Article: Tracking Nearly Every Aircraft With A Raspberry Pi

Interest Groups Amateur Radio
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"FlightAware is the premier site for live, real-time tracking of aircraft around the world, and for the last year or so, Raspberry Pi owners have been contributing to the FlightAware network by detecting aircraft flying overhead and sending that data to the FlightAware servers.

“Until now, these volunteers have used Raspis and software defined radio modules to listen in on ADS-B messages transmitted from aircraft. With FlightAware’s new update to PiAware, their Raspberry Pi flight tracking software, Mode S transponders can also be detected and added to the FlightAware network.”

“Tracking Nearly Every Aircraft With A Raspberry Pi”

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This is very cool, but I have a hard time understanding how this has enough precision to trilaterate positions accurately.

Well…ok, I did the math, and with only a 100ns accuracy, you can resolve 30m in pure time-of-flight for the radio signal.

299,792,458 m/s * 100e-9 = 30m

I guess we get so used to messing around in nanoseconds, that it’s easy to forget that light literally inches along in that timescale!

Even a 10us accuracy would be around 3km, which is a lot better than no position information at all.

Pretty cool!

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I am more surpised that they can get clocks synchronized well enough to get accurate time-difference between ground stations. Even using something like an NTP server to synchronize them, I would have thought there would be too much drift between the different clocks.

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NTP can only really provide millisecond-level precision. There has to be something else going on to get precise multilateration over the intertubes.

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I was basing my assumption on marginally iffy GPS sync signals.

You can get 10ns if you get a really nice time reference module. Below that you would usually do a bunch of averaging possibly with an oven-controlled crystal oscillator and a rubidium time source or something high-end like that.

But that is a good call, because this would require some interesting hacking to get the Pi to support it well, and using it to time the RF inputs.