Comment by sbierwagen | Hacker News Reader

Comment by sbierwagen | Hacker News Reader

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  basically shining a very bright light

GPS satellites broadcast 500 watts from a 13dbi antenna.

But they're far away. Really really far away, to the tune of 21,000km. (The Earth is about 12,000km wide.)

By the time the signal makes it to the surface of the planet, the inverse square law has punched it down to -130 dBi, which is well below the thermal noise floor of room temp electronics. GPS receivers pull a useful signal out of a storm of random noise using deep information theory magic, which is vulnerable to several clever low power attacks.

Even a dumb jammer doesn't need much power, though.

  one could program a missile to follow the light...

http://en.wikipedia.org/wiki/AGM-88_HARM

"The AGM-88 High-speed Anti-Radiation Missile (HARM) is a tactical, air-to-surface missile designed to home in on electronic transmissions coming from surface-to-air radar systems."

>pull a useful signal out of a storm of random noise using deep information theory magic

Any keywords I can look up as to how this is done?

It's called spread-spectrum. The two types are frequency hopping and direct-sequence. GPS uses direct sequence.

http://en.wikipedia.org/wiki/Spread_spectrum

http://en.wikipedia.org/wiki/GPS_signals#Demodulation_and_de..., currently paragraph 4 describes pulling out the data.

Basically, even though the signal is below the noise floor, part of the signal is the output of a pseudo-random number generator with known properties, so by setting up your own PRN, and then trying different (time) alignments, the correct alignment will stick out, despite the noise (which all averages out). Once you have the time alignment, you can read the rest of the signal, and then you can tell how far you are from that satellite ((it's clock - your clock) / c). Repeat for other satellites and you're done.

Can't find any good references, but that's what our information theory lecturer said! Also, it explains why a hot start is quicker than a cold start (if you know where you are, and the accurate time, you know what alignments to try to start looking for the satellite signals.)

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