zzz wrote:
Over the counter Gps Units are not allowed to be 100% accurate,as per Federal Reg.
Error was removed back in 2000. However, here a re a few snippets from Garmins Website.
From Garmin's Website:
Certain atmospheric factors and other sources of error can affect the accuracy of GPS receivers. Garmin® GPS receivers are accurate to within 15 meters on average.
Sources of GPS signal errors
Factors that can degrade the GPS signal and thus affect accuracy include the following:
Ionosphere and troposphere delays — The satellite signal slows as it passes through the atmosphere. The GPS system uses a built-in model that calculates an average amount of delay to partially correct for this type of error.
Signal multipath — This occurs when the GPS signal is reflected off objects such as tall buildings or large rock surfaces before it reaches the receiver. This increases the travel time of the signal, thereby causing errors.
Receiver clock errors — A receiver's built-in clock is not as accurate as the atomic clocks onboard the GPS satellites. Therefore, it may have very slight timing errors.
Orbital errors — Also known as ephemeris errors, these are inaccuracies of the satellite's reported location.
Number of satellites visible — The more satellites a GPS receiver can "see," the better the accuracy. Buildings, terrain, electronic interference, or sometimes even dense foliage can block signal reception, causing position errors or possibly no position reading at all. GPS units typically will not work indoors, underwater or underground.
Satellite geometry/shading — This refers to the relative position of the satellites at any given time. Ideal satellite geometry exists when the satellites are located at wide angles relative to each other. Poor geometry results when the satellites are located in a line or in a tight grouping.
Intentional degradation of the satellite signal — Selective Availability (SA) is an intentional degradation of the signal once imposed by the U.S. Department of Defense. SA was intended to prevent military adversaries from using the highly accurate GPS signals. The government turned off SA in May 2000, which significantly improved the accuracy of civilian GPS receivers.
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Back to my analysis.
Most hand held GPSs sample at a 1 second update rate...One particular problem with running is a change in direction, particularly 180 turnarounds. That will effect the receivers inaccuaracy substanially. Because it could miss the updates and think your at the same spot in the turnaround. Multipath in a city environment where GPS signals are bouncing off of skyscrapers can fool the receiver in thinking it's in one position, when it's actually in a different spot. Large and Tall buildings can block signals all together, which render the GPS inneffective.