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How Geocaching Works
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Ah, GPS accuracy. We all have trouble with it from time to time. Our GPSrs are state of the art electronics. They seem almost magical! So why, after all of our hard work ensuring the best coordinates possible for finders, do we still end up with logs like. “Your coords are off by 20 feet!”? Well it turns out that there are many reasons for two different cachers on two different days to have coordinates that may be as much as 20 to 30 feet off. Below I’ve briefly (and very simplistically) listed some of the major causes of signal error. I’ve also listed some links at the bottom of the page for those of you who wish to read more about the magical technology that drives our sport.
There are no perfect coordinates - only perfect for that particular time and space.
Sources of Errors in GPS
One factor influencing the accuracy of the position determination is the "satellite geometry". Simplified, satellite geometry describes the position of the satellites to each other from the view of the receiver. If a receiver sees 4 satellites and all are arranged for example in the north-west, this leads to a “bad” geometry. In the worst case, no position determination is possible at all, when all distance determinations point to the same direction. Even if a position is determined, the error of the positions may be up to 100 – 150 m. If, on the other hand, the 4 satellites are well distributed over the whole firmament the determined position will be much more accurate
The satellite geometry is also relevant when the receiver is used in vehicles or close to high buildings or dense foliage. The larger the obscured part of the sky, the more difficult the position determination gets. The error in the position determination caused by the satellite geometry also depends on the latitude of the receiver.
Although the satellites are positioned in very precise orbits, slight shifts of the orbits are possible due to gravitation forces. Sun and moon have an influence on the orbits. The orbit data are controlled and corrected regularly and are sent to the receivers in the package of ephemeris data. Therefore the influence on the correctness of the position determination is rather low, the resulting error being not more than 2 m.
The multipath effect is caused by reflection of satellite signals (radio waves) on objects. It was the same effect that caused ghost images on television when antennae on the roof were still more common instead of todays satellite dishes. For GPS signals this effect mainly appears in the neighbourhood of large buildings, tall trees or other elevations. The reflected signal takes more time to reach the receiver than the direct signal. The resulting error typically lies in the range of a few meters.
Another source of inaccuracy is the reduced speed of propagation in the troposphere and ionosphere. While radio signals travel with the velocity of light in the outer space, their propagation in the ionosphere and troposphere is slower. These layers refract the electromagnetic waves from the satellites, resulting in an elongated runtime of the signals and therefore inaccurate readings. Civilian receivers are not capable of correcting unforeseen runtime changes, for example by strong solar winds. Electromagnetic waves with lower frequencies are slowed down more than electromagnetic waves with higher frequencies. If the signals of higher and lower frequencies which reach a receiver are analysed with regard to their differing time of arrival, inaccuracy results.
Clock inaccuracies and rounding errors
Despite the synchronization of the receiver clock with the satellite time during the position determination, the remaining inaccuracy of the time still leads to an error of about 2 m in the position determination. Rounding and calculation errors of the receiver sum up approximately to 1 m.
In the normal life we are quite unaware of the omnipresence of the theory of relativity. However it has an influence on many processes, among them is the proper functioning of the GPS system. It’s beyond the scope of this cache page to go into detail describing relativity but the theory of relativity says that time moves slower the stronger the field of gravitation is. This affect the GPS signal and in turn their accuracy.
The errors of the GPS system are summarized in the following table. The individual values are not constant values, but are subject to variances. All numbers are approximative values.
Ionospheric effects ± 5 meters
Shifts in the satellite orbits ± 3.5 meter
Clock errors of the satellites' clocks ± 2.5 meter
Multipath effect ± 1 meter
Tropospheric effects ± 0.5 meter
Calculation- und rounding errors ± 1 meter
BASIC FACTORS OF GPS ACCURACY
Last Updated: on 8/18/2013 10:11:45 AM Pacific Daylight Time (5:11 PM GMT)
Coordinates are in the WGS84 datum