Selective Availability

This is obsolete info. SA was turned off in 1998, so this info no longer applies. Someday I'll update this page.

'Selective availability' (SA) is built in variable error that the US military makes the GPS satellites transmit. It's theoretically to keep Saddam from buying a $100 GPS unit from America, strapping it to an ICBM and sending it our way with great accuracy. The error is changing all the time, every second. Spot readings in the same place with the same GPS will change slightly every reading. 95% of the time the position shown on your GPS receiver is supposed to be off 100 meters or less, (like an atom bomb is going to matter much if it's 100 meters off target!). The other 5% of the time the error can be more, but won't necessarily be. 100 meter error means you are within a circle 200 meters in diameter. SA is supposed to be shut off and accuracy to be within just a couple meters or so by the year 2,006 (maybe sooner).

The really funny part about the above is that SA was turned off during the Desert Storm War of '94 (reportedly, the Dept. of Defense is mum on this topic). It seems we didn't have enough Military GPS units available (which filters out SA) so many of our people had to use civilian units. The Military decided that our guys and gals knowing for sure where they were was more important than the few hostiles that might have our units knowing where they (and us) were (and there are supposedly enough military GPS units available that they won't have to turn it off again). Now, if they turn it off in wartime, when its theoretically needed the most....

Use your "Back" button or UTM Coordinate Basics or Coord Explanation

Copyright © 1996 - 2006, and legal fine print
Revised: 3 August, 2006
http://www.canoebase.com/wildcamp/sa.htm