13

Navigation—A General Overview

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Fig. 13.8 a GNSS satellite constellations and b GNSS segments

User segment contains the GNSS receivers to compute the position using the

signals receivedfromthesatellites. Thesignals transmittedbytheGNSSsatellites use

the CDMA and spread spectrum principles. GNSS has different signals for military

and civilian users.

The assumption that “velocity of light is constant” and “travel in straight line”

gives some error in the computation of time difference due to refraction of signals in

the Tropospheric and Ionosphere regions, and also due to refraction. These errors are

with respect to time and place due to varying medium characteristics. Similarly some

other errors due to clock drift, receiver noise etc. will add to the errors in position

computation. Hence additional augmentation systems are developed to increase the

accuracy of GNSS. NavIC/IRNSS and QZSS are regional satellite navigation systems

covering only India and Japan respectively, with limited number of satellites. The

main drawback of GNSS for positioning or navigation is interference, multipath and

other errors like ionosphere, drift in clocks, loss of signals etc., which degrade the

accuracy of the position. Of course GNSS cannot be used in the indoor navigation.

RADAR (Radar)—Radio Detection and Ranging

Radar radiates the energy into space in the form of electromagnetic signals. The

properties of radiated electromagnetic energy have made it possible to measure the

range or the distance of the object under investigation. When these electromagnetic

waves reflected by any object, these signals are received again by radar and calculates

the distance between radar and that particular object [8]. Electromagnetic waves

travel in the space with the speed of light (300,000 km/s), meaning it maintains a

constant speed. Radar measures time difference between the transmitted pulse and

reflected pulse. By using the constant speed and time measurements, it permits to