Gullible Morons Passing on Bad Info

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Earlier today, I was eating lunch with a friend while lamenting that the GPS on my phone is glitchy. I had a GPS app loaded, which is telling me that it saw the GPS constellation of satellites (about 10) while I was sitting still inside the restaurant. We walked out, and I was remarking about how the GPS lost track while moving, when some high school kids around the corner made a random remark:

"Mister! You may want to turn that GPS off! Government can track you with it!"

For a moment, I was about to turn back and tell him that he's being stupid, that he had no idea how GPS actually works, but then, I thought better. You can't convince a conspiracy theorist that there is no conspiracy. His mind is already made up.

Besides, there is slight bits of truth in that statement, crazy as it sounds.

E911 service is always on for any cell phone sold in the US. This means if you dialed 911, the other side will receive your location (if available).

However, a GPS by itself doesn't send anything. GPS is a PURELY PASSIVE DEVICE. it only receives. It is NOT A BEACON.

A cell phone on the other hand, can be tracked even withOUT the built-in GPS, by triangulating the signal off multiple towers. This is called aGPS. Some WiFi equipped cell phones can also look for WiFi hotspots with known locations and thus know the current location.

But this goes back to a fundamental question: how *does* GPS receivers work? How can they calculate your location on earth based on just receiving signals from the sky? Well, it has to do with the GPS satellites in the sky (together, they're known as a "constellation").

The guys who created GPS was inspired by the Sputnik. Sputnik puts out a 1 Hertz beep, and you know its orbit. By listening to the signal, and calculate its doppler effect, you can work out where on Earth you can approximately, but it's not very accurate, as there's no cross check.

So GPS uses multiple satellites.

Each satellite has a unique orbit, and its transmission has a unique signature. It is also VERY VERY precise in its broadcast time-wise. Thus, if you have an accurate clock, you can measure the signal coming off one satellite. You know what pulse rate it *should* be. If you then measure its pulse rate as received by you, then you know the doppler shift. You know its velocity, thus, working backwards, you can calculate your approximate latitude. By working off math off at least THREE satellites in the constellation, you can get both lattitude AND longitude, and if you get a fourth one, you can get height. The more satellites you can "fix" the better your accuracy.

So that's the rant. :)