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)
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
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