned to positions 200, 202 . . . and another to intervals 201, 203... to give effectively double the number of positions. Confusion between received pulses is discussed and is mitigated by selection of position rate, attenuation by distance, and the use of pulse pair interval coding. General apparatus at a station.-At each station the general apparatus includes a transmitter, receiver, decoders, synchronizing circuits, gates and pulse generators, Fig. 9 (not shown), and the apparatus for an aircraft station which includes altitude coding is described in more detail, Fig. 10 (not shown); there are included (i) amplitude control on the receiver to eliminate low-amplitude pulses and (ii) a known voltage-controlled oscillator. The apparatus may be used in a TACAN type of system. A circuit for producing altitude-coded pulses consists of a voltage feed (proportional to altitude) from a barometric altimeter passing through a gate controlled by the B pulses to a phantastron yielding a pulse delay as a function of altitude, Fig. 11A (not shown). Other data e.g. heading may be encoded; in an alternative circuit, the altimeter shaft position is digitally encoded, Fig. 11B (not shown). All stations can transmit on the same carrier frequency. Range measurement, Fig. 12A.-Range is measured to other aircraft in the same altitude layer or to a selected ground station; " own " start pulse B o  causes a phantastron 194 to produce a gating pulse (Fig. 12B, c) of a duration less than the interval between two transmission positions and at a time determined by the voltage from a transducer 190 so that the correct transmission pulse is passed through a gate 196 as one input to a bi-stable circuit 198 (Fig. 12B, d) the decoded A or G pulse from the other station (Fig. 12B, e) terminating the output of circuit 198 (Fig. 12B,f) and the duration of the latter pulse giving the required range since all stations are synchronized. When an aircraft wishes to investigate the safety of an intended movement to another altitude layer, transducer 190 produces a gating pulse corresponding to the new altitude layer and other aircraft are informed of the impending change. Further circuit details in block diagram form are described: the synchronizing circuit includes a pulse counter associated with a clock pulse generator and gates operated according to whether the synchronization error is leading or lagging, Figs. 13A, 13B, 14 (none shown); a pulse position selector for generating B, A and G pulses and fed from a further counter in the synchronizing circuit, Fig. 17 (not shown); an interrogation pulse generator utilizing a random pulse generator, Fig. 18 (not shown); an automatic frequency control of the clok pulse generator of the synchronizing circuit, Fig. 20 (not shown); pulse coding and decoding circuits, Figs. 23, 24 (not shown); and the radio transmitter-receiver which incorporates timevaried gain in the receiver, Fig. 25 (not shown).