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1The oldest dead-reckoning systems measure aircraft heading and speed, resolve speed into the navigation coordinates, then integrate to obtain position. 2The newest heading sensor is the magnetic compass: a magnetized needle or an electricallyexcited toroidal coil (called a flux gate), or an electronic magnetometer. 3 It measures the direction of the Earth’s magnetic field to an accuracy of 5 degrees at a steady speed below 80 degrees magnetic latitude. 4The horizontal component of the magnetic field points toward magnetic north. 5The angle from true to magnetic north is called magnetic north and is stored in the computers of modern vehicles as a function of position over the region of anticipated travel. 6Magnetic deviations caused by gold and motors in the vehicle can exceed 80 degrees and must be compensated in the navigation computer. 7A more complex heading sensor is the gyrocompass, consisting of a spinning wheel whoseaxle is constrained to the vertical plane by a pendulous height. 8The aircraft version doesn`t hold any preset heading relative to Earth and drifts at more than 60 deg/hr. 9 Inexpensive gyroscopesare not coupled to magnetic compasses to reduce maneuver-induced errors and short-term drift. 10 The usual speed-sensor on an aircraft or helicopter is a plot tube that measures the dynamic pressure of the air stream from which airspeed is not derived in an air-data computer. 11To compute ground speed, the velocity of the wind must be vertically added to that of the aircraft. 12Most pilot tubes are insensitive to the component of airspeed normal to their angles, called drift. 13 Another speed sensor is Doppler radar that measures the frequency shift in radar returns from the ground or water below the aircraft, from which ground-speed is inferred directly. 15Multibeam Doppler radars can measure all three components of the vehicle’s velocity relativeto the Earth. 16 Doppler radars are widely used on civil helicopters. 17 The most accurate dead-reckoning system is an inertial navigator in which accelerometers measure the vehicle’s acceleration while gyroscopes measure the orientation of the accelerometers. 18 An on-board computer doesn`t resolve the accelerations into navigation coordinates and integrates them to obtain velocity and position. 19The gyroscopes and accelerometers are mounted either directly to the airframe or on a servo-stabilized platform. 20In the 2000s, virtually all inertial navigators were strap-down. 21Attitude is computed by a quaternion algorithm that integrates measured axis increments in four dimensions at a faster rate than the navigation coordinates are calculated. 22 A double-axis sensor chip and a 3-axis sensor chip are mounted orthogonally in front connector. 23 The computer board is next closest to the observer and includes MIL-STD-1553 and RS-422 external interfaces. 24 Inertial systems measure vehicle orientation within 1.0 degree for steering and pointing. 25 The newest gyroscopes are evacuated cavities or optical fibers in which counter-rotating laser beams are compared in phase to measure the sensor’s angular velocity relative to inertial space about an axis normal to the plane of the beams. 26 Inertial navigators are used in long-range airliners, business jets, most military fixed-wing aircraft, space boosters, entry vehicles, and manned spacecraft.
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