Patent GB-955175-A (GB)
Title: Inertial system alinement
Inventor: FISCHER JOHN JOSEPH
IPC: G01C25/00
Filed: 1961-05-05
Abstract: 955,175. Gyroscopic apparatus. NORTH AMERICAN AVIATION Inc. May 5, 1961, No. 16506/61. Heading G1C. An inertial navigation system for aircraft having axes X 1 , Y 1 , Fig. 1a, which, before take-off, are displaced through an error angle Ï from the desired directions X o , Y o , is aligned by moving the aircraft 10 from point 11 to point 12a so that at the latter position the aircraft has its transverse displacement zero, determining the transverse displacement as measured by the inertial navigation system as a measure of Ï and moving the axes accordingly. The aircraft is moved approximately along the line 12 from point 11 ensuring that it is on the line 12 at point 12a, the lateral displacement L as measured by the system being indicative of Ï. Line 12 may be an accurately surveyed line on a runway and the system, Fig. 5, has three modes; A for fast-levelling and set-up prior to take-off, B for take-off during which the alignment error is measured and C azimuth correction and normal operation. In the locally earth level system shown, during normal operation a gimbal mounted platform 16 is maintained horizontal and orientated so that the X axis points in a fixed direction, e.g. North, by means of three single degree of freedom gyroscopes 20, 21, 22 having sensing axes along the X, Y and Z axes respectively, the outputs of pick-offs 23, 24, on gyroscopes 20, 21, being fed via azimuth resolver 26 and amplifier demodulators 27, 28 to the gimbal servomotors 30, 31 whilst the output of pick-off 25 on gyroscope 22 is fed, in the C mode of operation, through amplifier demodulator 29 to servomotor 32. The platform 16 also has mounted thereon a pair of accelerometers 43, 44 whose sensing axes lie along the X and Y axes respectively and feed signals to computor 15 which calculate position coordinates, velocity signals V x , V y  and the gyroscope correction signals W x , W y  and w, which are fed through amplifiers 45, 46, 47 to the respective torques 40, 41, 42 to allow for earth rotation and possibly earth ellipticity. Before take-off the aircraft is in position 11 centred on and aligned along the accurately surveyed line 12 and the system is switched to mode A. The latitude and longitude are set to the appropriate values by conventional means, a reset mechanism maintains the V x , V y  outputs of the integrators in computor 15 at zero and a manual control 81 is operated to set shafts 82, 83 at a position corresponding to the true azimuth of the line 12. Due to the synchro generator 88 connected between the inner and outer gimbals 17,18, and synchro control transformer 85 comparing the position of manually set shaft 82 with shaft 91 on inner gimbal 17, a difference signal is fed through switch S1, in mode A, and amplifier demodulator 29 to Z axis gimbal servo 32, so that the platform is slaved to the airframe in a position determined by shaft 82. The Z axis gyro 22 is slaved to the platform 16 by feeding signals from pick-off 25 via demodulator 92, switch S2 and amplifier 47 to the gyro torques 42. Servo integrators 95,96, comprising amplifier demodulators 97, 98 driving motors 99,100, which in turn drive tachometer generators 101, 102 and the arms 103, 104, of potentiometers 105, 106 or other suitably energized storage devices, have their output shafts 93, 94, maintained at zero by means of servo action through switches S3, S4, in mode A. If the accelerometers 43, 44 provide output signals, these are indicative of levelling errors of the platform 16 about the X and Y axes and suitable outputs from computor 15 feed amplifiers 45, 46, to correct the levelling. On switching to mode B, the accelerometer integrators are freed to indicate V x , V y  outputs, the slaving of the Z axis gyro 22 to the platform is disconnected by switch S2, the synchro slaving to the manual control 81 is disconnected by switch S1, the servo action zeroing of shafts 93, 94 is disconnected by switches S5, S6, and the takeoff run is started. During mode B a resolver 111, which has been positioned to the true bearing of line 12 through shaft 83 and which receives velocity signals V x , V y  from computor 15, supplies signals to servo integrators 95, 96, indicative of the velocities along line 12 and transverse thereto, V R , V L , respectively. The positions of shafts 93, 94, thus indicate distance R along and L transverse to line 12. When the aircraft reaches point 12a, the signal fed through detector 114 overcomes the bias provided by a source of potential 115 and the resultant signal in relay coil 117 switches the system from mode B to mode C. Thereupon the shafts 103, 104, are returned to zero through servo action and during the return to zero, a signal passes through switch S11 now in the C position, and amplifier 47 to the torquer 42 of the Z axis gyro 22 so that the platform is moved through the angle Ï. In a modification the transverse distance L is divided by R and fed to the system as the correction angle Ï.