Patent GB-978710-A (GB)
IPC: G01C21/16
Abstract: 978, 710. Navigation systems. GENERAL PRECISION Inc. March 15, 1963 [March 21, 1962], No. 10390/63. Heading G1C. An inertial navigation system comprises a platform with mutually perpendicular X, Y and Z axes, the X-axis being adapted to be aligned with true north, or other reference position, the Z axis being adapted to be aligned with the vertical, and inertial means e.g. at least one accelerometer, to produce a signal representing the platform velocity along at least the X-axis, velocity combining means to produce a signal varying in accordance with the difference between the velocity represented by the accelerometer signal and the velocity of the platform along the X-axis determined from a received Doppler signal, and means responsive to the output of the velocity combining means to pivot the platform about the Z-axis to reduce said output The platform is mounted to be freely rotatable with respect to the aircraft frame by a conventional four-gimbal arrangement. Two gyros are mounted on the platform, one being adapted to maintain the system aligned with the vertical (vertical gyro) and the other adapted to maintain alignment with true north (azimuth gyro). The gyros are connected with servomechanisms which maintain the platform alignment and two accelerometers are also provided to sense accelerations of the platform along the X and Y axes respectively, the output signals of the accelerometers being fed to the system. Fig. 2 shows an overall block diagram of the system and the pilot may select by means of a control panel 11 either a flight align programme 13 or a ground align programme 15. Each programme follows an automatic sequence of operations. The ground align programme will give more accurate information for the first part of the flight, but requires 15 mins. on the ground to set up before takeoff. In emergencies the flight align programme 13 is used. When the programme 13 is initiated it operates the system in a coarse alignment phase 17. Power is applied to the gyro rotors for 15 secs. and they are then allowed to coast whilst alignment takes place. As soon as taxiing begins an acceleration switch switches the system to a phase 19 in which is operates in an undamped Schuler mode, true north shared in azimuth, and ground speed outputs are obtained from the platform accelerometer outputs. On receiving a Doppler signal, the programme 13 switches the system to operate in a flight fine level alignment phase 21, which refines the level achieved during the phase 17. In this phase a damped Schuler operation is used making use of the reference Doppler signal as previously described. Finally the programme 13 switches the system to operate in a flight fine azimuth alignment phase 23 in which the error in velocity is fed to both the vertical and azimuth gyros. This causes the azimuth gimbal to align itself to true north. The system continues to operate in the phase 23 supplying navigational data. The coarse alignment phase 17 may be skipped, so that the programme can be initiated during flight, but the system then takes longer to reach the steady state. When the ground align programme 15 is selected it initiates first the phase 17 which operates as in the programme 13, and this is followed by a ground fine levelling alignment phase 25 and ground fine azimuth alignment phase 27. These phases are closely related to their counterparts 21 and 23 in the programme 13 except that no Doppler reference is required. As soon as taxiing begins the system is switched to operate in an undamped Schuler mode phase 19 until a Doppler signal is received when the systemswitches to a damped Schuler mode phase 29 in which navigational information is supplied. For flights over polar regions, the pilot may select a phase 31 in which the system operates in a grid mode and is unslaved from its true north co-ordinate. After operation in the phase 31, it is desired to again operate in a true north slaved mode the system is switched to operate in an offset gyrocompassing flight phase 33 in which the system is gyrocompassed to the offset angle assumed during operation in the phase 31. The system for each of the phases of operation is described in more detail in the Specification allowances being made for the rate of rotation of the earth and the meridian convergency rate.