Don't count on GPS

[Mark Krawec]

Driving around Kyushu one afternoon, I happened to wander into a section of a national park where, according to the GPS, I vanished off the face of the earth - the pointer indicating position & facing disappeared, the map stopped updating, everything froze. Several kilometres later we reappeared.

If that can happen in Japan, which you'd think is just about the most wired place in the world, it can happen anywhere.

[Epoch]

GPS doesn't depend on local "wired" ability. It uses satellites in relatively high orbit. Your local antenna needs to be able to "see" at least two satellites, and three+ is practically necessary for a good fix.

Probably what happened is that you entered an area where your antenna couldn't see very much of the sky. The classic example would be a canyon or grotto -- GPS signals can't penetrate rock very well, so you just have a narrow view of the sky, and if there don't happen to be any sattelites in that line of sight, well, the GPS can't work.

I'm told that particularly thick vegetation can sometimes do the job, and so might some weather conditions.

Alternately, your local unit may have glitched.

But locality is pretty irrelevent to GPS. A canyon with a narrow view of the sky right in the middle of Tokyo would be worse than the flat, open plains of the Sahara desert, hundreds of miles from the next human.

[Grop]

I think GPS is being increasingly accurate, and I guess existing problems will be solved by 2010. Of course it can not detect every road change, they have to be recorded by some local authority or field agent. And I guess you may expect local civil authorities to report changes as they get used to this tool.

BUT it is based on satellites that detect your position and reply to your requests. In a modern setting with a little bit of paranoia (such as cyberpunk or Chtulhu 2006) you may expect the CIA, or a corporation that owns satellites, or anything else to use this system to watch you, or to lie to you about your position.

[Epoch]

BUT it is based on satellites that detect your position and reply to your requests. In a modern setting with a little bit of paranoia (such as cyberpunk or Chtulhu 2006) you may expect the CIA, or a corporation that owns satellites, or anything else to use this system to watch you, or to lie to you about your position.

It is not based on satellites that "detect your position," nor do they reply to your requests. Your GPS is emphatically not capable of generating a signal that those satellites could even detect.

The satellites continuously broadcast a signal themselves, which lets your GPS know their position and their time. When it sees two or more satellites, it can use trigonometry to figure out its position. The GPS itself doesn't broadcast anything.

[TavishArtair]

So, if the car battery breaks down, that means the GPS is out?

That isn't too difficult to arrange in a horror setting.