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GPS accuracy prediction


Martti
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Did you konw that the accuracy of the GPS receiver depends on the amount of visible satellites and their geometry? When measuring the coordinates of the cache you should be sure that the conditions are optimal. Nowadays in Tampere area the critical time is just after noon.

 

This can be checked using software Mission Planning, which is available for download at

ftp://ftp.thalesnavigation.com/software/winprism/mp/

 

To use that software you need also updated almanac file which is available at ftp://ftp.thalesnavigation.com/almanacs/

 

Martti

 

[This message was edited by Martti on April 12, 2003 at 12:42 AM.]

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quote:
Originally posted by Martti:

Did you konw that the accuracy of the GPS receiver depends on the amount of visible satellites and their geometry?

Martti

 

[This message was edited by Martti on April 12, 2003 at 12:42 AM.]


 

No, I did not know this. I assume that all this talk of accuracy has to do with the myth that Earth is a giant steel ball bearing. icon_cool.gif

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Are you reading my mind? I knew there must be software like these and was about to ask. I have been fighting with couple of caches under woods and unfortunately while being in those places there has always been a very small amount of satellites visible and they have all been just above me so there was actually no use for GPSr.

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> Maybe this article - DPGS on Garmin receivers - could be of use?

 

In Finnish point of view I would like to mention the FOKUS service which is provided by Digita Oy via the Finnish radio broadcast station network. The GPS differential correction signal is transmitted in RDS channel on Radio Suomi frequencies. Using this service the GPS accuracy is better than 2 m at 95 % of time. This service is not free, you need a special RDS receiver and a license from Digita Oy. Sweden and some other European countries have similar services. It is - as far as I know - also available in some areas in US.

 

Mainly for ships there is another (and free) differential correction service in Finland as described (in Finnish) provided by Finnish Maritime Administration

 

Martti

 

[This message was edited by Martti on April 23, 2003 at 07:59 AM.]

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This is not actually accuracy prediction matter but monitoring problem. I have seen several times even with good satellite geometry and GPS performance following thing: I mark a waypoint in same physical location several times and they make a group with little variance. Then I turn the unit off and on again. If I now mark the waypoint couple of times in the same location I get a group again but the distance between these two groups may exeed significantly the accuracy reading of the unit.

 

This thing has also helped me some times while looking for a cache: the offset has led me to the right stone or at least near it after a long seeking in wrong point. Has anybody noticed anything like this? I use the basic yellow eTrex.

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Few times the GPSr has showed two 'zero-points' about 15-25 meters away from each others but this was propably caused by environment (buildings, cliffs, foliage...)

GCF6D8 resides on quite open place and the same thing applied there too. The second spot was correct, did not check the first one again after finding the stash.

Next time I have finding problems I'll try this reboot system, it sounds interesting. Got to examine anyway.

 

[This message was edited by Erwast on May 13, 2003 at 11:23 PM.]

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I have similar experiences, for example with Highway Star. It was a bright day, and the satellite geometry was good. My GPS unit directed me constantly to the wrong location. The deviation was about 20 meters. The spot did not change as I spent some 30 to 45 minutes there and checked the reading regularly. Some days later, I came back, and my GPS guided me directly to a different place, the right one.

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