For anyone wanting to build their own GPS receiver, here's a fully open source project that explains a great deal of the theory too: http://www.aholme.co.uk/GPS/Main.htm
It's wild reading old issues of VHF Communications and finding advertizements for buying the PCB's for this project.
So I,guess people really did try to reproduce it as well.
Can you explain what you mean by that? The only thing I can think of is that GPS has 2 modes : one civilian and other military use. But military use-case uses encryption so civilians can't use that mode anyway.
GPS was built primarily for the military and the signals available for civilian use were crippled to reduce accuracy. GPS receivers that provided high accuracy fell under the ITAR rules (ie you needed proper licensing to trade them)
Over time such restrictions have largely been lifted, but there are still US export controls on GPS receivers with particular features, such as those designed for use in high speed aircraft, those able to decode the still-military-only encrypted signals that piggyback GPS and provide greater positioning precision, or those designed for use in rockets/UAVs.
So, it's reasonable to assume that if you were to build your own, and do too good of a job, you would accidentally become subject to arms trade regulations, and that's probably not a place you want to unintentionally find yourself, particularly if you're publishing it as open source on the Internet :)
> those able to decode the still-military-only encrypted signals
How? I was under the impression that military-only signal was encrypted. And if someone breaks that encrpytion, blame should go to the poor handling of encryption rather than the person breaking it? Analogy : if you leave a classified document on the train and a passenger reads it, whose fault is it?
The Kraken RF project, which has multiple SDR radios next to each other, had demo code of a passive radar: the slight time delay by which signals arrived at the antennas could be used to detect large metal bodies (IOW: planes) in the sky.
They took down the demo code they were informed that they were violating ITAR regulations.
I'm relatively certain that the US government will be unmoved by your argument that it would be their fault if you built a GPS receiver that could decrypt the precise positioning signals ;)
Well, consider another analogy: a person leaves their door unlocked and another person goes in their home. It is still trespassing, even though the physical barrier has been intentionally disabled.
I'm not sure even what section of US code to check but it is certainly plausible that you can (for example) find a surplus device that decodes the signals on EBay and it's actually illegal to do it. You can find radios on EBay that broadcast on bands that are unlawful to use (or unlawful without certain licenses, &c).
I never really thought of that. That's a pretty interesting restriction. Although any party with access to warheads that can fly 1000+ mph probably can bypass the GPS restriction, no?
There was a guy in NZ (Bruce Simpson) who detailed on his blog how to make a DIY cruise missile. I think he got politely asked to stop doing so at some point.
BPS.Space on youtube is working on a DIY space-capable rocket and in a recent video he mentioned that he is not doing this as a tutorial and that his guidance system likely already wanders into ITAR territory, and thus he's self-censoring which parts he shares and which parts happen off-camera.
Ultimately these kinds of regulations are fairly silly because a sufficiently determined smart person can recreate the covered technologies from scratch, but here we are.
Any sufficiently smart person can accomplish the same original work as any other sufficiently smart person.
But when the details of these things are published or otherwise made openly available, it doesn't take nearly as many smarts to duplicate these accomplishments.
Quite often, that's good: It's easy for a dullard like me to build a circuit or to re-use some clever assembler code when someone else has published it for my own tinkering around the house. In this way, it's a pretty great world to live in; it is often very simple to stand on the shoulders of giants and get some things done that I could probably never do on my own.
But sometimes, that's bad: We don't live in a perfect world. Enemies exist. Things like ITAR can't prevent a sufficiently smart person from doing anything, but they do make it a lot harder for them to get started.
correct. and the idea here is to put up at least some form of barrier to entry, because any good guidance is also nuclear warhead delivery guidance at a certain point.
I launched a high altitude balloon as part of a summer school program over a decade ago and we checked on edge cases. Off the shelf GPS are supposed to not work beyond a certain height 18,000 m and/or speed 515 m/s to be a barrier for use as a weapon. Some hardware treat that as AND; some treat that condition as OR. The term to look up is “CoCom Limits”.
