How GPS Watches Work

GPS can trace its origins to the very birth of spaceflight; the principles behind it came together as American scientists scrambled to track Sputnik, the satellite launched into orbit by the Soviet Union in October of 1957. Researchers from Johns Hopkins University's Applied Physics Laboratory, Drs. William Guier and George Weiffenbach, discovered that they could track early satellites' positions based on the Doppler Effect: Signals transmitted from a moving satellite would appear to distort as it approached or moved away from the receiving station. By measuring the nature and severity of the distortion, a person could calculate the orientation of the satellite's orbit.

APL company researcher Frank McClure suggested that the tracking system could also work the other way around: By tracking a satellite with a precisely known orbit and evaluating the resulting Doppler shifts in its signal, a person on the ground could identify his position anywhere he could pick up the signal. The U.S. Navy expanded the system into the Navy Navigation Satellite System, which was actively guiding ships by 1964. (For more on the history of GPS, take a look at our article on How GPS Receivers Work.)

Military-sponsored research continued into the 1970s and '80s as scientists tried to improve the signal precision, reliability and speed of satellite navigation. Technology switched from tracking Doppler shifts to using triangulation and signal timing: The longer a signal took to reach a receiver, the farther away the satellite was at the moment the signal was sent. GPS was a tried-and-true tool for all branches of the U.S. military and its allies by the 1980s, when the U.S. government began allowing widespread civilian access to the technology [source: Parkinson].

Early civilian GPS units were large and heavy, limiting their usefulness for on-the-go users. But they suffered from a much more crippling drawback: They weren't all that accurate. To prevent enemies from accessing the navigation signals, the U.S. military employed dithering: intentional interference with the satellites' timing signals that unauthorized users couldn't decode. This limited civilian GPS accuracy to about 300 feet (100 meters). It wasn't until the year 2000 that the U.S. government finally removed the dithering, making accurate civilian navigation possible. GPS then became a useful tool for anyone who needed to know where they were, how fast they were traveling or how to get where they wanted to go [sources: Parkinson; Navigon].

So that took care of the accuracy issue, but how did GPS unit manufacturers create products of a reasonably portable size?

The access to dither-free GPS technology coincided with a revolution in electronics miniaturization -- think about how much less a modern laptop computer weighs than the one you were using in 1996. GPS developers embraced innovations such as surface mount technology (SMT), in which tiny components are soldered directly to a circuit board (rather than using wire leads to make connections), and robotic assembly that helped shrink the timing and signal processing chips at the heart of GPS receivers into miniscule components. Today they're almost too small to see clearly without magnification [source: Poole].

Researchers further shrank GPS units by applying fractal geometry -- which involves shapes made up of repeating, expandable patterns -- to design powerful receiver antennae that literally folded into miniscule spaces. This important (and once bulky) piece of the GPS receiver package became small enough to hide in the slim case of a smartphone -- or within the body of a watch [source: Rusu and Baican; Mikkola].

The first GPS watches were essentially small versions of handheld GPS units, measuring about 2 to 3 inches (about 50 to 75 millimeters) long and about 1 inch (25 millimeters) wide, with wrist straps attached. Even though they were indeed smaller and lighter than most other GPS units from the early 21st century, they were still large enough to make running long distances with one -- not to mention running to the coffee shop after a workout with the thing on your arm -- more trouble than it was worth for many athletes [source: Wollman].

Sports that hinge on knowing one's location relative to a target, such as golf, adopted GPS early in this miniaturization process. For example, well-known golf champion and course designer Greg Norman became an adviser with golf-related GPS products producer Inforetech in 2002 [source: GPS Industries]. But by and large, it took two technological steps for consumers to start adopting GPS watches on a wide scale.

To become really useful (and hence marketable), GPS watches had to first become, well, watchlike. That point was finally reached around 2010, and as of 2012, you could buy a GPS watch the same size, shape and general style as any other fashion watch on the market. Some models, such as a 13-millimeter (0.51-inch) thick prototype released in February 2012 by Epson, were actually smaller than many non-GPS sports watches [source: Tucker].

Small size does not mean a small number of features, though, and here's where GPS watches really became useful training tools. Knowing exactly where you are at any given moment is nice, but unless you're in the middle of nowhere or trying to choose a club in the middle of an unknown fairway, the application is limited. GPS watch manufacturers saw this, and expanded their products' versatility by adding in features from other sports devices such as bike computers, pedometers and heart rate monitors.

