According to the government and GPS receiver manufacturers, expect your GPS unit to be accurate within 15 meters (49 feet), but you can get more accuracy by using a GPS receiver that supports corrected location data.
You should be clear about something before you start training with a GPS watch. You should not expect perfectly precise accuracy all the time. Speed and distance measurements are based on position data, so if the coordinates received from satellites is not entirely accurate, then the reported speed and distance data won’t be either.
The main cause of inaccurate position data is satellite signal reception. The most obvious obstruction of satellite signals is when there are large objects causing obstruction. Although modern GPS receivers are improving, you are not going to have accurate GPS readings unless you have a wide open view of the sky.
To most athletes this will not be critical as the GPS data is far more accurate than taking a guess, but your GPS watch should be treated as another tool that offers the training data required to fine-tune your workouts and performance in the same way that a heart rate monitor would, or an interval timer feature.
A couple of ways to address GPS accuracy issues are:
- Repeat and average: If you run the same course more than once, measure it several times and take the average over a number of readings
- Look for trouble spots: Keep a close eye out for areas on the course that might affect accuracy.
- Compare with other tools: A service like MapMyRun.com can verify the accuracy of the distances of your favorite courses
Recently the leading watch manufacturers have upgraded their watches to use the SiRFstar III chipset, which supports corrected location data, providing much better accuracy over legacy watches.
Corrected information is broadcast over radio signals that come from either non-GPS satellites or ground-based beacons.
Two common sources of more accurate location data are
• Differential GPS (DGPS)
• Wide Area Augmentation System (WAAS)
SiRFstar III watches connect to the WAAS network. The WAAS network is specified to have a worst-case accuracy of 7.6m laterally and 7.6m vertically, but independent testing has shown it to be well within these limits.
WAAS is an air navigation aid developed by the Federal Aviation Administration to augment the GPS network, with the goal of improving its accuracy, integrity, and availability.
The system has a series of ground-reference stations throughout the United States. These monitor GPS satellite data and then send the data to two master stations — one on the west coast and the other on the east coast. These master stations create a GPS message that corrects for position inaccuracies caused by satellite orbital drift and atmospheric conditions.
The corrected messages are sent to non-NAVSTAR satellites in stationary orbit over the equator. The satellites then broadcast the data to GPS receivers that are WAAS-enabled.
Under certain conditions — say, when weak WAAS satellite signals are being received or the GPS receiver is a long way from a ground station — accuracy can actually worsen when WAAS is enabled.
Selective Availability (SA)
The GPS user didn’t always have a high level of accuracy. GPS satellites transmit two types of radio signals: C/A-code (coarse acquisition) – intended for consumer GPS units and P-code (precision) intended for use by the U.S. military.
In the 1970’s, studies showed that the C/A-code signal was more accurate than the U.S. government had intended. Originally it was thought to provide accuracy to within 100 meters, yet experiments showed it to be in the range of 20-30 meters.
To reduce the accuracy of C/A-code, the U.S. government developed Selective Availability (SA). SA adds errors to the NAVSTAR satellite data and prevents consumer GPS receivers from providing an extremely precise location fix.
On May 2, 2000 however, SA was turned off permanently and accuracy of civilian GPS users went from 100 meters to 15 meters. The decision to turn off SA on a global scale was related to the U.S. military’s ability to degrade the C/A-code on a regional basis.
For instance, during the invasion of Afghanistan, the American military jammed GPS signals in Afghanistan to prevent the Taliban from using consumer receivers in operations against American forces.
|Lateral Accuracy||Vertical Accuracy|
|GPS Specification (SA on)||100 m||150 m|
|GPS Measured*||2.5 m||4.7 m|
|WAAS Specification||7.6 m||7.6 m|
|WAAS Measured†||0.9 m||1.3 m|
* The actual measured accuracy of the system (excluding receiver errors), with SA turned off, based on the findings of the FAA’s National Satellite Test Bed, or NSTB
† The actual measured accuracy of the system (excluding receiver errors), based on the NSTB’s findings.
Accuracy of GPS Sports Watches
When you purchase a GPS watch, consider the chipset of the GPS. Many older GPS watches have inferior GPS capability; they will take longer to acquire a satellite signal and the signal strength will be lost more easily in these older models.
We have listed below tables of GPS watches that have superior GPS chipsets and those that have older chipsets with poorer GPS reception.
GPS Watches With Modern GPS Chipsets
These are GPS watches that by all accounts have modern chipsets equivalent to the SiRFstarIII. The GPS on these watches will lock onto GPS and WAAS satellite signals faster and will be more reliable around dense areas like forests or canyons.
|Garmin Forerunner||205||SiRFstarIII||Product Link|
|RS800CX with G3 GPS||SiRFstarIII||Product Link|
|Timex||S & D T5E701||Navman||Product Link|
|Bodylink T5F011||Navman||Product Link|
|Bodylink Trailrunner T5J985||Navman||Product Link|
|Bodylink T5G311||Navman||Product Link|
GPS Watches With Older GPS Chipsets
These are GPS watches that have older chipsets. The GPS on these watches will be adequate, however it will take longer to acquire a satellite signal and will less reliable around densely populated areas. The signal is liable to drop out wherever there are objects obstructing the signal reception.
|Garmin Forerunner||101||Unknown||Product Link|
|Suunto||X9/i and X9M/i||Unknown||Product Link|
For more information on GPS watches, visit our GPS watches information page.
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