Gps Accuracy

[gcorbett]

Garmin iQue 3600 specification quoted GPS accuracy of 15 meters, subject

to accuracy degradation to 100m 2DRMS under the U.S. Department of Defense

Selective Availability Program. Do you know the development of this

program and how it will effect geocaching?

[+tozainamboku]

Garmin iQue 3600 specification quoted GPS accuracy of 15 meters, subject

to accuracy degradation to 100m 2DRMS under the U.S. Department of Defense

Selective Availability Program. Do you know the development of this

program and how it will effect geocaching?

 

The Selective Availability Program was used by the U.S. Department of Defense to degrage the accuracy of civilian GPS signal. The U.S. military would get the full accuracy of GPS while civilian users would be degraded. On May 1, 2000 a presidential order turned off Selective Availability. Civilian GPS users immediately saw an increase of accuracy. 2 days later the sport of Geocaching began . The government could conceivably turn the Selective Availability Program back on in an emergency, but it is unlikely.

[+briansnat]

The government could conceivably turn the Selective Availability Program back on in an emergency, but it is unlikely.

 

I could be wrong, but I understand they can turn on and off regionally if the need arises.

[+budd-rdc]

The government could conceivably turn the Selective Availability Program back on in an emergency, but it is unlikely.

 

I could be wrong, but I understand they can turn on and off regionally if the need arises.

I'm curious about this, since there have been tales of strange behaviors where coordinates would jump in hundreds of feet in apparently GPSr friendly environments. Any expert opinions?

[+Tonylama]

I could be wrong, but I understand they can turn on and off regionally if the need arises.

 

Apparently they can selectively turn on and off in a fifty foot circle, right over most of the cache I seek.

Edited March 9, 2006 by Tonylama

[StarPuller]

The U.S. and Canadian governments (among others) have set up Differential GPS (DGPS) stations to transmit correction signals. They are broadcast on existing marine beacon frequencies around 300 kHz. The DGPS system is available for use without a fee. There are other competing systems, both ground based and satellite based, that provide correction signals for a subscription fee - however are probably not worth the expense.

 

DGPS signals were originally developed by the U.S. Department of Defense (DOD) under their Selective Availability (SA) program (no longer in effect). These signals were meant to correct GPS in case of deliberate degradation of GPS signals by the DOD. DGPS signals are still used today to enhance the accuracy of GPS units. Keep in mind, these systems were not installed to make your unit perform less, it was installed to improve governmental positioning. They just decided to make the technology available to the population.

 

You can get additional information about this from the U.S. Coast Guard's website for locations and status of their differential stations: US Coast Guard Website.

 

The Garmin GBR 21 and GBR 23 (both are no longer in production or available) were optional receivers designed to receive the U.S. Coast Guard's differential signals and supply these signals to your Garmin GPS unit. When connected to a compatible Garmin GPS, the GBR 21 and GBR 23 will increase its accuracy to under 10 meters when within receiving range of differential beacon stations.

 

The development of WAAS has also made a significant difference to the reliability of positioning with a GPS. I believe you have WAAS on your unit, as most new GPS's will. It is a fact DGPS is no longer in use and can be turned on again, however with new breakthroughs in technology it will do little disrupt your GPS positioning capabilites.

 

You can get more information on this topic from a Google search of DGPS. If you have any additional questions let me know. Hope this helps.

[+PFF]

Garmin iQue 3600 specification quoted GPS accuracy of 15 meters, subject

to accuracy degradation to 100m 2DRMS under the U.S. Department of Defense

Selective Availability Program.

 

I have been using the Garmin iQue 3600 for about 18 months and it is very accurate. I have checked it numerous times against known points, such as US Government benchmarks.

 

However, you should know that the unit will "fudge" several hundred feet on the display to put you on the nearest road. You can disable this with the ROUTING preference. And, there is a preference which will put your position at the exact Latitude and Logitude, without stretching to the nearest highway. This is especially valuable in "off road" situations.

 

And here's a quirk: While standing exactly on the NC-SC state line (as evidenced by a government boundary marker), the iQue 3600 showed me as being 200+ feet north of the line. My teammate's Magellan showed him 200 feet south of the line.

 

That is what led me to search for internal controls to minimize distortion caused by "consumer convenience" programming.

 

-Paul-