V Skim the text in a small group to fill in the spaces with correct prepositions.
1 On aircraft, mapping radars and optical sensors present an image __ the terrain __the crew, whereas __ unmanned aircraft, navigation must be autonomous. 2 Automatic map-matchers have been built since the 1960s that correlate the observed image __ stored images __ patches __ distinctive terrain, choosing the closest match __ update the dead-reckoned state vector. 3 Since 1980, aircraft and cruise missiles measure the vertical profile __distinctive patches __ terrain below the aircraft and match it __ a stored profile. 4 Updating __ the matched profile, perhaps hourly, reduces the long-term drift __the inertial navigator. 5 The profile __ the terrain is measured __ subtracting the readings __ a baro- inertial altimeter and a radio altimeter. 6 An on-board computer calculates the cross-correlation function __ the measured profile and each __ many stored profiles __ possible parallel paths __ the vehicle. 7 The on-board inertial navigator usually contains a digital filter that corrects the drift __ the azimuth gyroscope as a sequence __ fixes is obtained. 8 Hence, the direction _ flight through the stored map is known, saving the considerable computation time that would be needed __ correlate __ an unknown azimuth __ the flight path. 9 The most complex mapping systems observe their surroundings __ digitized video (often stereo) and create their own map __the navigated space. 10 __ the 2000s, optical mappers are being developed that allow landings __ fields that are not equipped __ electronic aids.
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