[time-nuts] GPS first LO need to be locked?
jimlux at earthlink.net
Thu Mar 30 19:05:34 EDT 2017
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)
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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