How to Choose a GPS Receiver

Originally intended as a navigational aid for the US military, GPS technology is now used for everything from tracking ships on the ocean to helping backpackers find their favorite campsites. It can also be found factory-built into cars, watches, two-way radios, PDAs and more. With all these options, how do you know which unit is right for your needs? Let's take a look.

Understanding the Technology

How It Works

GPS stands for Global Positioning System, which pinpoints locations on land and sea using a network of 24 orbiting satellites operated by the United States Department of Defense. Satellites are data transmitters, beaming waves of information down to earth. GPS technology captures a combination of three or more satellite-transmitted waves to triangulate the GPS receiver's position on earth.

Early on, a system of "selective availability" prevented private citizens from full access to GPS technology, limiting the accuracy of consumer-owned receivers to 100 meters. But in 2000, the U.S. government removed selective availability, substantially improving GPS accuracy.

WAAS: A Big Improvement

In 2003, the government opened the door for more improvements in accuracy by making the Wide Area Augmentation System (WAAS) available. Originally designed as an aid to aircraft navigation, this uses ground-based stations to provide GPS signal corrections. A WAAS-enabled GPS unit can achieve position accuracy of better than three meters under ideal conditions. This is particularly true if the unit has a high-sensitivity processor chip such as the SiRF Star III.

However, be aware that real-world conditions are rarely "ideal." Tree cover, canyon walls and tall buildings can all block signal reception, and moving satellites aren't always in optimal position. So it's advisable to expect accuracy of 20 to 30 feet, even with the best WAAS-enabled GPS. If you're an international traveler, know that WAAS is currently functional only in North America.

Advantages and Limitations

GPS technology works day or night, rain or shine. Theoretically, a high-end GPS unit can help you find your way through an old-growth forest in the dead of night. To do so, however, requires an excellent antenna that can capture reception under heavy tree cover. Night travel also requires a backlit screen, and backlighting quickly drains battery power.

This serves as a reminder that while a GPS unit enhances your navigational abilities, it should NEVER be considered a replacement for a map and compass and the knowledge of how to use them. The fact remains that a GPS receiver is an electronic device: Batteries run out; electronics break down. Always go prepared with the "old technology" of a compass and map.

GPS in Other Devices

GPS technology is inspiring new, convergent technologies. Some two-way radios with GPS capabilities let you maintain real-time position-tracking of other people in your party who are carrying the same device. Some GPS-enabled cell and satellite phones are also capable of this.

Geocaching

GPS units can be fun as well as useful. Check out geocaching (gee-oh-cashing). A high-tech version of hide-and-seek, this game involves using your GPS to find the locations around the world of hidden caches placed by other geocachers! Read our article on geocaching.

The Four Basic Functions

When using a GPS receiver on a hike, there are four functions that are of particular importance.

• Giving a Location: The foundation of GPS technology is its ability to accurately triangulate your position by receiving data transmissions from multiple orbiting satellites. Your location is given in coordinates, such as latitude and longitude or Universal Transverse Mercators (UTMs). NOTE: The UTM coordinate system was developed by the military. Because it lays out a map in a square grid with all gridlines 1,000 meters apart, it's especially effective for quickly pinpointing map locations. Most topo maps have UTM gridlines printed on them.
• Point-to-point navigation: In GPS terms, a "waypoint" is a destination or an intermediate location along the way. On a hike, you can mark a beginning waypoint at the trailhead. Then, if you have the coordinates for the campsite you're headed for (taken from a map, resource book, website, mapping software program or other source), the GPS unit can give you a straight-line, point-to-point bearing and distance to your destination. Of course, trails rarely follow a straight line, so the GPS' bearing will change as you go. The indicated distance to travel will also decrease as you approach your goal.
• Plot navigation: By combining multiple waypoints on a trail, you can move point-to-point with intermediate bearing and distance guides. Once you reach the first predetermined waypoint, the GPS receiver can automatically point you to the next one or you can manually do this. Waypoint-management software, which comes with most GPS units, simplifies the process of obtaining and entering coordinates for many trails. By inputting coordinates in advance, you can enhance your trail navigation before you even leave home.
• Keeping a track: One of the most useful functions of a GPS unit is its ability to lay a virtual "breadcrumb trail" of where you've been, called a "track." In GPS parlance, this is different from a "route," which is a plan for where you're going. You can configure the GPS receiver to automatically drop "trackpoints" over intervals of either time or distance. To retrace your steps, simply follow the GPS bearings back through the sequence of trackpoints, just as you followed the sequence of waypoints on your route.

Key Features

The Screen

• Color screens can be helpful, especially in maritime environments. You'll want that lump on the screen to quickly distinguish between land mass and water depth. Other outdoor enthusiasts find that less-expensive grayscale (black-and-white) screens are just as suitable.
• Large screens are best for automobile navigation, and for some is the deciding factor when choosing a GPS unit. But sometimes less is more-a screen too cluttered with an excess of information can be a dangerous distraction.
• Illumination Backcountry hikers often look for illumination options. Can it be read in a variety of light conditions? Does it light up for nighttime or low-light use?

How's your eyesight? Select a screen type and size that is easily readable.

Antenna Configurations

Each type and style of antenna has its pros and cons-determining which works best for your planned adventures should be the deciding factor.

• Quad helix: A quadrifilar helix antenna is the most common in today's models. It is generally considered the best for use under tree cover. (Think of an eggbeater, whose spiral blade configuration is capable of creating greater splatter coverage than say, a wooden spoon.)
• Flat patch: If you tend to hike or travel in open country, consider this popular, less-expensive alternative to the quad helix. It works just as well, provided the antennas have the same aperture.

