io. Placing a satellite in orbit In view of the colossal amount of energy required to place a satellite in orbit it is necessary to ensure that the energy is used in the most effective way. This ensures that the amount of fuel required is kept to a minimum; an important factor on its own because the fuel itself has to be transported until it is used. Many satellites are placed into geostationary orbit, and one common method of achieving this is based on the Hohmann transfer principle. This is the method used when the Shuttle launches satellites into orbit. Using this system the satellite is placed into a low earth orbit with an altitude of around 180 miles. Once in the correct position in this orbit rockets are fired to put the satellite into an elliptical orbit with the perigee at the low earth orbit and the apogee at the geostationary orbit as shown in Figure 8.4. When the satellite reaches the final altitude the rocket or Satellites 229 Perigee Low earth orbit Transfer orbit Earth Apogee Final geostationary orbit Figure 8.4 Use of a transfer orbit to place a satellite in geostationary orbit booster is again fired to retain it in the geostationary orbit with the correct velocity. Alternatively when launch vehicles like Ariane are used the satellite is launched directly into the elliptical transfer orbit. Again when the satellite is at the required altitude the rockets are fired to transfer it into the required orbit with the correct velocity. These are the two main methods of placing satellites into orbit. Naturally it would be possible to place a satellite directly into geostationary orbit, but this would take more energy and would not be feasible. Path calculations Communication with satellites occurs via a direct line of sight path. As a result it is possible to calculate the signal levels which will be received by the satellite or the earth station assuming that figures like the transmitter power, antenna gains and receiver performance are known. To ensure that the signal levels are kept within working limi