Receiver clock error can be eliminated, however, by comparing times of arrival of signals from two satellites (whose transmission times are known exactly).

Receiver clock: GPS receivers are equipped with quartz crystal clocks that are less stable than the atomic clocks used in NAVSTAR satellites.

Only about three-quarters of the bias can be removed, however, leaving the ionosphere as the second largest contributor to the GPS error budget. By modeling ionosphere characteristics, GPS monitoring stations can calculate and transmit corrections to the satellites, which in turn pass these corrections along to receivers. The delay also depends on how close the satellite is to being overhead (where distance that the signal travels through the ionosphere is least). The ionosphere density varies thus, signals are delayed more in some places than others. Upper atmosphere (ionosphere): As GPS signals pass through the upper atmosphere (the ionosphere 50-1000km above the surface), signals are delayed and deflected.These errors are minimized by calculating clock corrections (at monitoring stations) and transmitting the corrections along with the GPS signal to appropriately outfitted GPS receivers. NAVSTAR satellites use atomic clocks, which are very accurate but can drift up to a millisecond (enough to make an accuracy difference). Satellite clock: GPS position calculations, as discussed above, depend on measuring signal transmission time from satellite to receiver this, in turn, depends on knowing the time on both ends.A complete list of the sources of User Equivalent Range Errors, in descending order of their contributions to the total error budget, is below: User Equivalent Range Errors (UERE) are those that relate to the timing and path readings of the satellites due to anomalies in the hardware or interference from the atmosphere. How, then, can you minimize errors associated with mapping-grade receivers? A sensible start is to understand the sources of GPS error. For example, if your assignment was to GPS 1,000 manholes for your municipality, you probably wouldn't want to set up and calibrate a survey-grade receiver 1,000 times. Still, there are lots of mapping applications for which it's not practical to use a survey-grade unit. In general, GPS users get what they pay for.ДЄs the market for GPS positioning grows, receivers are becoming cheaper. But, if you were to invest several thousand dollars in a dual frequency, survey-grade receiver, your error circle radius might be as small as a centimeter or less. If you used a slightly more expensive WAAS-enabled single frequency receiver, your error ellipse might shrink to one to three meters or so (WAAS makes use of both the satellite signals and a network of ground reference stations to increase accuracy for more on WAAS, see the FAA WAAS site). If you used a very low cost GPS receiver, the radius of the circle you drew might be as much as ten meters to capture 95 percent of the points.

The answer depends in part on your receiver. What would the radius of that circle or ellipse be? (In other words, what is your receiver's positioning error?) Now, imagine drawing a circle or ellipse that encompasses about 95 percent of the points. What do you suppose the plot would look like?Дo you imagine a cloud of points scattered around the actual location? That's a reasonable expectation. Next day, plot the 144 coordinates your receiver calculated. Turn it on and record its position every ten minutes for 24 hours. Try this thought experiment (Wormley, 2004): Attach your GPS receiver to a tripod.

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