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Gps Accuracy Experiment - Any Volunteers?

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I'm looking for volunteers to fill in a form for me for a period of thirty days. At the end of this time I'll collaborate all the data and make it available to anyone who wants it. The purpose of the experiment follows on a visit that myself and cache fan did to Faerie Glen Nature Reserve today.


We both had the same GPS (Garmin Vista) and both got similar readings, yet my original data, taken on a different day and time were different to todays readings. :ninja:


Whilst our GPS readings differed slightly they were not drastically different which leads me to believe that the time of day and day of week may affect the readings, which in turn may affect our geocaching.


If you're interested in participating in this little experiment, please email me at bombtech_sa@yahoo.com


All you need to do is take a GPS reading at the same place and time (home??) twice a day for thirty days and note which satellite numbers are used by the GPS to fix your location. You may take as many readings as you like. I have compiled a form ready to be filled in, all you have to do is print it. You can either email the stats at the end or fax them to me with your GPS details. (make and model)


Hopefully, I'll have a very full inbox in the days to come. :)



I'll nag QFC and see if he will post the data on his site :D

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Hi, I can help, it would be interesting to see if the time of day makes a difference. It should not, as the GPS system makes use of satelites in a geo-stationary orbit. (edit - this last bit is incorrect, I mistakenly thought they were in geostationary orbit - check out this link Trimble - Positions


I suspect that the weather conditions may be to blame, more so than anything else - presume that there are no other obstacles to receiving the satelite signals. Maybe add a column to your page for weather conditiones (clear sky, windy, slight overcast, overcast, rain, thunder storms)?


Check out this link for more:

Trimble GPS

Edited by perdix
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I will do it as often as possible, but If I understand what you are trying to do, it won't matter too much if someone misses out on a day or two, Here is some info I got from the wonderful wikipedia:


Global Positioning System:

The GPS system uses a satellite constellation of at least 24 active satellites in intermediate circular orbits. The constellation also includes three spare satellites in orbit, in case of any failure. Each satellite circles the Earth exactly twice each day at an altitude of 20,200 kilometres (12,600 miles). The orbits are aligned so at least four satellites are always within line of sight from almost any place on Earth. There are four active satellites in each of six orbital planes.


The flight paths of the satellites are measured by five monitor stations around the world (Hawaii, Kwajalein, Ascension Island, Diego Garcia, Colorado Springs). The master control station, at Schriever AFB, processes their combined observations and sends updates to the satellites through the stations at Ascension Island, Diego Garcia, and Kwajalein. The updates synchronize the atomic clocks on board each satellite to within one microsecond.


The GPS receiver calculates the orbit of each satellite based on information encoded in their radio signals, and measures the distance to each satellite, called a pseudorange, based on the time delay from when the satellite signals were sent until they were received.


In order to measure the delay, the satellite repeatedly sends a 1,023 bit long pseudo random sequence; the receiver calculates an identical sequence from a known seed number, and shifts it until the two sequences match.


One of biggest problems for GPS accuracy is that changing atmospheric conditions change the speed of the GPS signals unpredictably as they pass through the ionosphere. The effect is minimized when the satellite is directly overhead and becomes greater toward the horizon, as the satellite signals must travel through the greater "thickness" of the ionosphere as the angle increases. Once the receiver's rough location is known, an internal mathematical model can be used to estimate and correct for the error.


Because ionospheric delay affects the speed of radio waves differently based on their frequencies, the second frequency band (L2) was used to help eliminate this type of error. Some military and expensive survey-grade civilian receivers can compare the difference between the L1 and L2 frequencies to measure atmospheric delay and apply precise corrections.


GPS signals can also be affected by multipath issues, where the radio signals reflect off surrounding terrain- buildings, canyon walls, hard ground, etc. This delay in reaching the receiver causes inaccuracy.

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Thanks QFC, now if we can tell how many satellites are generally above RSA we should be able to work out when the best cache placing times are to get the best accuracy.


What your above post also says is that we will need to have a look at how many sats are above and how many are towards the horizon. So perhaps we can number the sats according to the abbreviation A for above and H for horizon.


Anyone know of any fixed survey markers in Gauteng??

I know there is one in the waterfront in Cape Town but surely that cant be the only one in the country?????

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Anyone know of any fixed survey markers in Gauteng??



Check out my note at Cornwall Hill Memorial


Last time I looked, there were just under 30 000 (Thirty thousand) trig beacons in the country, of which this one at Eucaliptus is a fine example:



Also, there are Town survey marks in some built-up areas (Pretoria included) which looks something like this:

inside.jpg, closed.jpg


If you wish, I can send you some of the files in a format you can read.

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Great to hear Wolkynou, the more differences the better. What times did you take the readings??

I have been using two different GPSr's and the differences are incredible.


There have been days when the weather has been really miserable and only 3 sats picked up on the Vista. Fortunately the SIrf chip did a little better and got 4.


The more I test, the more I understand!!!

And its honestly amazing that we actually find any caches :rolleyes::P

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Took the readings at 13:00 and 18:00 ... had a call earlier today from other cachers ... they found my on cache on quite different coordinates ... My question now: Must I really ammend the coordinates that I originally listed the cache or must I keep the original coords ... will changing them help others to find it quicker ... Think I'll keep the originals

Edited by Wolkynou
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When I started caching a few years ago, I checked the "drift" of my location over a day, taking several readings every 30 minutes. I repeated this kind of exercise a few months later, but in the second case I downloaded the GPSr's location data every second, for about three hours, using the NMEA option on my unit, and some freeware GPS software to plot the location.


A quick summary of my findings is that the two factors that affected my readings most are:


* number of satellites being picked up (with only three, I have been out by up to 150 metres on occasion!!!!)


* satellite geometry (if they are all in one line, accuracy is compromised; my units warns me about poor geometry)


Since my unit is the original one used on Noah's Ark, a Magellan Trailblazer XL, steam-powered!!, I can't really comment on how it's accuracy might compare to much newer units. The sensitivity is definitely lower, so foliage affects my unit much more severely than newer models. The processing power is also probably a lot lower, so the algorithms to calculate the location are possibly not as good, in order to cut down on calculations.


However, none of this has stopped me from successfully finding caches, nor placing some. Also, I have to admit that I generally use a magnetic compass to locate the cache, so once I'm about 50 metres from it, I triangulate with the compass and then switch off the GPSr. The compass has definitely improved my ability to locate the cache quickly.

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