GPS receivers appear everywhere these days, integrated into smart phones, vehicle dashboards, cameras, and more. They're also shrinking—a variety of wrist-top models are available that track speed, distance, and location. There's a reason, however, why manufacturers dub these smallest GPS units "wrist-top computers" instead of "watches." They still require a linebacker-sized wrist to fit comfortably and are much chunkier than their timekeeping brethren. When will we see GPS watches small enough to fit us all?

A GPS receiver is composed of three basic components: antenna, processor, and battery. The antenna receives the signals from orbiting GPS satellites and must be capable of detecting incredibly faint transmissions (GPS signal strength at Earth's surface is less than a million billionth of a Watt). Different types of antenna are used; patch and quad helix are two of the most common in consumer devices. They are all relatively small (most less than 20mm wide), and even tinier versions are available. Smaller antennas can compromise reception, however, creating a trade-off in size versus performance.

The processor is the magic computer chip that translates GPS signals into location. These are available in microscopic dimensions, commonly described relative to the size of match heads. (The smallest currently available are 5mm wide, the size of a single match head.) They have also become increasingly powerful. Even with their reduced size, today's processors quickly identify and interpret multiple satellite signals simultaneously. They also decipher weak or intermittent transmissions, improving the sensitivity and accuracy of today's GPS receivers despite only modest improvements in antenna performance over the years.

The battery powers the whole operation—and creates the biggest limitation on size. The high-powered processors on GPS receivers operate constantly when tracking your location. This incessant activity rapidly drains even the most efficient batteries; wrist-top GPS units typically last only 5 to 10 hours on a full charge. In order to further shrink GPS units, designers would have to reduce battery size (and battery life), a trade-off consumers may not be ready to accept.

The smallest wrist-top GPS units currently hover around 75 grams (just under 3 ounces); a typical digital watch weighs half that. In the years ahead, however, expect GPS processors to become more energy-efficient and batteries to last longer. The GPS shrink-down will continue. Sometime in the coming decade, you won't even notice you’re wearing one.