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https://axleos.com/building-a-gps-receiver-part-1-hearing-whispers/
I decided to try my hand at decoding these GPS signals, guided by the vague end-goal of plucking out my position from peanuts. I learned that the GPS signals that facilitate our mapping apps are ever-present, around us at any altitude, in any weather conditions, at all times.
This sounds cool in the abstract, but the tangible reality is staggering. These signals are all around me as I write this. They're all around you as you read it. The world is soaked in these whispers, repeating themselves endlessly for anyone willing to listen.
You can find out exactly where you are, from thin air, anywhere at any time, by learning to speak the language of the electromagnetic waves flowing over your skin. These waves have been a constant and quiet companion for most people's entire lives.
[...] All that said, it's not as though there's a cacophony of navigation data swarming around you, deafening if you could just hear it. In reality, the GPS signals surrounding you are astoundingly weak. To take an analogy: imagine a normal light bulb, like the one that might be above you now. Pull it twenty thousand kilometers away from the room you're in, and have it flash, on, off, on, off, a million times a second. Imagine straining your eye to watch the shimmer of the bulb, two Earths away, and listen to what it's telling you.
[Ed's Comment: Links to subsequent parts of this series are included in the source article]
(Score: 2) by VLM on Thursday April 18, @04:15PM
I just logged into my kiwisdr, the admin page is on port 8073 I always forget that, and in the GPS tab it has five satellites in green acquisition and it has a vast dump of data, the interesting part is my ADC clock runs at 66665994 Hz according to approx 95K GPS fix calibration activities. It can use navstar and galileo, unsure about the others.
Something I would find interesting would be a full featured GPS RX. Its interesting reading articles about all the new codes and new bands and new features and would be interesting to decode and see that data; however its very hard to find a commercial GPS RX that provides that detailed information. "Heres your lat/long and maybe alt and maybe some satellite data best of luck bye". Whereas I'd be amused to actually see ephemeris data or code streams.
Imagine if you could be the first guy to ever decode the "new" L2C code or the new L1C code. L1C is supposed to have a pilot carrier. I remember reading about that over a decade ago. Signal L1Cp (still gets spread by a ranging code but it doesn't have a data message). Deployment of new protocols takes time when it literally requires new satellites to be built and launched so I can't complain about the delay.
I don't entirely understand the new (is it new?) NSCP / NSCD codes that are in L1C. Transmit invalid data to prevent jamming by transmitting fake data that's not real or something, huh what?
I remember a "big" outcry asking whats in the "reserved" undocumented third frame of ephemeris data. There are entire chapters of the spec right after frame 3, marked "reserved" and the consensus at the time was its not "reserved" so much as "redacted" so something secret and interesting is being broadcast to the entire world but very few people indeed have the ability to decode and see it, just "mysterious .gov users" and I suppose every software defined GPS out there. Conspiracy theory that the .gov and FCC discourage the certification of civilian SDR-based GPS to prevent people from seeing the "reserved" byte ranges in the GPS data stream.
Ironically, like most classified stuff, its probably very boring. Maybe a literally worldwide pager system for secret squirrel types.
The idea of software GPS might not be novel and there's a large number of GPS software implementations, but the four-piece doc series is really cool to read and that is novel and worth the time, so overall a cool story.