Electronic Navigation – GPS – Basic Navigator Functions
Source: Mariners Learning System, By Captain Bob Figular
Six basic functions are common to almost all GPS units although the terminology and the operating procedures required to use them vary from one manufacturer to another.
The most fundamental of all is position, given in terms of latitude and longitude. If you plot your position on a chart it is relatively easy to measure the range and bearing to a waypoint. Your GPS receiver can do this too, except that it uses trigonometry, rather than geometry.
By comparing your vessels current position with its position a few minutes ago, you could work out the direction and distance you have travelled and, from that, work out your speed over the ground. These two are standard functions on your electronic GPS and are usually called CoG (course over ground), and SoG (speed over ground). CMG and SMG for course made good and speed made good are common variations, as are TRK and VEL for track and velocity.
In practice, most GPS receivers use a more sophisticated technique for working out CoG and SoG, but both functions can be adversely affected by positioning errors and become increasingly suspect at low speeds. On a stationary boat, in particular, the CoG display is likely to fluctuate randomly.
One way to improve the accuracy of CoG and SoG is to take an average over a period of time, and most of the better GPS receivers have a damping function based on this principle. ‘High’ damping uses a long time interval and gives a steadier and generally more accurate reading, but is less responsive to genuine changes of CoG and SoG.
The final standard function is known as cross track error, usually abbreviated to XTE. XTE can only be used when the GPS has been given a route consisting of two or more waypoints. This is due to the fact that XTE it is an indication of how far your present position is from the straight-line track joining the two.
On all but a few particularly sophisticated GPS systems, the man overboard function effectively ‘freezes’ the position at which the man overboard button is pressed, and stores it as a waypoint. It may be tempting to use this to guide the boat back to the spot at which the victim fell overboard, but you should remember that by the time you get back to the spot where the person fell in the water they will have drifted away from it with the wind and tide. The real value of the man overboard function is that it can allow you to give the Coast Guard an accurate time and position, from which they can calculate the persons most likely movement.