The boat GPS has become indispensable. So what is the GPS system about? The US Department of Defense (DOD) operates the NAVSTAR system. The system consists of 24 satellites that are placed in six polar orbits, so that at least four will always be visible above the horizon at any time. GPS position fixing involves the use of a mathematical principle called trilateration. The GPS receiver calculates a position from a number of satellites. This requires satellite ranging to measure the distance from the satellites, accurate time measurement, the location of all satellites, and correction factors for ionosphere conditions.
When the GPS is powered up this initializes with the closest satellite and ephemeris or almanac data being downloaded into memory. A period of at least 15-20 minutes is often required to stabilize a precise and accurate position and verify the status of satellites, availability etc although it takes a much shorter period in general. After switching off a GPS, the last position is retained in memory. If your position remains within 50nm, prior to the next power up, a position will generally be available within approximately 3-5 minutes. The receiver monitors for data from other satellites in view. After acquisition of data, it locks on to a satellite to commence the ranging process. Based on the data on positions and times the receiver calculates a position solution and displays this on the screen.
This system was developed by the U.S. Coast Guard to improve and enhance GPS accuracy levels from the nominal accuracy levels of 15 meters to about 1-3 cm. The system uses a number of fixed position land station that calculates theoretically correct distance and signal travel times between the station and each satellite and any errors, such as such as SA. The required error correction factors are then transmitted to all mobile GPS receivers in the area with beacon receivers, in particular marine GPS. This correction signal is applied to the position measurement solution of the GPS.
The accuracy of GPS is important when the best fishing location of all time is to be returned to. The Precise Positioning Service (PPS) is primarily for military use and is derived from the Precise (P) code. The P code is transmitted on the L1 (1575.42MHz) and L2 (1227.60MHz) frequencies. PPS fixes are generally accurate within 16 meters spherical error. The Standard Positioning Service (SPS) is for civilian use and is derived from the Course and Acquisition (C/A) code. In general accuracy is within 9-10 feet (3 meters) 95% of the time. This can degrade in poor conditions to around 25 feet (8 meters). Many factors come into play including space weather, and GPS interference issues. If you have GPS problems than it will be necessary to do some GPS troubleshooting. More great information from fishing and boats.