Sensor-driven pedometers count steps and calculate the distance traveled based on a preset length of stride and the total steps taken. Many GPS pedometers offer measurement of speed and location as well as mileage. GPS pedometers need contact with at least three and preferably four satellites to calculate this information, according to Pedometers.com. If objects block the view of satellites in the network, the GPS stops generating data. In open areas outdoors, GPS pedometers work well, but if you exercise indoors or between high-rise buildings, performance suffers.
History
Originally intended for military use, the network of satellites in the Global Positioning System now provides accurate geo-location data to any civilian user with the right technology. GPS units measure the time required for a signal to travel from a satellite to the receiving unit. Comparing the transmission time of pulses from at least three satellites allows calculate of latitude and longitude. With connection to four satellites, accuracy improves and GPS measurement of altitude becomes possible. GPS pedometers reduce the physical size of the device and sometimes offer standard sensor-driven pedometer functions as well, according to University of Florida IFAS Extension.
Accuracy
The accuracy of the GPS readings depends on how many satellites the unit can contact, and upon the algorithms used to refine the raw data. Simpler units capable of receiving four channels of satellite data refine horizontal location to within 10 m or about 33 feet. Advanced GPS units measure position to within about 5-1/2 feet. Several popular GPS pedometers rate accuracy as equal to or less than the 10 m resolution, according to research conducted by Computing Research at the University of Florida.
Problems
When buildings, hills or trees block transmissions, GPS pedometers store the last known position and wait for another set of signals. When enough satellites move back into view, calculations resume. GPS units estimate travel through these blank spots with a straight-line, point-to-point calculation. GPS pedometers won't work inside buildings and don't record simulated travel on exercise bicycles or treadmills. Some units require precise positioning on the body for best reception of signals.
Accessories
Some manufacturers of GPS pedometers and GPS watches provide sensor units that attach to one of the owner's shoes, according to Family and Consumer Sciences at Texas A&M. After measuring average stride length and setting the device to count each step as that distance, owners can use the GPS devices to calculate mileage indoors, outdoors or on treadmills. The sensor units transmit pulses to the GPS unit with each footstep. Like ordinary pedometers, the GPS version calculates distance based on the total number of steps. In variable terrain expect accuracy between plus or minus 10 percent.
Device Differences
Consult the owner's manual of the GPS pedometer for the most reliable information about its accuracy. One popular GPS pedometer measures only speed and distance and calculates to within two percent of actual. More complex models that also compute location might claim accuracy to within 10 m, according to the Garmin website. Some include standby modes that halt readings unless the owner moves at least 35 feet, and shut themselves down after a predetermined interval of inactivity. Using a GPS pedometer for short walks could generate inaccurate readings, but over longer distances accuracy improves.
References
- University of Florida Extension: An Introduction to GPS
- Mobile Pervasive Computing Research: A Brief Survey of Physical Activity Monitoring Devices
- Suunto GPS Pod User's Guide
- Garmin Forerunner 205/305 User's Guide
- The Anatomy of a Pedometer: GPS vs SDM
- Garmin: Garmin® Sets Pace for U.S. Record-holders Deena Kastor, Ryan Hall in Training for Chicago and New York City Marathons
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