[time-nuts] GPS first LO need to be locked?
kb8tq at n1k.org
Thu Mar 30 19:56:31 EDT 2017
> On Mar 30, 2017, at 7:05 PM, jimlux <jimlux at earthlink.net> wrote:
> On 3/30/17 10:32 AM, Bob Camp wrote:
>> 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 ….
> 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.
>> 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.
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