Global Positioning System (GPS) receivers use satellites to fix your location and track your movement. GPS units provide you with information on your current position and directions to reach your destination. Your GPS can be used for fun too; the sport of geocaching is popular all over the world.

The Global Positioning System consists of 24 Earth orbiting satellites operated by the U.S. Department of Defense (DoD). The satellites broadcast a stream of data to military, civilian, and commercial users on Earth. Your GPS receiver uses these signals

to triangulate your position. A position fix can be determined with three or more satellites in view. If locked onto four or more satellites your GPS can also determine your altitude. Although from my experience, the elevation fixes leave a little to be desired in terms of stability and accuracy, especially when compared with the outstanding horizontal position accuracy of today’s units.

The accuracy of the GPS position fix depends on several factors, including the number of “visible” satellites and whether the satellites are widely spaced or close together in the sky. Wide spacing between satellites, that is “good” geometry, yields more accurate positions than those computed from a narrow cluster of satellites.

In the U.S. the Wide Area Augmentation System (WAAS) provides corrections to suitably equipped receivers enabling accuracy of less than three meters 95% of the time. WAAS was developed for aircraft to use precision GPS assistance when making landing approaches. An earlier correction system, differential GPS (DGPS),enables 3 to 5 meter accuracy and is used mainly by mariners. Without any corrections, typical GPS accuracy is about 15 meters.

Waypoints and Routes

One of the most useful features of your GPS unit is its ability to capture and save information about locations along your route, called waypoints. You can note important locations, such as where you left your car at the trailhead or the location of your camp, and have the GPS guide you back to them later. You can put waypoints in ordered groups called “routes.” Since straight line travel isn’t always possible you can use a route to define each leg of your trip and have your GPS guide you from waypoint to waypoint as you move along your route.

GPS Receiver Types

GPS receivers are available that cater to different uses. For the outdoorsman small,

lightweight, handheld units are best. Some have color screens and can be loaded with topographical maps. Most are water resistant and will not be bothered by a little rain or a splash during a stream crossing. There are even some GPS units that can float, ideal for those traveling by canoe or kayak.

For the motorist there are GPS navigators pre-loaded with road maps, Points-Of-Interest (POIs), and software that calculates the driving directions to get to your destination. Nearly all units for road navigation come with color touch screens for easy operation. A nice feature in some road units is a separate FM band radio receiver that decodes the Traffic Message Channel (TMC) data sent on radio

subcarriers. These units can plot traffic problems along your route and even automatically re-route you around the traffic jams and road closures. Cool huh?

GPS receivers designed for maritime use feature large screens and electronic chart atlases for plotting your vessel’s position on a standard nautical chart. Some marine units are part of integrated data display system that combine GPS position plotting, navigation, and radar display. The modern Automatic Identification System (AIS) is made possible by GPS. AIS equipped vessels broadcast their GPS coordinates, and course and speed information to other vessels in the area. AIS receivers decode and plot the position data from other ships, giving a comprehensive display of all the vessel traffic in your area.

When looking for a GPS unit here are some important points to consider:

• How will you use it? Trail or road navigation? Or perhaps both?
• What degree of mapping detail do you need? For back country hiking a GPS unit that can display topographical maps is a real asset.
• How much memory do you need? If you need to carry a lot of high resolution maps, want large POI databases, or need to store a lot of waypoints and routes, then you’re going to need plenty of memory. Some units offer a memory expansion slot that accepts an SD/MMC type memory cards.
• Other integrated gadgets? A GPS can only tell you directions when you are moving (it simply calculates the direction of travel). For help with direction finding you may opt for a unit with a built-in electronic compass. Barometric altimeters can also improve the functionality of the unit.

Your old fashioned map & compass are NOT obsolete!

Despite rumors to the contrary, the map and compass are not relics of the past. GPS receivers are complex electronic systems that rely on other complex electronic systems. GPS’s can break, the batteries can run flat and satellite reception can be disrupted. The skillful navigator always employs multiple tools for determining position and direction. Make sure you carry, and know how to use, a traditional map and compass.

GPS for Sport

With your GPS unit in hand you can participate in Geocaching, a fun, high-tech treasure hunt enjoyed the world over. The point is to use your GPS to locate caches secretly hidden in publicly accessible areas. The caches usually contain some trinkets that you can trade and a logbook that you sign to prove you found the hidden cache. You can find caches in your area and track your accomplishments on the Geocaching website. Just Google “geocaching” to find out more.