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Show HN: Building a GPS receiver

Have you ever noticed that your Maps app still works during a flight? It can feel illicit, like someone just forgot to turn off the signal, and that watching yourself crawl along the earth should be done without drawing undue attention. A few months ago I learned that there were only around 30 GPS satellites serving the entire planet. This piqued my interest, because it reminded me of the 13 root DNS servers from which all resolution flows. Perhaps GPS has a similar design in which the ‘source of truth’ is diluted by several layers of signal repeaters? I navigated to gps.gov, and was presented with this delightful image. I became even more excited to learn about what these satellites do!

It’s quite remarkable: some papers are entirely founded on tweaking the equation in effectively one line of code, and exploring how this impacts the acquisition performance characteristics. I’m also using a binary search sort of approach to converge on the Doppler shift that gives the strongest correlation spike for each satellite in view. All said and done, we’ve built a detector to determine which GPS satellites are in the sky above us, as well as a rough approximation of their phase (or time delay), and Doppler shift (or relative velocity).

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