Patent GB-1271746-A (GB)
Title: Improvements in or relating to aircraft navigation systems
IPC: G01S1/50
Filed: 1969-06-05
Abstract: 1,271,746. Aircraft navigation computers. BUTLER NATIONAL CORP. 5 June, 1969 [10 June, 1968], No. 28589/69. Heading G4G. An aircraft navigation system for flying courses offset from a radio (VOR) transmitter corrects for slant-range (SR) errors without using an altitude input signal. In Fig. 2, the broken line indicates the ground track, and the continuous line the desired flight path, of an aircraft 10 flying towards a waypoint 12 and using slant-range (DME) and bearing (VOR) signals from a transmitter 11, the track being offset from the transmitter by a distance y at the point of closest approach CPA. Positional error arises because the aircraft uses a signal corresponding to slant-range (marked SR) instead of horizontal range and this error, which is a function of height, reaches a maximum as the aircraft passes the transmitter. A computer for correcting this error without the use of an altitude signal produces analogue solutions of the following equation which may be derived from the Fig. 2 diagram The angle γ is derived from the radial (VOR) angle by subtracting the track bearing and adding 180 degrees. In equation II, y>0 when 180<γ<360 and y<0 when 0<γ<180. It is assumed that altitude remains constant, whereby dh/dt=0. Velocity V is equal to dx/dt. The slant-range computer of Fig. 5, receives a signal dγ/dt on line 60 from a differentiator 22, Fig. 4, a signal γ on line 31 from a VOR receiver 32, Fig. 4, a signal V on line 39 from an airspeed device 70, Fig. 4, and a signal y on line 17 from the navigational computer of Fig. 4. The various elements in box 71 combine these signals to produce the RHS of equation II as a signal on line 111 and this signal is fed through adder 100 and integrator 101 to adder 102 the output of which, on line 19, is the corrected radial angle. Similarly, signals SR and d(SR)/dt are fed respectively over lines 42, 50 to box 72 which produces a signal corresponding to the RHS of equation II, this signal being fed through adder 112 and integrator 113 to produce a corrected off-course signal on line 28. In the navigational computer of Fig. 4, the slantrange computer 16 is shown connected to DME receiver 34 and differentiator 22 for supplying SR functions, and to VOR receiver 32 and differentiator 24 for supplying radial angle functions. The signal output on line 28 represents the corrected SR and is fed to a meter movement 291 which controls a vertically movable indicator 53A. The corrected radial angle signal is fed over line 19 to an electromechanical transducer which positions an OBI differential and the input windings 236, 237, 238 of a control transformer 239. The output winding 241 is coupled to a servomotor 251 which positions the windings of a resolver 233. The servomotor 251 also controls a tachometer 253 having an output winding 256 which forms part of a negative feedback path 258, 259 to the motor amplifier 245, whereby a null-seeking servo arrangement having L.P. filter characteristics is provided. A signal proportional to the corrected SR is applied via lead 18, servo system 46, potentiometer 214 and winding 232 to the resolver 233 where it is resolved into N-S and E-W components. The range # and bearing # of the waypoint from the transmitter are respectively set on the potentiometer 271 and resolver 401. Track resolvers 277 and 402 are coupled through a servo arrangement 312, 313, 315, 316 to a control transformer 314 and synchro transmitter 328 which is manually set according to the track of the aircraft. The resolvers 277 and 402 serve to rotate the axes of the waypoint bearing ## and the resolved slant-range so that they are presented relative to the track. The distance y is derived from one of the windings of resolver 402 and fed through amplifier 285, detector 286, and damping circuit 292 to the SR computer 16. A signal proportional to the displacement of the aircraft from the selected track to the waypoint is derived from the winding 281 of resolver 277 and fed through amplifier 283, detector 284 and damping circuit 298 to the meter movement 297 which controls a horizontally movable indicator 54A. The various filters and damping circuits serve the purpose of limiting the response rate of the instrument 52A. Switch 274 and potentiometer 273 provide scale setting.