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Almost Understands GPS

The new TV series Almost Human deserves a full review. I'll get there eventually. In the meantime, one anecdote… (mild spoiler alert) In the last episode we saw a key plot point was that a particular circuit board was discontinued because it could be "easily located by the leakage from its GPS chip" or some such thing. So the good guys pull up a "GPS map" and pinpoint the board's location.

The number of misconceptions here is pretty staggering, but the chief one is one most people, including many technologists, don't get. The GPS devices here on earth do not send a signal. At all. The GPS satellites up in space do not receive a signal. At all. It's the exact other way around. The technical name of the devices here on earth is "GPS receiver". The satellites are GPS transmitters.

It's easy to understand the confusion, up to a point. Presumably people believe that their GPS device somehow transmits a signal into space, which a GPS satellite "sees" to identify the device's position; the satellite then sends this position information back to the device.

Nope. What actually happens is more interesting. All the GPS satellites send a signal down to Earth at exactly the same time from perfectly known places. The GPS receiver on Earth sees these signals arrive at slightly different times: some satellites are farther from the receiver, so their signal takes longer to get there. If the receiver gets signals from at least four satellites, it can use the satellites' known position and their measured relative distance to compute its position. It's a form of 3D triangulation.

So…yeah. The Almost Human episode makes no sense. However, now you know (if you did not before) how GPS works. Fob