IV Scan the text with a partner or in a small group to decide if the sentences are false or true.

1 Navigation systems can be categorized only an absolute navigation systems.

2 Absolute navigation systems measure the state vector withregard to the path

traveled by the vehicle in the past.

3 These are of three kinds: radio systems, dead-reckoning system and celestial

systems.

4 Radio systems consist of a network of transmitters in satellites.

5 Navigation Systems25-3 to the known positions of the stations in the navigation

coordinate frame.

6 The vehicle’s velocity is measured from the Doppler shift of the transmissions or

from a sequence of position measurements.

7 The second of the absolute navigation systems, celestial systems, measures the

elevation andazimuth of celestial bodies relative to local-level and true north.

8 Electronic star sensors are not used in special-purpose high-altitude aircraft and

in spacecraft.

9 Manual celestial navigation was practiced in the air for millennia.

10 Dead-reckoning navigation systems do not derive their state vector from a

continuous series of measurements beginning at a known initial position.

11There are three kinds: those that measure vehicle heading and either speed or

acceleration and those that measure emissions from continuous-wave radio

stations whose signals create ambiguous “lanes”.

12 Dead reckoning systems mustn`t be updated as errors accumulate and if electric

power is lost.

13 The only dead reckoning radio system, Omega, was decommissioned in 1999.

14 Lastly, mapping navigation systems observe and recognize images of the

ground, profiles of altitude, sequences of turns, or external features.

15They compare their observations to a stored data base, often on compact disc.