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.