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1 On aircraft, mapping radars and optical sensors do not present an image of the terrain to the crew, whereas on unmanned aircraft, navigation must be autonomous. 2 Automatic map-matchers have been built since the 1990s that correlate the observed image to stored images of patches of distinctive terrain, choosing the closest match to update the dead-reckoned state vector. 3 Since 1990, aircraft and cruise missiles measure the vertical profile of distinctive patches of terrain below the aircraft and match it to a stored profile. 4 Updating with the matched profile, perhaps hourly, reduces the short-term drift of the inertialnavigator. 5 The profile of the terrain is measured by subtracting the readings of a radio altimeter. 6 An on-board computer calculates the cross-correlation function between the measured profile and each of many stored profiles on possible parallel paths of thevehicle. 7 The on-board inertial navigator usually contains a digital filter that corrects the drift of the azimuth gyroscope as a sequence of fixes is obtained. 8 Hence, the direction of flight through the stored map is known, saving the considerable computation time that would be needed to correlate for an known azimuth of the flight path. 9 The most complex mapping systems observe their surroundings by digitized video (often stereo) and create their own map of the navigated space. 10In the 1900s, optical mappers are being developed that allow landings at fields that are not equipped with electronic aids.
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