Many GPS watches can sync with other devices using wireless signals, such as ANT+, a small, energy-efficient system. These additional devices often measure biometrics: bodily systems measurements such as heart rate, which athletes can use to gauge the intensity of their workouts. Other sensors included with GPS watches may include shoe-mounted cadence monitors that track the rate of a runner's stride and bike-mounted sensors that detect wheel speed and the force that a rider is exerting through the pedals. Computer software that comes with these watches lets users download array of workout data collected by the watch either wirelessly or through a USB port. By entering basic information such as height, weight and age into the watch, the tech-savvy athlete can have a precise, sport-specific tool, a 21st-century version of the athlete's training diary.

This combination of location and biometric data is the meat of what makes a GPS watch worth the expense. But as in many tech-device scenarios, it's easy to get overwhelmed by the options available. GPS watch prices vary, but with prices in early 2012 ranging from $100 to more than $400 for various models, they're still not a small expense. How can you choose the most appropriate GPS watch for your needs?

You wouldn't buy a sailboat if you wanted to run whitewater rapids. In the same vein, you'll want to make sure a GPS watch meets your anticipated needs before you spend your hard-earned money on it. Will you be using the watch for one sport, or do you want one that's useful for a variety of activities? Are you more concerned with having data you can download to your computer and track over time, or would you prefer a device that gives you information as you need it in the field? Do you go places where getting lost could be dangerous, or do your activities keep you on well-marked trails and roads? Your answers to all of these questions will narrow your watch choices.

Take that last question, for example: If you're an avid hiker or cross-country skier who likes to wander into the backcountry, it would certainly be helpful to have a GPS watch with a moving-map display, which shows you where you are in relation to other waypoints. The trade-off for this is size: The bigger display necessary to show map details means this type of watch may be too cumbersome for you if you mainly want something light and sleek to track splits in your next marathon [source: Geek.com]. Likewise, a rugged-sport athlete such as a skier or mountain biker might want to avoid a watch with a touchscreen interface, exchanging the sleek, button-free look for something that won't get recalibrated if it's unexpectedly whacked by a tree branch. A runner, on the other hand, might find that a touchscreen is perfect for quickly checking data.

GPS watch manufacturers list their models' features on their Web sites, giving you a quick, easy way to compare features and price across the entire market. As you narrow your search, though, it helps to get a live perspective. Talk to the folks at your local bike shop, running store or hiking outfitter, and don't be afraid to pester friends about their GPS watches. The first-hand tips and real-life reviews you'll get will leave you well prepared to make the right purchase to complement your chosen activities.

The benefits of basic GPS functions are clear if your passion is outdoor adventures such as backcountry hiking, sea kayaking or hunting; these tiny computers are tailor-made to help you get into the wilderness and make it back with less risk of getting lost. Just remember to pack a map and compass, and know how to use them; adventure lore is full of stories about GPS batteries dying at the worst possible moment.

Sports- and fitness-oriented users can also use wayfinding features to try a new running route or for training while traveling. But that only scratches the surface of what you can do with this high-tech training tool.

A GPS watch can measure your performance with a level of precision that was once only the domain of researchers and professional athletes. It's nice to know how long it took you to complete a half marathon, but a GPS watch can tell you exactly where in the 13.1-mile (21.1-kilometer) course you were fastest -- and slowest. With the data-download capability that's pretty much standard on these devices, you can compare each run to see trends in your performance. Maybe you'll notice that you lose too much time after Mile 10, or that you're not climbing hills as fast as you did on the same courses last year.

Once you identify these kinds of weak spots in your performance, the combined features in a GPS watch can give you on-the-fly feedback to make your training more specific and efficient. Many cyclists, for example, spend the first part of their preseason building cardiovascular strength with long, steady effort. This helps build a foundation for the rigors of sprinting and hill training later in the year. Tracking a "moderate pace" using perceived effort -- how you feel during the workout -- is a variable measure at best. Equip yourself with a GPS watch that can read data from a heart rate monitor, cadence sensor and wheel speed sensor, however, and you can fine tune every mile of your ride, keeping your effort within a precise range of speed, heart rate and time.

The results of this kind of specific training can be dramatic. If you're strapped for time, you'll now be able to get the most out of every minute of your workout. You'll consequently spend less time worrying about the balance of sport, family and work. And when game day arrives, you'll know exactly how hard you can push yourself, whether it's on the race course, trail or track. This is technology that will help you train smarter, not just harder.