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 dop

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.