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Nothing at all: http://www.aholme.co.uk/GPS/Main.htm http://www.aholme.co.uk/GPS/Main.htm
by philipkglass 9y ago
Nothing at all: http://www.aholme.co.uk/GPS/Main.htm http://www.aholme.co.uk/GPS/Main.htm
- ifoundthetao 9y agoBoy do I miss those types of sites. I need to find more of them.
- userbinator 9y agoHere's another, very detailed 4-part "build your own GPS" article from the early 90s: http://lea.hamradio.si/~s53mv/navsats/theory.html http://lea.hamradio.si/~s53mv/navsats/theory.html No FPGAs, RPis, or SDRs --- it's mostly done in discrete logic with a 68000 doing the back-end processing. As a bonus, he also implemented GLONASS. Furthermore, the software takes up less than 32KB of ROM.
- makomk 9y agoNote that while well explained, this is a very 90s way of implementing a GPS receiver. As I understand it modern receivers use FFT-based correlators that have much better cold-start performance than the old-fashioned design because they can test very large numbers of possible correlations at once (generally hundreds of thousands of them).
- userbinator 9y agoTechnically speaking, such a receiver would be classified as a munition and prohibited from export out of the US: https://en.wikipedia.org/wiki/GPS#Restrictions_on_civilian_use https://en.wikipedia.org/wiki/GPS#Restrictions_on_civilian_u... But practically speaking, almost all GPSes are made outside of the US and then imported...
- throwaway613834 9y agoHow do these folks do the math? Do they understand general relativity? Or do they just pull the equations from somewhere?
- cnvogel 9y agoThere is no general relativity involved in calculating your position. Solving for your position is basic geometry, if you want to have better resolution, have good model for the effects of the atmosphere on the radio waves. http://www.aholme.co.uk/GPS/user_position_solution.pdf http://www.aholme.co.uk/GPS/user_position_solution.pdf
- throwaway613834 9y agoHuh, is the whole thing a myth then? I'd definitely read that GR is necessary, but now that I'm searching I see mentions of it being a myth.
- dingaling 9y agoI remember this question arising on HN about a year ago. GR is a necessary consideration in the design of the satellite system. See page 5 of DTIC document: Relativistic Effects in the Global Positioning System http://www.dtic.mil/dtic/tr/fulltext/u2/a158720.pdf http://www.dtic.mil/dtic/tr/fulltext/u2/a158720.pdf Without correction the drift due to combined coarse relativistic effects would be on the order of 100,000 nanoseconds per DAY. In fact the first NTS-2 satellite was launched with the clock unadjusted for GR, due to doubters, but after three weeks the syntheziser had to be activated to compensate. The calculated uncorrected drift on that clock alone was 38,000 nanoseconds per day. As the document notes, to further refine Navstar to greater levels of precision would require tackling yet another set of tougher relativistic effects, plus more mundane atmospheric influences.
- c517402 9y agoIn "Exploring Black Holes" by Taylor & Wheeler, project A is titled 'Global Positioning System', where they say locating your position accurately depends crucially on General Relativity.
- michaelt 9y agoBefore the satellites are launched, their atomic clocks have to be tuned to offset for time dilation. GPS satellites are all tuned this way, and it has the desired effect. However, for people implementing GPS receivers, the signal arrives already corrected for time dilation. So the people making that part of the system don't have to know about general relativity at all :)
- ufmace 9y agoThat's really cool! I'd like to build something like that One Of These Days, when I have more free time. Also gives you the point - if one guy can build a receiver on his own... I mean, I don't want to make it easy for rogue nations and terrorist groups to get ICBM-mounted nuclear weapons, but with the difficulty of building and testing the missiles and bombs, it seems a little silly to expect restricting GPS receivers to accomplish anything.