Standford has/had a course that is available online on GPS/GNSS and a lot of the nitty-gritty details: * https://www.youtube.com/playlist?list=PLGvhNIiu1ubyEOJga50LJ...
Thank you for this, this looks great!
Show HN: Building a GPS receiver
101–110 of 135 posts
Re: Show HN: Building a GPS receiver
#102Wow, 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.
Re: Show HN: Building a GPS receiver
#103> I’ll be starting a new job next week which isn’t as amenable to publishing side projects What does a job have to do with publishing hobby projects?
Some jobs insist on owning everything you do even in your free time, and will fire you for having personal projects.
(Other jobs may just take up a lot of energy and/or time.)
Re: Show HN: Building a GPS receiver
#104"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.
Re: Show HN: Building a GPS receiver
#105> I’ll be starting a new job next week which isn’t as amenable to publishing side projects What does a job have to do with publishing hobby projects?
Some jobs insist on owning everything you do even in your free time, and will fire you for having personal projects. (Other jobs may just take up a lot of energy and/or time.)
> owning everything you do even in your free time
Where and how is that legal? You're supposed to be an employee, not a slave.
Re: Show HN: Building a GPS receiver
#106"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…
Kind of. The P code itself is publicly available in publicly available specification IS-GPS-200. They are xor-ed by W code, which like the sibling comment says, is cryptographically generated, which runs at a lower rate. This results in P(Y) code signal, which is then transmitted over the satellites.
If you know whats the P code roughly is, which you can know from L1CA code and handover words, you can try using both possible W code value (its 0 or 1) and that W code practically dissappear, until the W code move to its next iteration and you need to guess again. This strategy is called semi-codeless tracking.
People did this so they can access the signals at L2 frequency (1227.6 MHz), which because its at a different frequency than L1 (1575.42 MHz), they got slowed down by different amount through the ionosphere. So by comparing the signals together, you can remove the ionospheric effects and get more accurate position.
These days you don't need all of this, GPS now have public signals at L2 and now smartphones have these fancy dual-frequency receivers
Re: Show HN: Building a GPS receiver
#107Earlier quoted context omitted.
Some jobs insist on owning everything you do even in your free time, and will fire you for having personal projects. (Other jobs may just take up a lot of energy and/or time.)
> owning everything you do even in your free time Where and how is that legal? You're supposed to be an employee, not a slave.
Unfortunately, most countries don't have, or don't enforce, penalties for companies claiming rights they don't have. So that's the first part: Companies claim, often using vague or misleading wording, to have more rights than they can legally claim.
And what they can claim varies but can go quite far. For example, even employee-friendly California with a law specifically to limit far reaching clauses (https://law.justia.com/codes/california/2011/lab/division-3/...) allows terms in employment agreement that assign to the employer the ownership of "invention[s] that the employee developed entirely on his or her own time without using the employers equipment, supplies, facilities, or trade secret information" as long as they "Relate ... to the employers business, or actual or demonstrably anticipated research or development of the employer". For a megacorp, that can be basically everything.
The second part is that in some countries you can be fired for any reason, or no reason, except for specific protected reasons. "We didn't like your blog post and are worried that you might accidentally reveal company secrets in one of them in the future, so we're letting you go" would be legal in many places. "All social media or personal web site content has to be reviewed to make sure you're not leaking company secrets" is also a demand some companies make.
Likewise "we want you to give 100% at work and your side project clearly shows you're spending too much time elsewhere so we don't believe you're working to your full potential", even though stupid, would be a perfectly legal justification to fire someone in many countries. Or just the manager silently thinking that and silently retaliating through e.g. denying promotions.
Re: Show HN: Building a GPS receiver
#108Does it only support the US-made GPS (NAVSTAR GPS) or other GNSS systems like Galileo as well?
Re: Show HN: Building a GPS receiver
#109Earlier quoted context omitted.
> owning everything you do even in your free time Where and how is that legal? You're supposed to be an employee, not a slave.
Unfortunately, most countries don't have, or don't enforce, penalties for companies claiming rights they don't have. So that's the first part: Companies claim, often using vague or misleading wording, to have more rights than they can legally claim. And what they can claim varies but can go quite far. For example, even employee-friendly California with a law specifically to limit far reaching clauses ( https://law.ju…
Are you using "many countries" to mean "many US countries"? What you're describing is rather unheard of here in Europe. I wouldn't even call that California example anywhere near "employee friendly", it's outright ridiculous.
Re: Show HN: Building a GPS receiver
#110I find it amazing that they managed to design and build all this in the 1970s when computers were only in their infancy and SDR was complete science-fiction.