GPS Navigation – Accuracy

GPS navigation has revolutionized travel in the wilderness. Having a mobile navigation unit that can pinpoint your location anywhere on the globe can be very handy for finding a campsite, following a trail, returning to a prime hunting location, and many other applications. And it can save your life.

One of the biggest confusions about gps navigation systems concerns accuracy. There seem to be many different statistics and methods of reporting accuracy. Some appear to be better than others. Here are a few of the methods you’ll find.

Many portable gps navigation systems report “EPE” or “Estimated Position Error”. This number usually isn’t very accurate. It is based on the positions of the each navigation satellite, which changes constantly. At times the gps satellites are positioned relative to where you are on the globe in a highly optimum arrangement, other times not. The EPE takes this into account and calculates the possible position error due to this factor.

The Department of Defense, which operates the gps satellite navigation system, uses a method of reporting accuracy called “SIS” or “System in Space”, which calculates the total system accuracy based on a measure of the signals sent from the satellites. Like the EPE, its of limited use for determining the accuracy of your GPS navigation system “on the ground”.

The reason that neither of these methods are very helpful is that the biggest source of error of a portable GPS navigation system is the atmosphere, and more so in particular the ionosphere. Other factors which influence accuracy are the time of day, solar activity, and even humidity levels.

As far as the overall accuracy of a portable GPS unit, the DoD estimates this to be better than 50’ 95% of the time. This figure is referred to as “2drms accuracy” (2 times the distance root mean square). There are many popular manufacturers (you may have a Garmin GPS navigation system). Some use a different standard of accuracy, such as “RMS” (Root Mean Square), or “CEP” (Circular Error Probable). These numbers will report a greater accuracy, less than 50 feet, but for a lower percentage of times, so their reliability is more questionable.

In practical application, another big factor affecting GPS portable navigation is blocked signals. The signals from the GPS Sat Nav system cannot pass through mountains, buildings, people or heavy tree cover. To get an acceptable level of accuracy from your GPS unit you must be receiving a signal from a minimum of 4 satellites. Further, satellite signals can be reflected by tall buildings or canyon walls, which can “confuse” your portable navigation device. In this case you may have to move around to pick up the necessary number of satellite signals before you can rely on the position reported by your GPS unit.

The best bet of ensuring the highest accuracy of your portable GPS navigation system is to make sure you have a clear view of the sky overhead, and are tracking at least 4 satellites. With these factors in mind, most consumer GPS units will get you within 25’ of your position or destination most of the time.

The author, Mike Wreggitt, is an avid outdoor enthusiast, and big fan of GPS navigation systems. Find out more about GPS navigation at http://GpsForHiking.net.



One Response to “GPS Navigation Part One- Accuracy”