GPS errors

A number of conditions can reduce the accuracy of a GPS receiver. From a top-down perspective (from orbit down to ground level), the possible sources of trouble look like this:

• Ephemeris errors: Ephemeris errors occur when the satellite doesn’t correctly transmit its exact position in orbit.
• Ionosphere conditions: The ionosphere starts at about 43–50 miles above the Earth and continues for hundreds of miles. Satellite signals traveling through the ionosphere are slowed down because of plasma (a low density gas). Although GPS receivers attempt account for this delay, unexpected plasma activity can cause calculation errors.
• Troposphere conditions: The troposphere is the lowest region in the Earth’s atmosphere and goes from ground level up to about 11 miles. Variations in temperature, pressure, and humidity all can cause variations in how fast radio waves travel, resulting in relatively small accuracy errors.
• Timing errors: Because placing an atomic clock in every GPS receiver is impractical, timing errors from the receiver’s less-precise clock can cause slight position inaccuracies.
• Multipath errors: When a satellite signal bounces off a hard surface (such as a building or canyon wall) before it reaches the receiver, a delay in the travel time occurs, which causes an inaccurate distance calculation.
• Poor satellite coverage: When a significant part of the sky is blocked, your GPS unit has difficulty receiving satellite data. Unfortunately, you can’t say that if 50 percent (or some other percentage) of the sky is blocked, you’ll have poor satellite reception; this is because the GPS satellites are constantly moving in orbit. A satellite that provides a good signal one day may provide a poor signal at the exact same location on another day because its position has changed and is now being blocked by a tree. The more open sky you have, the better your chances of not having satellite signals blocked. Building interiors, streets surrounded by tall buildings, dense tree canopies, canyons, and mountainous areas are typical problem areas.

If satellite coverage is poor, try moving to a different location to see whether you get any improvement.