Show HN: Building a GPS receiver

Shortly after publishing my iOS 4 jailbreak last October[1], I got to work on my next hobby project: a from-scratch homebrew GPS receiver, which can solve the user’s location solely from billions of radio antenna samples.

I took a commodity SDR (alongside the Python standard library and numpy) and built a signal processing pipeline that can detect and track GPS satellites over many minutes, drop and pick up satellites as they come in and out of view, and precisely determine the user’s position and clock inaccuracy.

All told, gypsum can go from a cold start to a fix on the user’s position, and the precise time, in less than a minute of listening to the antenna. I went on a journey of learning how to detect and track satellite signals that are literally too quiet to hear, and I hope that some of the magic comes through in the posts!

After implementing this myself and walking the long road of getting it working, I’m left completely stunned by the brilliance of GPS, across so many axes. I hope you enjoy the read!

On a more personal note, I’ll be starting a new job next week which isn’t as amenable to publishing side projects, and therefore this will be my last publicly-published project for some time. I’ve had great experiences making and sharing projects on here, and I’m really grateful for the positive feedback that’s been shared!

[1]: https://news.ycombinator.com/item?id=37736318

Show HN: Building a GPS receiver
axleos.com

Whenever I see "from scratch," I'm always curious to see how from scratch the author actually means so I'll admit I was a bit disappointed to see that the hardware was just RTL-SDR. Still, the protocol decoding was very interesting and the result is great.

> GPS was launched in 1978, which was 45 years ago at time of writing. Five billion people are currently under 40 years old, so well over half the world’s population has never existed in an environment but this.

A note based on this. While GPS was around since 1978 the signal was intentionally degraded with a process known as "selective availability" until 2000. This largely rendered GPS unusable for many many purposes, definitely useless for road navigation, it had some limited utility in areas like backcountry navigation and was definitely useful for marine navigation.

> gypsum can go from a cold start to a fix on the user’s position, and the precise time, in less than a minute of listening to the antenna

This is very impressive and outclasses what I see even commercial receivers doing today, do you have any idea how? I remember on road trips in the early 2000s I would have to sit on the side of the road and wait for the GPS receiver to get a fix (a 15-20 minute process, when it worked) before we could leave. Or, more likely, my mother would just start driving with paper maps.