3 ms·

I 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

by cnvogel 13y ago

I 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/ 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/ 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.

sliverstorm 13y ago
Many standalone recievers are just that- completely standalone. They just spit out a string of coordinates and other information on configurable intervals.
Florin_Andrei 13y ago
There is some sort of international agreement, originally intended to prevent the use of off-the-shelf GPS components in building cheap missiles. The agreement prevents all GPS receivers from providing data when their altitude is higher than some number (10 km I think), and their speed is above some number (hundreds of km/h IIRC). Some GPS vendors read the above in terms of an OR function, instead of an AND function. In other words, their receivers stop functioning above that altitude no matter what the speed is. That prevents some off-the-shelf GPS receivers from being used to build stratospheric balloons, since these things can climb all the way to 30 km altitude, or more. Other GPS receivers are built on an AND function, and work well with strato balloons. Sometimes it's a bit tricky to tell if a given receiver is an AND or an OR. I assume, if you build a DIY GPS receiver, you can even ignore any such limitation altogether, right? That might well be illegal where I live, I'm not sure, and I have no intention to build such a thing anyway. I'm just asking in principle. EDIT: This is the COCOM limit: In GPS technology, the phrasing "COCOM Limits" is also used to refer to a limit placed to GPS tracking devices that should disable tracking when the device realizes itself to be moving faster than 1,000 knots (1,900 km/h; 1,200 mph) at an altitude higher than 60,000 feet (18,000 m).[2] This was intended to avoid the use of GPS in intercontinental ballistic missile-like applications. http://en.wikipedia.org/wiki/CoCom http://en.wikipedia.org/wiki/CoCom
- cnvogel 13y ago
  You can certainly ignore both limits in your homemade-receivers, but if you ever want to ride faster than 1900km/h you will have to account for an increased doppler shift (I hope I'll not help any terrorists by explaining that ;-) ), and even though I (in the end) realized that this is not as dramatic as thought at first, just let me write down some calculations: Doppler shift is the change of the percieved frequency when either source or receiver move towards another, or away from another. Canonical example: Sound of a emergency vehicle's horn when passing by. Normally you never see the satellites ride directly towards you (or away from you) but the path is pretty orthogonal to your line-of-sight. So you only account for a fraction of the 14'000 km/h speed of the GPS satellites to cause frequency shifts of the signal. (Max. example listed on the homepade-GPS page is ±5 KHz of doppler, which is relatively 3.3×10^−6 of the GPS frequency, even though the GPS satellite rides at 1.8×10^−5 the speed of light). If you'd fly with 1900 km/s (1.8×10^−6 of the speed of light) directly towards the GPS bird, this movement will increase the frequency you receive by 1.8×10^−6 × 1,5 GHz = 2.7 kHz. So please increase your doppler search range from ±5 KHz to ±8 KHz to be able to successfully steer your homemade cruise missiles at that the COCOM limit speed, increase more if faster. This will slow down your time-to-first-fix, obviously. The other limit of 10km is easier to ignore, though. It would only become relevant once you can no longer exclude some solutions to the formulae that lie outside of the GPS satellite's orbit (imagine four coplanar GPS birds used for calculation). And in practice proabably it's sane to assume a position that is not wider than 100km from the earth's surface, again, if you don't plan to ride your DIY receiver to the ISS, or maybe even geostationary orbit.

by cnvogel 13y ago

sliverstorm 13y ago

Many standalone recievers are just that- completely standalone. They just spit out a string of coordinates and other information on configurable intervals.

Florin_Andrei 13y ago

There is some sort of international agreement, originally intended to prevent the use of off-the-shelf GPS components in building cheap missiles. The agreement prevents all GPS receivers from providing data when their altitude is higher than some number (10 km I think), and their speed is above some number (hundreds of km/h IIRC). Some GPS vendors read the above in terms of an OR function, instead of an AND function. In other words, their receivers stop functioning above that altitude no matter what the speed is. That prevents some off-the-shelf GPS receivers from being used to build stratospheric balloons, since these things can climb all the way to 30 km altitude, or more. Other GPS receivers are built on an AND function, and work well with strato balloons. Sometimes it's a bit tricky to tell if a given receiver is an AND or an OR. I assume, if you build a DIY GPS receiver, you can even ignore any such limitation altogether, right? That might well be illegal where I live, I'm not sure, and I have no intention to build such a thing anyway. I'm just asking in principle. EDIT: This is the COCOM limit: In GPS technology, the phrasing "COCOM Limits" is also used to refer to a limit placed to GPS tracking devices that should disable tracking when the device realizes itself to be moving faster than 1,000 knots (1,900 km/h; 1,200 mph) at an altitude higher than 60,000 feet (18,000 m).[2] This was intended to avoid the use of GPS in intercontinental ballistic missile-like applications. http://en.wikipedia.org/wiki/CoCom http://en.wikipedia.org/wiki/CoCom

cnvogel 13y ago
You can certainly ignore both limits in your homemade-receivers, but if you ever want to ride faster than 1900km/h you will have to account for an increased doppler shift (I hope I'll not help any terrorists by explaining that ;-) ), and even though I (in the end) realized that this is not as dramatic as thought at first, just let me write down some calculations: Doppler shift is the change of the percieved frequency when either source or receiver move towards another, or away from another. Canonical example: Sound of a emergency vehicle's horn when passing by. Normally you never see the satellites ride directly towards you (or away from you) but the path is pretty orthogonal to your line-of-sight. So you only account for a fraction of the 14'000 km/h speed of the GPS satellites to cause frequency shifts of the signal. (Max. example listed on the homepade-GPS page is ±5 KHz of doppler, which is relatively 3.3×10^−6 of the GPS frequency, even though the GPS satellite rides at 1.8×10^−5 the speed of light). If you'd fly with 1900 km/s (1.8×10^−6 of the speed of light) directly towards the GPS bird, this movement will increase the frequency you receive by 1.8×10^−6 × 1,5 GHz = 2.7 kHz. So please increase your doppler search range from ±5 KHz to ±8 KHz to be able to successfully steer your homemade cruise missiles at that the COCOM limit speed, increase more if faster. This will slow down your time-to-first-fix, obviously. The other limit of 10km is easier to ignore, though. It would only become relevant once you can no longer exclude some solutions to the formulae that lie outside of the GPS satellite's orbit (imagine four coplanar GPS birds used for calculation). And in practice proabably it's sane to assume a position that is not wider than 100km from the earth's surface, again, if you don't plan to ride your DIY receiver to the ISS, or maybe even geostationary orbit.