GPS: The practical application of recording your GPS position under way.

Although the Global Positioning Satellite (GPS) system has transformed the way we all approach coastal navigation in practice, it is worth remembering that your GPS computer does what it does with only one piece of information … and that is where you are at any given moment in time.

It knows this much more accurately than any terrestrial method of taking fixes and accomplishes this trick in a curious way. Each satellite that has a line of sight to your boat (i.e. is above the horizon and not obscured by mountains) tells your GPS exactly what time it is using a fabulously accurate set of atomic clocks. By the time that information has traveled from the satellite to your GPS, it is no longer that time because it takes time for the signal to travel.

We know how fast radio waves travel, so using the difference between the time the Satellite says it is and the time it actually is, we know how far it is to that satellite

If there was only one satellite, that would not be terribly useful, because, in theory, I could be anywhere on a sphere with the satellite at its centre and a radius equal to that distance.

However, if I can see more than one satellite, then I can do the equivalent of a triangulation only using spheres. If it can see at least four, then I can pinpoint pretty much exactly where I am and that is what you GPS computer does.


If I was in space, and not buried under a few miles of atmosphere, that would be enough. However, the atmosphere is capable of mucking this calculation up by affecting the speed at which the signal travels.

To address this, the system has been enhanced by adding a set of earthbound stations who send signals through the atmosphere to test what effect it is having at any one time.

These then calculate a correction to the time difference calculation and send that to your GPS computer. This additional service is called WAAS.

Speed over the ground (SOG), Bearing to waypoint (BTW), Cross Track Error (XTE) and Distance to Waypoint (DTW) are all useful pieces of navigational information calculated by your chart plotter as it constantly updates and records your boat´s position and compares it to where you wanted to be. Although they are all simple mathematical calculations you could carry out yourself by taking fixes and plotting them on paper charts, it is unlikely you could ever achieve the same degree of positional accuracy as your chart plotter. More to the point, who would wish to spend all their time at sea continuously plotting their position?

The chart plotter may provide us with a reliable navigational fix at a touch of a button, but you should not rely entirely upon it alone. They have a nasty habit of failing at the most inopportune moment. So it is a sensible idea to combine your reliance on electronic navigation, with some traditional paper chart plotting at regular intervals. So, if the worse happens and you do happen to lose your chart plotter, you have a recent reliable positional fix on your paper chart from which to work from.

In the next few paragraphs we show you how you should use GPS information to manage your chart navigation under way.

1. The first and simplest thing you can do is to plot your position on a chart to confirm you are where you think you are (Figure 1). Obviously, if you don´t have a chart, then the information gleamed from your GPS is useless.

GPS - Ploting your position on a chart with Salt Water Experience

2. If you take your position again in (say) fifteen minutes time, you can then plot the second position and determine the track you have made good over the ground (Figure 2).

With this information you can quickly read off the distance made good, calculate your speed, and determine your heading.

GPS - Ploting your track on a chart with Salt Water Experience

3. If you continue to record your GPS position, you can determine your actual track made good with the course you planned to steer between two waypoints. This is known as cross track error XTE (Figure 3). It will show you the effect that wind and tide may be having on your track over the ground based on the bearing you are steering.

GPS - Cross track error (XTE) on a chart with Salt Water Experience

4. If you are making a passage through dangerous waters, such as near rocks, you can draw a safety margin line on the chart you do not want to cross (Figure 4) and then use Cross Track Error information to track the course made good so you stay within the corridor you have penciled in on your chart. For example, you may set the boundaries of the safe corridor´s line to be 0.05 miles from the track. If the XTE approaches 0.05 miles you know you are in danger and can correct your course before you sail into trouble.

GPS - Ploting your position on a chart with Salt Water Experience

Of course modern electronic Chart Plotters are simply computers that relieve you of the tedious task of continuously plotting your position on paper charts. They can instantly calculate and report your speed over the ground (SOG), bearing to waypoint (BTW), bearing (BRG) and Cross track error (XTE) by continually comparing two or more position fixes.

It is good standard practice, however, to supplement your reliance on your electronic navigation equipment by regularly plotting your position on a paper chart as well, for the following reasons:

a). Electronics do fail. Mother boards blow, relays corrode and, if that is not enough, lightning strikes close by can fry all the electronic equipment nearby, including the battery powered spare GPS in your pocket.

b). Many of us use plotters which have small screens of less than eight inches across. The chart detail that can be shown and read easily at this scale is very limited. There is also the question of how up to date the electronic charts are.

We use and rely upon electronic navigation equipment as much as you. But we also always record our progress on paper charts as well. Do you?

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