13

Navigation—A General Overview

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c.

Micro-machined silicon accelerometers consists of two parts, sensors and

electronic interface. Both play crucial role in the performance of the accelerom-

eters. These are also called as MEMS accelerometers. MEMS accelerometers

are mainly two types. Mechanical accelerometers with MEMS. The second type

of accelerometers measures the frequency change in vibrating element due to

the change in tension called SAW accelerometers. MEMS accelerometers will

have light weight and small size and also takes less power.

Gyroscopes

Three gyroscopes are used to measure the angular rate of the object in three orthog-

onal axes. Mechanical gyroscopes with gimbals have a lot of friction. The high

precision bearings and good quality lubricants are used to reduce friction. Their cost

is also very high. However, they do take some time to warm up. Most of these chal-

lenges of mechanical systems have been removed in the modern inertial systems

with introduction of strapdown systems. The inertial sensors are rigidly attached to

the body of vehicle i.e. sensors are strapped down to the body. Different types of

gyroscopes like Fiber Optic Gyroscopes (FOG), Ring Laser Gyroscopes (RLG) and

MEMS based gyroscopes are available to use in strapdown navigational systems.

a.

Fiber Optic Gyroscope (FOG) measures the angular velocity with the inter-

ference of light. FOG consists of light source and large optical fiber coil. The

light beam from light source split into two beams, and they propagate simul-

taneously along the optical fiber cable in the opposite directions as shown in

Fig. 13.5. When the sensor experience a rotation, the beam propagation in the

same direction of rotation will travel longer path or distance as compared to

the beam propagating in the opposite direction as shown in Fig. 13.6. This is

known as Sagnac effect. Here the phase or frequency difference induced between

two waves due to the Sagnac effect is proportional to the angular velocity. The

frequency difference is measured in a laser resonator and phase difference is

measured in the Interferometric Fiber Optic Gyroscope (IFOG). The accuracy

Fig. 13.5 Fiber Optic Gyroscope (FOG)