4 ms·

Homemade GPS Receiver

329 points by samlittlewood 15y ago

Duckpaddle2 15y ago
Really cool project, seeing hackers who really understand the technology makes for great reading! Thanks for sharing this!
- VladRussian 15y ago
  great combination of electrical engineering, core physics (classical and if you want it - GPS does touch SR and GR), programming (at app and VSDL levels). Such projects should be a regular part of lab curriculum for any engineering student (including programmers).
samlittlewood 15y ago
Also, this article references an older project that has (for me) the clearest description of the theory of GPS: http://lea.hamradio.si/~s53mv/navsats/theory.html http://lea.hamradio.si/~s53mv/navsats/theory.html
caf 15y ago
Cold start to lock in 2.5 seconds is pretty damn good.
- moe 15y ago
  It's an order of magnitude better than my Samsung Galaxy S can do (when it gets a lock at all...).
  - caf 15y ago
    Yes - it's because of the clever design that means that it doesn't use the Almanac data, so it doesn't need to wait for it to download.
jgrahamc 15y ago
I guess this is what it takes to get round the CoCom limit: http://blog.jgc.org/2010/11/gaga-1-cocom-limit-for-gps.html http://blog.jgc.org/2010/11/gaga-1-cocom-limit-for-gps.html
- calloc 15y ago
  Having done two balloon launches, it is not that hard to find resources that specify what GPS modules will work and what ones won't work. We used a Garmin GPS 18 and it worked flawlessly (other than our night launch, where we recorded really low freezing temperatures and we think that our GPS froze because it stopped giving us data). Our first launch got to 92k+ ft and the GPS kept reporting without issues.
jeremyarussell 15y ago
Thought this was great, will be trying to make one myself I think. Keep up the awesome work, I'm keeping your site bookmarked now. (I saw a whole bunch of other cool stuff.)
spitfire 15y ago
I wish I could upvote this several times over. This is hacker news a thousand times over, and makes up for all the dotcom, and ruby rubbish you have to wade through on this site. Well done sir.
- Create 15y ago
  What I cannot create, I do not understand -- as quoted in The Universe in a Nutshell, Richard Feynman The scientist describes what is; the engineer creates what never was. -- Kármán Tódor
- joshu 15y ago
  An absolute rockstar.
zb 15y ago
The L1 carrier is spread over a 2 MHz bandwidth and its strength at the Earth's surface is -130 dBm. Thermal noise power in the same bandwidth is -111 dBm, so a GPS signal at the receiving antenna is ~ 20 dB below the noise floor. This is slightly misleading; the bandwidth of the entire signal is 2MHz (it's a 1MHz chip). The bandwidth of the carrier is much narrower - it is above the noise floor typically by somewhere between about 15 and 50dBHz (you can see this quite easily on a spectrum analyser). The spread-spectrum part of the signal is indeed well below the noise floor. That minor quibble aside, this is a pretty awesome effort for one guy to do end-to-end.

329 points by samlittlewood 15y ago

Really cool project, seeing hackers who really understand the technology makes for great reading! Thanks for sharing this!

VladRussian 15y ago
great combination of electrical engineering, core physics (classical and if you want it - GPS does touch SR and GR), programming (at app and VSDL levels). Such projects should be a regular part of lab curriculum for any engineering student (including programmers).

samlittlewood 15y ago

Also, this article references an older project that has (for me) the clearest description of the theory of GPS: http://lea.hamradio.si/~s53mv/navsats/theory.html http://lea.hamradio.si/~s53mv/navsats/theory.html

caf 15y ago

Cold start to lock in 2.5 seconds is pretty damn good.

moe 15y ago
It's an order of magnitude better than my Samsung Galaxy S can do (when it gets a lock at all...).
- caf 15y ago
  Yes - it's because of the clever design that means that it doesn't use the Almanac data, so it doesn't need to wait for it to download.

jgrahamc 15y ago

I guess this is what it takes to get round the CoCom limit: http://blog.jgc.org/2010/11/gaga-1-cocom-limit-for-gps.html http://blog.jgc.org/2010/11/gaga-1-cocom-limit-for-gps.html

calloc 15y ago
Having done two balloon launches, it is not that hard to find resources that specify what GPS modules will work and what ones won't work. We used a Garmin GPS 18 and it worked flawlessly (other than our night launch, where we recorded really low freezing temperatures and we think that our GPS froze because it stopped giving us data). Our first launch got to 92k+ ft and the GPS kept reporting without issues.

jeremyarussell 15y ago

Thought this was great, will be trying to make one myself I think. Keep up the awesome work, I'm keeping your site bookmarked now. (I saw a whole bunch of other cool stuff.)

spitfire 15y ago

I wish I could upvote this several times over. This is hacker news a thousand times over, and makes up for all the dotcom, and ruby rubbish you have to wade through on this site. Well done sir.

Create 15y ago
What I cannot create, I do not understand -- as quoted in The Universe in a Nutshell, Richard Feynman The scientist describes what is; the engineer creates what never was. -- Kármán Tódor
joshu 15y ago
An absolute rockstar.

zb 15y ago

The L1 carrier is spread over a 2 MHz bandwidth and its strength at the Earth's surface is -130 dBm. Thermal noise power in the same bandwidth is -111 dBm, so a GPS signal at the receiving antenna is ~ 20 dB below the noise floor. This is slightly misleading; the bandwidth of the entire signal is 2MHz (it's a 1MHz chip). The bandwidth of the carrier is much narrower - it is above the noise floor typically by somewhere between about 15 and 50dBHz (you can see this quite easily on a spectrum analyser). The spread-spectrum part of the signal is indeed well below the noise floor. That minor quibble aside, this is a pretty awesome effort for one guy to do end-to-end.