How handheld GPS units work

The Global Positioning System (GPS) provides precise time and location data by using 24 Navigation Satellite Timing and Ranging (NAVSTAR) satellites. Each satellite completes one Earth orbit every 12 hours, continuously transmitting its position and a time signal, which a GPS receiver can pick up. A handheld GPS unit must receive data from four or more satellites in order to calculate a user's position, typically within 50 to 100 feet. Many factors can have an impact on accuracy, including atmospheric conditions, buildings, tunnels and heavy foliage. However, reviewers say that the latest high-sensitivity GPS receiver chips all but eliminate those issues.

Most GPS units use the Wide Area Augmentation System (WAAS). This system is based on a network of 25 ground-reference stations. Each station receives GPS satellite signals, corrects any errors, and then forwards corrected signals to a Wide Area Master Station, which makes some additional calculations and uploads the newly corrected data to a satellite. The message is finally broadcast on the same GPS frequency and picked up by GPS receivers capable of reading WAAS signals that are within the broadcast coverage area of the WAAS. The difference in accuracy can be significant: a WAAS-enabled GPS receiver can fix your position to less than 10 feet. While that might not make a big difference if you're driving in a car, it's a big deal if you're on the ground trying to find the turn to your base camp.

Most handheld GPS units can store waypoints, which are coordinates for a selected location. You can program waypoints while you're traveling, or you can program them before you leave home or camp. These markers aid the unit in plotting the routes that interest you -- sort of a virtual breadcrumb trail.

Here's what experts say to look for when considering a handheld GPS unit. Also see our companion report on auto GPS receivers and a separate report on sports watches, which covers GPS-ready watches.

  • Determine the map set and degree of accuracy you will need. Different models come with different types of maps, and some map sets are more detailed than others.
  • Avoid unnecessary high-end features. You can save some money if you do not need features such as a barometric altimeter, electronic compass or heart-rate monitor, all of which are found in costly, top-end GPS receivers.
  • Look for a waterproof or water-resistant handheld GPS receiver. As editors at GPSReview.net say, "The IPX-4 standard means that the device will stand up to water splashed on it from any angle. However, this does not mean you can drop it in a stream. For that you want a device that is IPX-7 waterproof." A unit that meets IPX-7 standards can withstand accidental dunks, but it isn't intended for swimming.
  • Twenty-route memory is standard, and some models can store as many as 50 routes. Make sure you get one with at least 20 routes. Keep in mind that GPS receivers with a memory-card slot offer expandable route memory.
  • Look for at least 500 user-entered waypoints. Some units can store many more.
  • Check size and weight. This is a very personal thing, but you should consider it if you are thinking about hauling a unit around in the wilderness. Lighter units come at the expense of screen size.
  • Make sure the unit you are considering has a feature that guides you from waypoint to waypoint. This feature is essential for hiking use.
  • Look for a unit that uses a 12-channel parallel receiver system or better. This will give you the best reception in wooded areas. Some GPS units can receive even more channels.

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