Patent GB-1325475-A (GB)
Title: Aircraft clock monitoring and synchronising system
IPC: G01S11/08
Filed: 1970-11-05
Abstract: 1325475 Radionavigation J P CHISHOLM 5 Nov 1970 [17 Nov 1969] 52779/70 Heading H4D In a radio (or acoustic) navigation and/or collision avoidance (CAS) system in which each mobile unit (e.g. an aircraft) P, Fig. 2, transmits coded pulse signals PP at predetermined instants in a timebase defined by a local clock 14 absolutely synchronized to a system-wide standard timebase to effect one-way ranging in a known manner, the synchronization accuracy of the mobile clock 14 is monitored at a reference (e.g. fixed ground) station A, Fig. 3, of known location producing corresponding pulse signals PA derived in a unit 36 from a clock 27 accurately synchronized to the standard timebase by:- (1) applying the signals PP from the mobile unit P as received at theee spaced reference (ground) stations A, B, C to a computer 25 (located at station A) which computes the position of the mobile unit P as given by the intersection of three hyperbolic iso-  chrones with foci at A, B, C as depicted in Fig. 1 (not shown) and then derives from said position the estimated propagation time # between the mobile unit P and the station A, and  (2) delaying the local signals PA from the unit  36 in accordance with the computed time # so that if the mobile clock 14 is accurately synchronized to the standard timebase then said delayed local signals PA(#) will coincide with the signals PP received at A from the mobile unit P as determined by a time comparator 40. Any synchronization error in the mobile clock 14 will result in a corresponding error output e from the comparator 40 which is relayed to the mobile unit P, e.g. by a telemetry system 23, 24, 17, 19, Figs. 3 and 2, for manual or automatic correction of the mobile clock 14. The location of each mobile unit P may be presented on a display unit driven by the computer 25. As described, the reference stations are fixed on the ground but they may be carried by moving objects (e.g. weather ships or aircraft) of known relation location. Locating station D.-If the chain of reference (ground) stations includes a fourth station D, Fig. 1 (not shown) connected to the computer 25, Fig. 3, and having a clock (corresponding to clock 27 at station A) which cannot be easily synchronized to the standard timebase either because (1) the precise location of station D is not known or (2) it is not in line-of-sight radio communication with the other ground stations, then the precise location of station D can be determined at said station as the intersection of:- (1) a first hyperbolic isochrone defined by the time difference between reception at D of synchronous pulse signals from a first pair of spaced mobile units in line-of-sight communication with all four ground stations A, B, C, D, with (2) a second hyperbolic isochrone defined by a second similar pair of mobile units (or by said first pair after waiting for them to move to different positions), the instantaneous position of each mobile unit being derived from the intersecting hyperbolic isochrones defined by the time differences between reception of a pulse signal from the mobile unit at different pairs of stations A, B and C. When the precise location of station D has been so determined then its clock can be synchronized to the standard timebase by comparing (in comparator 40):- (1) the time of reception at D, as given by D's clock, of a coded pulse signal from a single mobile unit (said time of reception being transmitted to the comparator 40 via a line D<SP>1</SP>), with (2) the corresponding estimated time of reception in standard time as derived by the computer 25 from the actual time of reception, in standard time, of said pulse signal at A and the known location of the mobile unit as determined above, any difference in said two reception times as given by the comparator 40 being applied (via line D<SP>1</SP>) to correct the clock at station D. In this latter mode of operation the coded pulses transmitted by the mobile unit need not be synchronized to standard time. Other groups of reference stations, e.g. (52- 54) and (56-59), Fig. 1 (not shown), may be similarly connected to respective computers (51, 55) corresponding to computer 25, Fig. 3, all the computers 25 (51, 55) being linked to a central computer (50) to establish a common standard time-base.