Use on Board Ships
On board, the GPS receiver stores the almanac data for continuous use. It also calculates, (based on the A, B and C data above) exactly how far the satellite is from the ship at any given instant. Three such satellites and the calculations give us an exact 2D fix on board- Latitude and Longitude. If there are four satellites used, one can calculate altitude as well. By keeping a record of the ship’s positions, another simpler calculation determines the ship’s speed, and the course it has ‘made good’ in the time between positions.
What is even remarkable about the GPS system is its accuracy; normally 10-15 meters for civilian systems, and a huge improvement on the older sextant centered methods. Another advantage of the system is that it is an ‘all weather’ one. Even when one cannot see a landmark 50 metres away in dense fog, the GPS will give you your position without any decrease in accuracy.
Installation is fairly simple. Fix the receiver to any convenient place on the navigating bridge, run the cable and fix the antenna to a suitable point outside, power the GPS receiver, feed in some ship-specific data (e.g., height of antenna above sea level) and that is about it. Some minor corrections may be necessary to plot GPS positions on a navigational chart at sea, though. This is because there is a difference in the datum and the projection of the chart being used. This correction is so small that it is often ignored.
GPS information at sea is often replicated on other navigational equipment on the bridge like radars, electronic navigational systems and communication systems resulting in convenient, easy and seamless navigation- essential in these days of minimum manning on ships.
Don’t throw away that sextant, though. If for nothing else, you still need it to pass any navigational exam.