Gps Accuracy

[Lary Fusco]

I have been away from geocaching for about a year now. recently I heard on th that gps data is accurate to within a few (very few) feet. As one newscaster said....millimeter accuracy.

 

Last I heard, accuracy was about 39 feet-correct? Or did I miss something major?

 

Lary Fusco

laryofvt@sover.net

[+DustyJacket]

Professional GPS like the backpack-sized Trimble with tripod mounted antenna and all thet, that the surveyors or government uses are milimetre (or is it centemetre?) accurate.

 

Recreational GPS receives (without cover) can be under 10 feet.

Edited November 25, 2003 by DustyJacket

[+nincehelser]

If I remember correctly, it's more like 15 feet. Perhaps 10 feet if you can get the WAAS signal.

 

If you consider elevation, the error margin is typically doubled due to the the unfavorable geometery.

 

There are more professional units that I believe are more accurate, but they typically require taking data for longer periods of time and do some post-processing of the data. I doubt if even that is in the millimeter range...at least for map coordinates...I can't imagine that maps are that accurate anyway.

 

I think they can get down to millimeters between specific points, though. Say they take a measurement at point A, they can take another measurement and say point B is this many millimeters away. Relating this to map lat/long is a different matter, however.

 

George

Edited November 25, 2003 by nincehelser

[Dan Vull]

If I remember correctly the original design specs were for 15M positional accuracy at mach 3. Of course the march of technology has helped refine these capabilities. All uncorrected GPS lives out in the 30-40 foot range, correct it with WAAS and it can be 10-15 feet, correct it with another dual freq survey grade unit utilizing real time radio modems and then you are talking cm. Then there is the world of post processed static data, relative accuracies using this method approach 1 part in 10,000,000. Not too shabby.

[qman2]

This is from GARMIN:

 

100 meters: Accuracy of the original GPS system, which was subject to accuracy degradation under the government-imposed Selective Availability (SA) program.

15 meters: Typical GPS position accuracy without SA.

3-5 meters: Typical differential GPS (DGPS) position accuracy.

< 3 meters: Typical WAAS position accuracy.

 

You can't get millimeter accuracy without DGPS

[Kerry.]

GPS accuracy can be different things to different people depending on the equipment and requirements.

 

Millimetre accuracy (even less) is certainly possible with the right gear used in the right way by the right people but can be dependent on time or distance. With post processing time can be the determining factor and in some cases data can be recorded for hours/days/weeks (even months) to achieve a specific result. With real-time differential then the limitation is distance and the type of differential. Many "different" types of differential also with different accuracy capabilities.

 

For what cachers generally use the Standard Positioning Service (SPS) accuracy is specified at less than 13 metres (~43 feet) 95% of the time, world average.

 

World worst case scenario is spec'd at less than 36 metres (~118 feet) @ 95%. The worst case scenario includes the possible loss of any 2 satellites from a standard 24 satellite constellation.

 

These accuracy figures are Signal In Space (SIS) and don't take into account the possible affect of atmospheric, obstruction or user issues.

 

Then there's also the other unknown 5%

 

Right now the constellation is fairly strong and in general "most" users "probably" get less than 10 metres "most" of the time and probably average less than 7 metres "much" of the time down to less than 5 metres "some" of the time.

 

Cheers, Kerry.

[rrostie]

I read in a usenet newsgroup that Selective Availability was turned off after WAAS became prevalent, this was several years ago.

[Kerry.]

I read in a usenet newsgroup that Selective Availability was turned off after WAAS became prevalent, this was several years ago.

When Selective Availability was discontinued (it hasn't really been turned off, as such) WAAS was still an infant and then stagnated for many years trying to find out its purpose in life.

 

Gee, if it hadn't been for the Transport department the FAA might have still been deciding if and when to actually declare it partially available for some roles and change the test flag. This only changed in July 2003, something like 3 years after SA was discontinued.

 

Cheers, Kerry.

[+GOT GPS?]

It depends on how much you want to spend on a good unit, and whether or not you use an amplified external antenna.

[Cachengrab]

This is from GARMIN:

 

100 meters: Accuracy of the original GPS system, which was subject to accuracy degradation under the government-imposed Selective Availability (SA) program.

15 meters: Typical GPS position accuracy without SA.

3-5 meters: Typical differential GPS (DGPS) position accuracy.

< 3 meters: Typical WAAS position accuracy.

 

All I can tell you for now is that during a few night raids this week my Rino 120's pointer circled the cache as I circled the cache. Clear sky and great reception.

[koz]

dumb question: i've got an older model and everyone always say my coords are "way off" e.g. 120 feet...question is are the older units much less accurate?...is sa "turned off" for older units?

[+GOT GPS?]

Please state the model of the GPS, since, we are left guessing on your older model.

 

You may need a firmware update on it, also you should stand at a spot for no less than 5 minutes before marking a waypoint to post for a cache you placed. You may need to upgrade to a 12 channel GPS(All new ones are at least 12 channel now).

Edited December 1, 2003 by GOT GPS?

[Kerry.]

Accuracy with older units "might" differ slightly but will still be less (generally) than the systems accuracy specifications and SA being a system function is discontinued for ALL units. Coords "way off" by 120 feet sounds like another issue/s other than and accuracy issue.

 

Cheers, Kerry.

Edited December 1, 2003 by Kerry.

[Vlad]

an 8 channel gps is going to give you around 15m (45 feet) accuracy best case - a 12 channel around 8m (25 feet) best case - 12 channel with waas/dgps 3m (10 feet) best case. emphasis on the best case...

[Bobthearch]

3-6 meters seems to be the norm for my unit. That's in the open desert. Tree cover, canyons, and clouds can greatly diminish accuracy.

 

My unit is a GPSMAP76S

 

Best Wishes,

Bob

[Kerry.]

an 8 channel gps is going to give you around 15m (45 feet) accuracy best case - a 12 channel around 8m (25 feet) best case - 12 channel with waas/dgps 3m (10 feet) best case. emphasis on the best case...

Why ?

 

Cheers, Kerry.