Patent GB-1235711-A (GB)
Title: Hierarchy clock synchronization
IPC: G01S11/08
Filed: 1968-09-24
Abstract: 1,235,711. Radionavigation. SIERRA RESEARCH CORP. 24 Sept., 1968 [2 Oct., 1967], No. 45292/68. Heading H4D. In a radio navigation and collision avoidance system comprising a plurality of fixed and moving (e.g. airborne) stations, each having an electronic clock nominally synchronized to a common standard time defined by the standard clock at a master (ground) station 1, Fig. 1A, i.e. all clocks in absolute synchronism, each station transmits the estimated time error E(l) of its own (local) clock, compares its own estimated error with the estimated errors received from the other stations and then resynchronizes its clock to match the clock in another station having a smaller estimated error E(o), so that eventually all the stations within radio range of one another will synchronize with the station having the smallest estimated error. As described, each clock defines recurrent 1 sec. epochs, each divided into 1024 time slots, each of duration 5125Á sees., each station being allotted a unique (local) time slot (identified by a corresponding address number) during which it transmits a coded range pulse R, Fig. 3, the distance from another station being normally determined by measuring the time interval between (1) reception of the range pulse from the other station and (2) a local reference pulse (i.e. so called one-way ranging) and resynchronization being effected by comparing an additional one-way distance measurement with the same distance as determined by an auxiliary secondary pulse radar (DME), i.e. two-way radar distance measurement. As shown in Figs. 3 and 4, each station also transmits during each local time slot telemetry data comprising:- (1) the identification (address number) of the local time slot, (2) the estimated local clock error E(l) as produced by a register 44, Fig. 1B, (3) the sync. address, i.e. the slot address of the other station selected for resynchronization therewith, (4) its own altitude and altitude rate. Figs. 1A and 1B show the block diagram of a single station comprising:- (1) a clock unit 23, 24, 26, 28, Fig. 1B, (2) a transmitter 16, Fig. 1A, for transmitting the range pulse R and telemetry data, (3) a receiver 14 for receiving corresponding signals from other stations which are applied to respective stores 22, 32, 40, 42, (4) a DME and transponder unit 18, (5) an altimeter 72. The estimated local error output from the register 44 is compared in a comparator 50 with the estimated errors of the other stations (from store 40) and if the estimated error from one of the other stations is less than the local estimated error then during the next local time slot the address number of said other station (as given by a slot selector 30) is transmitted as the sync. address data by the transmitter 16 and a coded interrogation pulse signal Q, Fig. 3, is transmitted by the DME unit 18. At the said other station the sync. address data received by the corresponding telemetry receiver 14 actuates an " own address unit " 68 to enable the transponder portion of the DME unit 18 which transmits:- (1) a reply pulse T, Fig. 3, in response to the interrogating pulse Q, and (2) a reference sync. pulse F at a predetermined position in the time slot as determined by the clock at the replying station, which are received by the DME unit 18 at the interrogating (local) station and applied to a computer 20 which determines the actual two-way transmission delay time 2 Td and offsets the received reference pulse F by the corresponding one-way transmission delay time Td. The resultant offset pulse F is then applied together with a corresponding local reference sync. pulse from the local clock unit 23, 24 to a time discriminator 60 which gives an error output representing the actual relative error E(a) of the two stations. If the actual error E(a) is consistent with the sum of the estimated local and other station errors E(l) +E(o) (from the comparator 50) over a predetermined number n of epochs as determined by a consistency check unit 52 then a " decision to synchronize" unit 56 is actuated thereby enabling a gate 58 to apply the actual error signal E(a) to correct the frequency and phase of the clock unit 26, 28. If the estimated local error E(l) exceeds a predetermined value as determined by a unit 38 then the unit 38 produces an output which (1) reduces the value of n in the consistency checker. 52 thereby speeding up the resynchronization process, and (2) actuates an alarm 39 (to indicate that the normal distance measurement is not reliable) or blanks the display portion of the computer 20. If the consistency checker 52 determines that the error is not consistent over n epochs then the resynchronizing process is repeated with another station selected by the selector unit 30 and if the inconsistency persists for a plurality of other stations then an alarm 54 is actuated to indicate that the system is failing. If a number of other stations have estimated errors of the same magnitude then the unit 30 selects the other station closest to the local station in altitude. The estimated local clock error E(l) is produced in the register 44 as the arithmetic or root mean square (Fig. 5, not shown) sum of:- (1) basic synchronization circuit delays, (2) the inherited error of the clock to which it was most recently synchronized, (3) a local clock drift error increment which constantly increases from the instant of most recent resynchronization, and (4) a factor dependent on the S/N of the received pulses which compensates for differences in rise time of pulses of different amplitudes. Initial coarse synchronization is effected by control signals from:- (1) a slot comparator 34 which compares the timing of the received slot address numbers with corresponding signals from the local clock unit 23, and (2) the unit 38 when the predicted local error is excessive.