Satellite Acquisition
	See also: 'How to send E-Mail using your Inmarsat-C Terminal'
	To operate any Inmarsat equipment is it essential to acquire the satellite signal (Common Signalling Channel), and "lock" on to that signal before the equipment can operate. In principle, the satellite has to be "acquired" either manually or automatically by pointing the antenna accurately towards the satellite for the chosen Ocean Region. Inmarsat-C equipment uses a small omni-directional type of antenna which only requires an unobstructed line-of-sight view of the satellite. Inmarsat-A, B and M Ship Earth Stations use high gain parabolic dish or phased array antennas which must be pointed at the required satellite quite precisely. Such antennas must be steered and gyro-stabilised against the ship's motion. The ship's gyro input is fed into the Ship Earth Station and allows the antenna to remain pointed at the satellite while the ship is moving. A manual means of antenna adjustment is usually available by which the azimuth and elevation of the antenna can be altered to "aim" at the satellite. The beamwidth of a typical antenna is wide enough that the antenna only needs to "see" the signal as the antenna is manually aimed towards the satellite for the terminal to acquire the common signalling channel signal. Once the equipment has achieved an initial coarse acquisition of the satellite, the electronic circuits within the equipment, (step track circuits) perform the fine adjustment to obtain the strongest signal and lock on to the satellite. Most Ship Earth Stations are able to adjust their antennas to find the satellite automatically after input of the ship's latitude and longitude. Where the ship's navigational system is linked directly to the terminal, the entire satellite acquisition sequence may be programmed to take place automatically at switch on. The full automated acquisition sequence may take several minutes to complete properly. In the event that the required satellite has to be acquired manually, the first step is to determine the required azimuth and elevation values appropriate to the terminal's position. The azimuth and elevation charts for the four Ocean Regions, are recommended for this purpose. The basic azimuth and elevation of the satellite in relation to the ships current position can be read directly from the chart. Atlantic Ocean Region West Azimuth and Elevation Chart [110 KB] Atlantic Ocean Region East Azimuth and Elevation Chart [110 KB] Indian Ocean Region Azimuth and Elevation Chart [106 KB] Pacific Ocean Region Azimuth and Elevation Chart [105 KB] It will be necessary, however, to make a correction for the actual heading of the ship since the charts are based on a heading of 000 degrees. The adjustment for the ship's heading should be made before adjusting the azimuth and elevation of the antenna. As the correct azimuth and elevation settings of the antenna are approached, the signal strength indicator will rise showing that a signal from the satellite is being received. The procedure then is to adjust the antenna manually for the best signal strength possible and let "Step track" circuits of the Ship Earth Station take over to continue tracking the satellite automatically.