Thanks everyone for your comments. It will be a GPSDP TBolt or Z3801 reference. I just wanted to eliminate some variables at this stage. Regards Paul WB8TSL
On Thu, Mar 30, 2017 at 7:56 PM, Bob kb8tq <kb...@n1k.org> wrote: > 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. > _______________________________________________ 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.