20-30 Feet Off Mystery Cache
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Difficulty:
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Terrain:
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Size: 
(regular)
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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!”?
The cache is NOT at the above listed cordinates..but
is 20-30ft away..now you have to find it..Lucky for you it is a
ammobox!:)
Plenty of parking available nearby....
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 have briefly (and very
simplistically) listed some of the major causes of signal error. I
have 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
"Satellite geometry"
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.
Satellite Orbits
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.
Multipath effect
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.
Atmospheric effects
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.
Relativistic effects
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 and rounding errors ± 1 meter
Be Sure to Visit This Cache Nearby.....Just A Few 100 ft.
Away!!!!!
"A Dam Site Better"
Charlottesville's Oldest Active Geocache!
Additional Hints
(No hints available.)