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Show HN: Building a GPS receiver

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Re: Show HN: Building a GPS receiver

#111

Post by mikewarot »

myself248 wrote: post #76
jeffbee wrote: post #39

Earlier quoted context omitted.

A person would have to have a quite flawed mental model of what GPS is to form the belief that it would stop working without data service, wouldn't they?

One of the interview questions where I work (we do automotive electronics) is "Explain how GPS works." It's directly relevant to the job, but it's also a neat opportunity to see how someone sizes up their audience, manages time and assumptions, etc. All those things are neat, but mostly what I've learned is that quite a lot of people, otherwise apparently reasonably smart and competent and toting a whole stack of pre…

If you think that's bad.... try asking someone where they think electricity comes from.

Re: Show HN: Building a GPS receiver

#112

Post by withinboredom »

OmarShehata wrote: post #36

Amazing! I also had exactly the same experience that led me to research this a few years ago, realizing that: - GPS works even in airplane mode (while on a literal airplane) - It works without cell service, or wifi, or anything - The United States of America controls the GPS constellation, and they can (and have!) turned off GPS off certain regions at will when necessary (which has prompted other countries to launch…

> The United States of America controls the GPS constellation

Heh, but they aren't the only global positioning system out there (nor the most accurate). EU, China, India, and Russia all have their own.

Re: Show HN: Building a GPS receiver

#113

Post by codyd51 »

amluto wrote: post #35

> Just one problem: you won’t find any SDR on the market that will claim to be able to sample a wave oscillating over a billion times a second. This was true, but not any more. You can get truly impressive “direct RF sampling” or “direct RF conversion” receivers that are more than fast enough for GPS. For example: Xilinx RFSoc: https://www.mouser.com/datasheet/2/903/ds889_zynq_usp_rfsoc_... A nice National Instrument…

This is amazing! I had no idea tech like this was available on the market. Thank you for the correction.

Re: Show HN: Building a GPS receiver

#114

Post by codyd51 »

tgsovlerkhgsel wrote: post #102

Wow, thank you! While I had a glimpse at various parts of the madness earlier, this is the first article that actually brought it to a level where I could convince my brain to understand it end-to-end.

That's so great to hear, thank you for the nice feedback!

Re: Show HN: Building a GPS receiver

#115

Post by codyd51 »

tylerchr wrote: post #24

Super impressive. Can’t agree more with the author that GPS is a stunningly clever engineering achievement. For those interested in the story of the development of GPS, I found “GPS Declassified” by Richard Easton to be an engaging retelling.

Thank you very much!

Re: Show HN: Building a GPS receiver

#116

Post by codyd51 »

sizzzzlerz wrote: post #18

Brilliant! I have no idea what the technical background of the author is but for anyone to tease apart the vast, complex, details of the GPS universe is a massive feat. Coupled with his ability to craft software to both assist his analysis and to implement the final solution, he has created a magnificent project. I've been studying GPS and worked with it professionally for a number of years and I still don't know eve…

I am really grateful for this nice comment, thank you! As a self taught programmer with no background in RF or EE, this project was definitely a stretch for me and involved lots of (sometimes frustrating) learning and experimentation. Looking back, I'm really glad for it, as it feels as though I've unlocked RF as a domain that I now understand a lot more tangibly and can use as a sort of tool. It reminds me of the things I love about computers themselves!

Re: Show HN: Building a GPS receiver

#117

Post by codyd51 »

noman-land wrote: post #6

I really love how this article is paced in real time from the first person as a learning adventure. Even down to the search terms used and the inner monologue. This is my absolute favorite kind of tutorial because you're not just being taught to fish, you're being shown how go about sourcing the parts to built your own fishing machinery.

Thank you for sharing this! It is sort of a vulnerable thing to write, because it reveals the shapes and bounds of my own ignorance, and the path I took to learning. I am really grateful to hear that you appreciated the approach.

Re: Show HN: Building a GPS receiver

#119

Post by codyd51 »

jeffypoo wrote: post #96

This was a delightful read. Reminded me of my time working on low power FHSS radio gear. We truly take modern wireless technologies for granted!

Thank you very much! I agree, they are incredible! I really had no idea until making this project, and it makes things like cellular phones so astoundingly impressive.

Re: Show HN: Building a GPS receiver

#120

Post by codyd51 »

tgsovlerkhgsel wrote: post #104
AlexanderTheGr8 wrote: post #95

"Interestingly, the only thing stopping civilians from using the P code is the knowledge of the value of its chipping sequence. If the formula to generate the P code was publicly known, there’d be nothing stopping civilian GPS receivers from locking on to it, with the exact same techniques as are used for the C/A code." I didn't finish reading the whole thing but was curious. Is there any way of brute forcing it or s…

I would assume that the chipping sequence is derived from a frequently-rotated cryptographic key.

I believe you are correct. My understanding is that the root P key is rotated daily, and needs to be manually uploaded to any military hardware that needs to use it.
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