MT Mimulous in SecondLife & Xraymancouk on Twitter asked:
Could the Americans just turn GPS off anytime they like? Can the US military restrict access to certain groups or individuals?
We put this to Professor Todd Humphreys:
Todd - Well, itís important to recognize that the US has a strong incentive to ensure that GPS is reliable, and thatís because a good deal of the world economy depends critically on GPS, especially for timing. So, any military initiated or US initiated denial of civilian GPS would most likely be a surgical pinpoint denial in some military area of operations. Itís possible but itís very unlikely because the US has such a strong incentive to make sure everybody understands GPS can be counted on.
Ben - As a blanket thing doesnít civilian GPS only work up to a certain altitude?
Todd - Well yes. Civilian GPS receivers are required to carry inside themselves certain checks against going too fast and going too high so that in the case that they were strapped to some intercontinental ballistic missile, they would fail to work. So thatís one way that the US is trying to prevent unauthorised use of GPS in a various purposes.
Ben - But obviously, assuming weíre not strapped to an intercontinental ballistic missile, we can still use it to find out where we are to go treasure hunting and to make sure we know where we are on a map.
Todd - Thatís right. All of those good activities!