s Satellites 223 miles a velocity of just less than 7000 miles per hour is needed giving an orbit time of about 24 hours. A satellite can orbit the earth in one of two basic types of orbit. The most obvious is a circular orbit where the distance from the earth remains the same at all times. A second type of orbit is an elliptical one. Both types of orbit are shown in Figure 8.1, where the main characteristics are shown. When a satellite orbits the earth, either in a circular or elliptical orbit it forms a plane. This passes through the centre of gravity of the earth or the geocentre. The rotation around the earth is also categorized. It may be in the same direction as the earth's rotation when it is said to be posigrade, or it may be in the opposite direction when it is retrograde. At any given time there is a point on the earth at which the satellite is directly overhead. As the satellite moves SO does this point, and the track that this traces out on the earth's surface is known as the groundtrack. The groundtrack is a great circle, i.e. the centre of the circle is at the geocentre. Geostationary satellites are a special case as they appear directly over the same point of the earth all the time and accordingly their groundtrack consists of a single point on the earth's equator. For satellites with equatorial orbits the groundtrack is along the equator. Satellites may also be in other orbits. These will cross the equator twice, once in a northerly direction and once in a southerly direction. The point at which the groundtrack crosses the equator is known as a node. There are two, and the one where the groundtrack passes from the southern hemisphere to the northern hemisphere is called the ascending node. The one where the groundtrack passes from the northern to the southern hemisphere is called the descending node. For these orbits it is usually found that the groundtrack shifts towards the west for each orbit because the earth is rotating towards the east underneath the satellite. For many orbit calculations it