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"Moving" co-ordinares

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The discussion and comments made by gerhardoosMPsa in this post - http://forums.Groundspeak.com/GC/index.php...t&p=3554024 - have prompted me to this topic.

 

Perhaps I need to start by asking a question. Does heat affect the accuracy of GPSr's at all? Here is the situation and I hope that some of the techno-buffs out there can help with solutions / explanations.

 

In January a cache [GC19160] was placed using 2 GPSr units to get the co-ordinates. An eTrex H and a Legend. The co-ords were published and the cache has since been found successfully by 8 cachers. Last weekend I visited the cache with another cacher and we encountered problems with the co-ordinates. 3 seperate GPSr units - an eTrex, Legend and Vista HCx - all showed the exact location of the cache - but 18m off!! ;) We checked and doube checked the readings, but it was confirmed that the published co-ordinates had now "changed".

 

The only thing that had changed was the ambient temperature. In January the temperature was in the region of 15'C when placed and last weekend it was about 51'C. Could that have been the reason? Will the co-ords "change" again when the temperature drops? Prior to last weekend the previous finds were all between January and March - when temperatures were at reasonable levels. :rolleyes:

 

I have not encountered this problem before and even in the height of the summer in the desert have managed to fnd caches - albeit within a radius of 10m which I considered acceptable.

 

Any comments would be appreciated.

 

Greetings from Qatar.

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Yes, this is a very interesting and I think I can learn from this topic. I also do not understand. Recently I climbed to the top of a mountain. I planted a cache and I was watching the accuracy of the GPS. The best I could get was at 6 meters. This is the last place where one should have problems and you should get to 4 meters accuracy. I turned on WAAS/EGNOS and no difference was noticed.

 

At some of the caches I had a clear view of the sky and I only got 6 meters accuracy. At one stage I told my brother that my GPS is faulty. He and I both have Quest 2 and we compared it. At the same point both GPS’s were giving the same waypoint. This eliminated the GPS as the problem. We then fitted the external aerial and the accuracy was back at 4 meters. This to me is pointing to weakness of signal. But why?

 

In the Gauteng area and MP the accuracy is 4 meters with a clear sky at most caches. Is there something I do not understand? If someone can explain this to me I will be grateful.

 

We learned something at the Western province. If there is any amount of mist or clouds then you are in for a battle. Is the signal that low?

 

gerhard

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In a way heavy cloud cov er can effect the signal quality so if it was cloudy maybe.

 

Here is an article that might give some light on it....

 

WAAS is implemented to overcome that though...

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The biggest single source of error in GPS posision is Ionospheric conditions, and to a lesser extent, humidity in the troposhere.

 

Temperature alone would'nt have the effect you experienced in Quatar, but it was probably caused by the compounded error sources, including bad satelite geometry.

 

Using more than one GPSr won't get better results either, as they are all using the same signals and the same algorithms to calculate a position. The only reliable way to better results, are with WAAS/EGNOS in countries that have ground stations (S.A. doesn't)

Or DGPS...

 

Try Goooogling too. :rolleyes:

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Thanks for the reply and explanation Jors. Any more ideas out there?

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A bit of a long winded explanation but here are all the factors that constitute to the accuracy of a GPS. Most of the clever people that work with these things everyday comes up with the same answer and that is that a civilian GPS (military GPS uses two signal frequencies) will have an accuracy of only about 15m. With WAAS or EGNOS enabled it will be reduced to 3m to 5m.

 

With the implementation of WAAS and EGNOS it s possible to set up „maps“ of the atmospheric conditions over different regions. The correction data are sent to the receivers, enhancing the accuracy considerably

 

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. Let’s assume the satellites are positioned in the north, east, south and west in 90° steps. Distances can then be measured in four different directions, reflecting a „good“ satellite geometry.

 

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 a weak 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 neighborhood of large buildings 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.

In the ionosphere in a height of 80 – 400 km a large number of electrons and positive charged ions are formed by the ionizing force of the sun. The electrons and ions are concentrated in four conductive layers in the ionosphere (D-, E-, F1-, and F2-layer). These layers refract the electromagnetic waves from the satellites, resulting in an elongated runtime of the signals.

These errors are mostly corrected by the receiver by calculations. The typical variations of the velocity while passing the ionosphere for low and high frequencies are well known for standard conditions. Theses variations are taken into account for all calculations of positions. However civil receivers are not capable of correcting unforeseen runtime changes, for example by strong solar winds.It is known that electromagnetic waves are slowed down inversely proportional to the square of their frequency (1/f2) while passing the ionosphere. This means that 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 analyzed with regard to their differing time of arrival, the ionospheric runtime elongation can be calculated. Military GPS receivers use the signals of both frequencies (L1 and L2) which are influenced in different ways by the ionosphere and are able to eliminate another inaccuracy by calculation.

The tropospheric effect is a further factor elongating the runtime of electromagnetic waves by refraction. The reasons for the refraction are different concentrations of water vapour in the troposphere, caused by different weather conditions. The error caused that way is smaller than the ionospheric error, but can not be eliminated by calculation. It can only be approximated by a general calculation model.

 

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.

 

The errors of the GPS system are summarized in the following table. The individual values are no constant values, but are subject to variances. All numbers are approximative values.

Ionospheric effects ± 5 meter

Shifts in the satellite orbits ± 2.5 meter

Clock errors of the satellites' clocks ± 2 meter

Multipath effect ± 1 meter

Tropospheric effects ± 0.5 meter

Calculation- und rounding errors ± 1 meter

 

Altogether this sums up to an error of ± 15 meters. Corrections by systems like WAAS and EGNOS, (Not available in South Africa as yet) which mainly reduce ionospheric effects, but also improve orbits and clock errors, the overall error is reduced to approximately ± 3 - 5 meters.

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Well what can I say? I asked for it didn't I. :huh: But then I knew that there had to be some geek out there that would respond and sure enough Quintus rose to the occassion. :huh:

 

Thanks for the explanation. Although it doesn't answer my question 100% it certainly has helped me to understand the way the receiver works and the various things that can affect its accuracy. I understand that WAAS is available in the Middle East. I cannot find any website that confirms it though, but when I enable or disable the setting on my Vista HCx there is a very distinct difference in accuracy shown. The normal 10m accuracy drops to 3m immediately when WAAS is enbaled so I am of the opinion that it does in fact work here.

 

Getting back to my original question then. Extremely high temperature effecting accuracy? From what I can gather from the various explanations above there could well be an effect due to temperature. However, this was only noticed on ONE cache location visited on that particular day. 3 other locations did not have the same "erroneous" reading. I will watch GC19160 with interest as the weather starts to cool down in October/November again. It will be interesting to see whether there is any difference in accuracy.

 

BTW, of the units involved with the readings taken only 2 had WAAS and 1 did not. The yellow eTrex without WAAS showed exactly the same "error" as the other 2 - namely 17m. Just has me thinking again. :o:o

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