s the angle at which the satellite appears above the horizontal. If the angle is too small then signals may be obstructed by nearby objects if the antenna is not very high. For those antennas that have an unobstructed view there are still problems with small angles of elevation. The reason is that signals have to travel through more of the earth's atmosphere and are subjected to higher levels of attenuation as a result. An angle of 5 degrees is generally accepted as the minimum angle for satisfactory operation. In order that a satellite can be used for communications purposes the ground station must be able to follow it in order to receive its signal, and transmit back to it. Communications will naturally only be possible when it is visible, and dependent upon the orbit it may only be visible for a short period of time. To ensure that communication is possible for the maximum amount of time there are a number of options that can be employed. The first is to use an elliptical orbit where the apogee is above the planned earth station SO that the satellite remains visible for the maximum amount of time. Another option is to launch a number of Satellites 225 satellites with the same orbit SO that when one disappears from view, and communications are lost, another one appears. Generally three satellites are required to maintain almost uninterrupted communication. However, the handover from one satellite to the next introduces additional complexity into the system, as well as having a requirement for three satellites. The most popular option is to use a satellite in what is called geostationary orbit. Using this orbit the satellite rotates in the same direction as the rotation of the earth and has a 24 hour period. In this way it revolves at the same angular velocity as the earth and in the same direction. As a result it remains in the same position relative to the earth. For the relative forces to balance and the satellite to remain a geostationary orbit it must be above the equator and have an altitude of 22 300 mile