 is necessary to consider the height of the satellite above the geocentre. This is the height above the earth plus the radius of the earth. This is generally taken to be 3960 miles or 6370 km. Velocity is another important factor as already seen. For a circular orbit it is always the same. However, in the case of an elliptical one this is not the case as the speed changes dependent upon the position in the orbit. It reaches a maximum when it is closest to the earth and it has to combat the greatest gravitational pull, and it is at its lowest speed when it is furthest away. For an elliptical orbit the centre of the earth forms one of the focal points of the ellipse as shown in Figure 8.1 (see above). It can also be seen that there are points where the satellite is furthest from the earth, and closest to it. These are important and they are called the apogee (furthest away) and perigee (closest). Generally the altitudes at the apogee and perigee are calculated from the geocentre. 224 Newnes Guide to Radio and Communications Technology Angle of inclination Angle of inclination Equatorial plane Plane of the satellite orbit North-south Geocentre Figure 8.2 Angle of inclination of an orbit A satellite may orbit around the earth in different planes. The angle of inclination of a satellite orbit is shown in Figure 8.2. It is the angle between a line perpendicular to the plane of the orbit and a line passing through the poles. This means that an orbit directly above the equator will have an inclination of 0 degrees (or 180 degrees), and one passing over the poles will have an angle of 90 degrees. Those orbits above the equator are generally called equatorial obits, while those above the poles are called polar orbits. Another important factor about the position of a satellite is its angle of elevation. This is important because the earth station will only be able to maintain contact with the satellite when it is visible as only direct or line of sight communications are possible with the satellite. The angle of elevation i