VI Scan the text to decide if the sentences are true or false.

 

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.