5 Navigation Systems
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5.1 The Earth
The shape of the earth and the distribution of material within the earth affect gravitational acceleration over the surface of the earth. Although these changes are relatively small, for the modelling of inertial navigation and the simulation of vehicle dynamics, these second-order effects should be taken into account. Similarly, magnetic variation also varies over the surface of the earth and, moreover, varies slowly over time. Magnetic compasses have been used in aviation since the early days of flying and heading is given in degrees magnetic on charts and in navigation displays.
5.1.1 Gravity
The earth is an oblate sphere; it is an ellipsoid with a difference between the equatorial radius and the polar radius, as shown in Figure 5.1, where R is the radius of the earth and the following values are used to define the shape of the earth:
Figure 5.1 The Earth Reference Ellipsoid.
Several equations are used to define gravitational acceleration, ...
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