Patent GB-870611-A (GB)
Title: Navigation system
Inventor: GREEN DAVID JORDAN
IPC: G01S1/02
Filed: 1957-10-19
Abstract: 870,611. Radio navigation. LEAR Inc. Oct. 19, 1957, No. 32695/57. Classes 40 (6) and 40(7) [Also in Group XXXVI] Relates to an aircraft navigation system comprising (1) a self-contained dead-reckoning system including means for integrating signals representing the velocity vector of the aircraft to give signals representing the position of the aircraft and (2) a known form of radio navigation system producing signals representing the position of the aircraft with respect to an external reference, e.g. the ground. According to the present invention the two sets of position signals are combined to give probability-weighted error signals which are integrated to give correction signals which are combined with the velocity vector signals to correct the position signals produced by the dead-reckoning system. As described the velocity vector signals comprise the true airspeed R' and heading # but alternatively they may be produced by a Doppler radar or by integrating accelerations. The radio position-finding system may use any suitable type of co-ordinates e.g. polar, hyperbolic, rangerange or angle-angle. As shown in Fig. 1 the heading and airspeed signals # and R' are combined in adding circuits 24, 26 with manually inserted wind signals to give ground track and ground speed signals #c, R'c which are converted in 28 to cartesian ground speed components R'ns, R'ew and integrated at 30, 32 to give corresponding cartesian distance travelled components # Rns, # Rew which are combined in adding circuits 34, 36 with manually inserted signals representing the initial position of the craft to give signals Rns, Rew representing the present position of the aircraft. Thes signals are converted at 38 into bearing #t and distance Rt from the ground reference co-ordinate centre of the radio position-finding system which system produces corresponding signals #r and Rr in a receiver 43. Any difference # between Rt and Rr is detected in a subtraction circuit 46 - and applied to a probability weighting network 50, Fig. 6, to produce a non-linear function of #, curve 94, Fig. 7, which, if positive, is integrated at 54 giving a correction signal # R which is combined with the airspeed signal R in the circuit 26; a corresponding heading correction signal # # is derived from #t and #r in the circuits 44, 48, 52 and is combined in 24 with the heading signal #, the net result being that the position co-ordinates produced by the dead reckoning system are brought into correspondence with the co-ordinates derived from the ground reference radio position finding system provided that the distance and bearing errors detected are within predetermined "confidence" limits of the dead reckoning system. As shown in Fig. 6, the error signal # from the subtraction circuit 46 is applied in parallel to (1) an amplifier 72 with a linear output-input amplitude characteristic #a/#, curve 92, Fig. 7, and (2) a varible gain amplifier 70 with a delayed A.G.C. feedback loop 74 giving a non-linear Eo/# characteristic, curve 90, Fig. 7, the two outputs being combined in a subtraction circuit 76 to give a resultant output (Eo-#o) having a characteristic curve 94 corresponding to the probability error curve, Fig. 5 (not shown), of the dead-reckoning system, the point O, Fig. 7, at which the curve 94 intersects the zero axis being the error confidence limit of the dead-reckoning system. The output (Eo-#a) is applied to the integrator 54 through a switch 78 controlled by a relay 80 which is controlled by the output #a from the amplifier 72. If the error signal s increases to the limit point 0 at a time to, Fig. 2, the relay 80 operates to open the switch 78 and to energize a motor in a timer gain control 82 which rotates a potentiometer to progressively reduce the gain of the amplifier 72 thereby progressively increasing the magnitude of the error limits Œ#x as shown by the curves 40, 42, Fig. 2, until the resultant (Eo-#a) is again zero at a time tx; the point X and the curve 94<SP>1</SP>, Fig. 7, show the error limit and the (Eo-#a)/# characteristic at the time tx. At the time tx the relay 80 operates to close the switch 78 thereby rapidly reversing the motor in the control unit 82 so that the confidence error limits Œ#x are rapidly reduced during a period tx-ty, Fig. 2. If AC signals are employed the integrators 30, 32, 52, 54 may be long-time-constant electromechanical integrators which will reduce noise effects and also provide a "memory" when the radio signals fail, i.e. the correction signals will remain constant at their pre-failure value; phase correction networks 56, 58 are provided to compensate for phase changes in the integrators. The operation of the system is discussed as an example of cross-correlation.