[time-nuts] GPS first LO need to be locked?

paul swed paulswedb at gmail.com
Thu Mar 30 20:35:44 EDT 2017


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 <kb8tq at n1k.org> wrote:

> Hi
>
>
> > On Mar 30, 2017, at 7:05 PM, jimlux <jimlux at 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 <paulswedb at 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
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