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HSH Yacht Articles

Do you rely on your GPS?


Jeremy R. Hood

As I write this, I am about to set out for Fort Lauderdale, Florida, to bring back to Clear Lake, a 50' ketch along with the owner of the boat. In checking out as much as I could about the boat before leaving, preparing the equipment that I would need to take, I mentally crossed off the list a GPS for I noted that there was one on the boat and in addition, the owner had a hand-held unit of his own.

How quickly we forget! When I set out to cross the Atlantic in 1988, an item high on my list of desirable equipment was a Navstar satellite navigation receiver which would enable me to get a fix anywhere in the world! The fixes came, I suppose, on average every hour though on occasions I had to wait for 5 hours for one. Just recently I took this 20 lb hulk of equipment off my boat and am only keeping it for nostalgia and as a potential museum exhibit. What cost hundreds of dollars is now worthless. All of this has happened because of the introduction in the last three years of the U.S. Defense Departments' Global Positioning System. Initially needed by the military, Congress determined that it should also be available for civilian use and so now we have it: world-wide coverage, 24 hours a day with an incredibly impressive accuracy

How does it work

The GPS system uses a total of 21 satellites in fixed orbits some 10,000 miles above the earth. Each satellite covers a track around the earth every 24 hours. The satellites broadcast two sets of signals continuously. There is a military signal which is secretly encrypted and not available for general use, and a civilian signal which is used by all commercial GPS receivers. After initial tests it was found that even the civilian coded signal (which gives less accurate information than the military one) provided an extremely accurate position and so the Department of Defense decided to introduce some small errors into the signals to reduce this accuracy. This error introduction is referred to as selective availability.

The satellites have aboard extremely accurate clocks and they broadcast not only a continuous update of their position but the time as well. The information from at least three satellites is needed for a GPS receiver to calculate a latitude and longitude position.

How accurate is GPS

On a number of occasions I have helped owners set up their new GPS receiver and enter positions into its memory. And whenever we are close to known marks such as buoys on the Houston Ship Channel, the accuracy is such that it is hard (or impossible) to distinguish on a chart between our position and that of the buoy. With this accuracy there is a danger of hitting a buoy rather than not finding it!

In distance terms, the accuracy of a GPS position is determined by a number of factors including the internal set-up of the receiver, the speed of the vessel and whether selective availability is turned on or off (occasionally it is turned off when the military wish to use the civilian code). A practical accuracy (with selective availability on) is to expect to be within 100 yards of a given position at least 95% of the time.

Differential GPS

Soon after the GPS system became fully operational it was realized that there was a simple way of overcoming the deliberately introduced errors known as selective availability. If a GPS receiver is fixed at a known position then the actual position of the receiver and the one shown on its display can be compared. The difference between the two is the error and if this error information is broadcast on radio frequencies, receivers equipped with the necessary equipment can obtain not only the original GPS signal but the error information necessary to correct the latitude and longitude. The system of transmitting this error information by radio is known as differential GPS and the U.S. Coast Guard are currently experimenting with this system using the already established marine radio beacons as their transmitters. Using differential GPS the accuracy of a position is usually within 10 yards rather than 100 yards.

Where are you

The position given by a GPS is shown as a latitude and longitude and this must be plotted on a chart to determine your position. More recent even than GPS is the advent of chart plotters (more on these in a latter column) which show a digital image of a navigational chart. When interfaced with a GPS your vessels position is shown on the screen and updated as you sail. Its pretty impressive!


Perhaps one of the most useful features of a GPS is the ability to enter waypoints. A waypoint is merely a position chosen by you when planning a route. Being able to sit at home with your GPS and chart and plan a voyage is ideal. You can plot an intended route on the chart, draw in the lines and check that there are no rocks, wrecks or other obstructions along the way. You can then enter positions along the route into your GPS. When underway, your GPS will provide a course and distance to the next waypoint along your route.

What can go wrong?

With a continual update of your vessels position it may seem that your worst fears are resolved and you will never have to worry about getting lost or going aground. Wrong! Though the system is impressive in its reliability you still have to expect errors and the warning shown on most U.S. charts is still as true in the age of GPS as it was a hundred years ago The prudent mariner will not rely solely on any single aid to navigation.

  1. Errors can occur in your receiver so that, suddenly, it fails to show a position. This could be caused by a lightening storm close-by.

  2. You may make an error in plotting a position on the chart so that it appears you are safe when you are not.

  3. You may have calculated a waypoint position wrongly (Yes, I've done this!)

  4. You may have calculated a waypoint position correctly but written it down wrongly.

  5. You may have calculated a waypoint position correctly, written it down correctly but entered it in to the GPS wrongly.

  6. Or perhaps one day the U.S. Defense Department decides to turn it off for a while!

And so as I pack my kit and prepare to leave, I have with me my sextant, sight reduction tables and a 1995 nautical almanac. I hope we have some good breeze!

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