Homemade GPS Receiver
holmea.demon.co.uk
Homemade GPS Receiver
1–10 of 67 posts
Re: Homemade GPS Receiver
#2That page is amazing. I'm astounded that someone can build a GPS receiver from scratch, not to mention writing the code that interprets the GPS signals. The difficulty is mind-boggling!
There's some wonderful quotes in the page that just hint at the complexity involved:
"Originally, by not transforming satellite positions from earth-centred-earth-fixed (ECEF) to earth-centred-inertial (ECI) coordinates, I was effectively ignoring Earth's rotation during the 60 to 80 ms that signals were in flight."
"The above solutions were generated without compensating for ionospheric propagation delays using parameters in page 18 of subframe 4 which should be applied because this is a single frequency receiver. Ionospheric refraction increases path lengths between users and satellites."
!
Re: Homemade GPS Receiver
#3That page is amazing. I'm astounded that someone can build a GPS receiver from scratch, not to mention writing the code that interprets the GPS signals. The difficulty is mind-boggling! There's some wonderful quotes in the page that just hint at the complexity involved: "Originally, by not transforming satellite positions from earth-centred-earth-fixed (ECEF) to earth-centred-inertial (ECI) coordinates, I was effecti…
Some of his other projects are pretty impressive as well - I'm taken by the FORTH computers myself.
Re: Homemade GPS Receiver
#4This is really amazing
I had no idea it was this complex. Well, in a way it is simpler than I imagined (since he's using an antenna with an amplifier), and the FI amplifier is "simpler" as well.
The most interesting parts for me is the demodulation, and the code phrases search using FFT.
No wonder you need a FPGA to do the heavy lifting.
Re: Homemade GPS Receiver
#5That page is amazing. I'm astounded that someone can build a GPS receiver from scratch, not to mention writing the code that interprets the GPS signals. The difficulty is mind-boggling! There's some wonderful quotes in the page that just hint at the complexity involved: "Originally, by not transforming satellite positions from earth-centred-earth-fixed (ECEF) to earth-centred-inertial (ECI) coordinates, I was effecti…
This is immensely impressive.
What strikes me is that the math and all the corrections would not be all that alien to a sextant user from 100 years ago. To determine position using a sextant you also start with an estimated position and an accurate clock. You compensate for drift during your measurements, for dip and refraction, you solve the equations for several celestial bodies etc.
Re: Homemade GPS Receiver
#6Here's another chap who designed his own GPS+GLONASS receiver back around 1992: http://lea.hamradio.si/~s53mv/navsats/theory.html
The best bit is that all the diagrams, schematics and PCB layouts are hand-drawn.
Re: Homemade GPS Receiver
#7I have to join in to say that this page (which exists since some time, only has recently be updated) is really impressive, and a very good educational material for everyone wanting to really understand the workings of GPS.
Nevertheless it's not the only attempt, bug a very good one, at "open source GPS" projects. There's at least http://www.gnss-sdr.org/ which is a more software-centric approach being able to work with different receiver frontends. And I've just found another FPGA project at http://www.witchnav.cz/.
Naturally the software-only solution will burn a lot of CPU in the correlators, so I'd expect a software solution to require a multi-GHz machine, whereas the GPS receiveres with FPGA (such as the one linked to) works easily on a pretty low-performing Raspberry-Pi.
Re: Homemade GPS Receiver
#8Hmm. I don't see any mention of whether this design complies with the COCOM Limits, an agreement that GPS devices should disable themselves when at an altitude above 60,000 feet and a speed faster than 1,000 knots.
Re: Homemade GPS Receiver
#9Hmm. I don't see any mention of whether this design complies with the COCOM Limits, an agreement that GPS devices should disable themselves when at an altitude above 60,000 feet and a speed faster than 1,000 knots.
Probably not
Which is funny, because to know you're higher/faster than allowed you need to calculate that from GPS data
In practice, not sure how this is achieved, but for speed this may even be a limitation of the circuit (for example, if you can't compensate for the doppler effect at that speed or has to add some other processing - still, the doppler effect should be small)
1000 knots is still +/- 1.5 Mach (at sea level) so Concorde and fighter jets should use a special GPS (but this is very fast)
Re: Homemade GPS Receiver
#10Hmm. I don't see any mention of whether this design complies with the COCOM Limits, an agreement that GPS devices should disable themselves when at an altitude above 60,000 feet and a speed faster than 1,000 knots.
Those limits are to prevent its use in intercontinental missiles. If intends to build one, I guess he'll have bigger problems than the speed at which his GPS receiver works. If you were thinking about the case where he sells it to someone making a missile, I guess that if someone is able of getting a missile capable of intercontinental flight, a GPS receiver won't be the missing bit.