Hi
> On Mar 30, 2017, at 7:05 PM, jimlux <jim...@earthlink.net> wrote: > > On 3/30/17 10:32 AM, Bob Camp wrote: >> Hi >> >> There is a limited tracking range for Doppler. You would need to stay inside >> that. >> > > Doppler is pretty big when the spacecraft is coming or going at the horizon, > about 5 kHz (out of 1.5 GHz, so 4-5 ppm). > Relatively speaking, GPS satellites are moving slowly (a few km/s) > So somewhere in the baseband processor code somebody said “we’ll handle +/- 5 KHz”. If your LO is < (say) 500 Hz it’s still inside the likely doppler handling range. If you want to do carrier phase then maybe you want to get a bit fancier …. Bob > > in LEO you're buzzing along at 7km/s, which is about 20-25 ppm. That is the > usual limiting case for bandwidth/tracking loops; you might want to go up to > 11-12 km/s so you can get things moving at escape velocity. > (there just aren't many people putting GPS on hypersonic projectiles - if > you've got the bucks to shoot something at Mach 45, you can probably afford a > custom GPS receiver) > > This is a bit tricky for older receivers because their tracking loop has to > acquire in the face of the Doppler uncertainty and the range (code phase) > uncertainty - there's a whole lore of optimum search strategies and how to > get the fastest time-to-first-fix. > > Does the first LO have to be locked to something? the signal you're > acquiring is MHz wide, so a 10ppm error in the LO frequency isn't a big deal. > Short term stability does help, while you're acquiring. > > But one of the things about GPS that made it attractive is that the local > clock can be pretty crummy. > >> Bob >> >> Sent from my iPhone >> >>> On Mar 30, 2017, at 9:46 AM, paul swed <paulsw...@gmail.com> wrote: >>> >>> I am curious if the first local oscillator on a GPS receiver must actually >>> be locked or coherent to the reference oscillator in the GPS receiver >>> typically running at some 10 MHz approximately. Or as long as the first LO >>> is quite stable it doesn't matter because the receiver can track the code. >>> This is a question for very classic receivers like Austrons, Odetics etc. >>> Discreet. Modern fully integrated receivers are not in question. >>> Thank you for your insights. >>> Regards >>> Paul >>> WB8TSL >>> _______________________________________________ >>> time-nuts mailing list -- time-nuts@febo.com >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