More on Antennas

With the addition of a high-sensitivity processor chip, such as the SiRF Star III, the performance of a GPS unit (regardless of antenna type) will be dramatically enhanced. This chip can resolve even very weak satellite signals, and will outperform other models under heavy tree cover.

In the meantime, keep in mind a general rule of thumb: a physically larger GPS receiver is likely to have higher-quality reception under forest canopy than a more compact unit, primarily because the larger units tend to use a quad helix antenna.

There are other antenna issues to consider as well:

• Built-in antennas are less likely to break off or get damaged during normal use.
• External antennas are less protected, but they can be re-positioned and manipulated to improve a receiver's pickup in some situations.
• Optional plug-in antennas are great when you want to use your GPS in a car or boat. They allow your GPS to pick up signals even if you're seated under a metal roof or hunkered down below decks and those satellite signals can't get through.

Internal Maps

One of the biggest variables between units (and prices) is how much preloaded map info they contain. You can look at a map of your general location right on your GPS screen, then use it to orient yourself to your surroundings. A unit might have a basemap of North America, backcountry details of major rivers, or street-level road maps. A limited number of international maps, particularly of Europe, are also available.

The mapping capability of some GPS units can be augmented via CD-ROM map collections as well as memory cards that are preloaded with maps.

Think about the future. Will you want a receiver that can adapt, especially when it comes to memory size and mapping capabilities?

Barometric Altimeter

While all GPS receivers can provide elevation as part of the information gleaned from the satellites, not all units come with a barometric altimeter. This is an "extra" that may well be worth considering. First of all, the barometric altimeter operates independently of the satellite signal. So even if you're hiking in a location where the signal becomes too weak to be reliable, you can still get an accurate elevation reading. And since the barometric altimeter operates off air pressure, it can also give you an idea of approaching weather changes by showing a graphic chart of barometric trends.

Other Considerations

Usability

Many of the most popular units have "simulation" modes that allow you to familiarize yourself with how they operate, even if you're inside an REI store where satellite signals can't be picked up.

Take a "test drive." Pushing the GPS buttons should become second nature. Are the directions simple to understand? If you lost the instruction manual, could you figure out the basics by yourself?

Try getting from one point on a map to another. How does the unit tell you which direction to go? On some models, it's as easy as keeping one line on the screen on top of another.

Another consideration is the computer interface. How easy is it to hook up the GPS unit to your computer for uploads/downloads? A USB port is becoming more common on GPS units, and will dramatically increase the speed of downloading maps and waypoints.

Keep your end use in mind-you may be selecting and ordering your GPS unit online from the comfort of your home office, but are looking forward to using your GPS for that saltwater fly-fishing trip on the Merrimack River. Put yourself there and imagine how you and your GPS will interact.

Power Supply

Keep in mind what type of batteries the unit will use and ask yourself the following: Will it work with more than one type? How many extra batteries will I have to pack in (and out) if I head out on a long trip?

• As with digital cameras, the battery life of a GPS unit depends on how many features it has eating up the supply. To combat the notoriously ravenous appetite of electronics, many GPS units have a "sleep" function that conserves energy when the GPS is not being used. Some simpler units lose temporarily stored information when the unit is turned off.
• When using your GPS receiver around town, consider rechargeable batteries. For trail use, especially on overnight trips, you may prefer lithium or alkaline batteries. Both are more reliable and generally last longer than rechargeables, so you won't need to carry as many backups. Lithium batteries make an especially good choice, as they have a longer shelf life and work well in the cold.
• Most modern GPS receivers have built-in back-up batteries to keep your stored information safe when the main internal battery runs out. This internal battery can be replaced by the manufacturer who is able to save and reload your hard-won navigation information.

Speed

GPS units search for and acquire location information constantly. But some units process data and provide updates far more quickly than others. In general, the more channels a GPS unit has, the faster it will compute and refresh its on-screen information. (When comparing models head-to-head, remember that a unit's performance in "simulation" mode may be far faster than its speed in the wilderness.) And it takes some units longer than others to re-acquire satellite signals once they've been turned off and back on. Look for "Time to First Fix" (TTFF) data when comparing models.

Size

Today's smallest GPS units are no bigger than a cell phone and fit in the palm of your hand. But weight and space savings often come at a cost, usually either a higher sticker price or fewer features. Smaller units often double-up functions on unlabeled buttons; larger models have dedicated and clearly labeled buttons for primary functions. Some smaller models compensate for the size limitations with a well-chosen feature: an oversized screen, perhaps, on an undersized unit.

Design

In evaluating the physical design of a unit, consider its durability. Look for pieces that can break off, buttons that can jam easily or outer casings that might crack when dropped. Most GPS units are waterproof rated, and some even float.

Cost

When comparing options, ask yourself what you want your GPS to do. You may be perfectly happy with a less-expensive model that includes only basic features. And remember that add-ons-cases, cords, CD-ROMS, mounting brackets, etc.-will add to the price as well as to the versatility of the unit you choose.

For those who plan to use their GPS in the car for road navigation, some features are worth particular consideration. Audible voice-prompted directions can be as much a safety factor as a convenience. Some states restrict mounting GPS devices, and if this true in your state, you'll need to rely solely on the unit's voice guidelines. Traffic compatibility and Bluetooth technology is available on some high-end units. These features offer effective rerouting around traffic snarls, and automatic, hands-free calling through a Bluetooth-enabled cell phone to numbers stored in the POI database.

In answer to the expanding needs of the GPS user, many GPS brands are now "bundling" their most popular units with the accessories most commonly asked for, taking the guesswork out of add-ons.

Contributors: Steve Wood, instructor/supervisor, REI Outdoor School, SF Bay Area; David Tynes, REI product information specialist.

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