Navigation and Seamanship (1+1)

Satellite Visibility and Acquiring the satellite signals

When the receiver is first turned on, it will display some type of satellite visibility page. The Satellite Page shows the satellites that the receiver thinks are currently visible from your location. The GPS constellation of satellites will always have a total of at least 24 satellites.. From any point on earth there should be at least 4 available in the sky above you at all times. The GPS satellites travel in orbits around the earth each with a period of 11 hours and 58 minutes. So depending on the time of day you go outside your receiver will see different satellites overhead. The same satellites will appear each day but their pattern in the sky will shift 4 minutes earlier each day. When a new satellite is launched or an old satellite is turned off, the receiver will be notified as soon as it tracks one of the other satellites in the constellation.

Each GPS satellite has a unique number that identifies it, sometimes called a PRN (pseudo random noise code number). The satellite number is shown on the bulls eye plot and on the bottom of the screen. The bulls eye represents the sky above you. Satellites shown in the center are directly overhead. Satellites shown near the outermost ring are near the horizon. At the bottom of the screen are some bars that show the strength of each received satellite signal. A taller bar indicates a stronger signal. If you are close to a building that blocks part of the sky you’ll see that the signals for satellites in the direction of the building will show a lower signal or possibly not be present at all. When the receiver tries to acquire a satellite its number will be shown on the bulls eye plot as white on black and the strength bar at the bottom will not be shaded. Once the satellite is tracked and its data downloaded, the bar will become shaded and the latitude and longitude of the current position is displayed.