[time-nuts] how good an oscillator do you need for a GPS simulator
jimlux at earthlink.net
Fri Dec 16 11:39:44 EST 2011
On 12/16/11 7:46 AM, Azelio Boriani wrote:
> A used Spirent is only 26K to 37K. Interesting: playing back bits from
> RAM... can it be that simple? Obvious: a DAC is required.
No.. you don't even need a DAC. The underlying waveform is a binary
code that is BPSK modulated.
there are a variety of commercial GPS record/playback units that
basically are single bit digitizers with no mixer (other than the
sampler with a well chosen sample rate to put the GPS signal at a good
place in the passband)
The GPS receiver we're flying for the CoNNeCT project samples the three
bands with a single bit at about 38.6 MHz, which aliases the L1 of 1575
down to about 9 MHz: close to fs/4, which is a convenient place. The
other bands alias to convenient places as well.
There is some art in picking a good sample rate... you want something
that aliases to somewhere convenient, and you want to not have the
Doppler push you past a folding point. Doppler from GPS satellite
motion to a stationary platform is on order of 5kHz max. For a receiver
LEO satellite (probably a worst case) you'd need to add a platform
motion of 7 km/s, which is about 10 kHz. Real worst case would be
something that's reached escape velocity, which I think is 40-50 kHz
So don't go picking sample rates that are exact fractions of L1. You'll
wind up in alias unwrapping hell.
For generating, though, sample rates that are a multiple of the chiprate
(1.023 MHz) might be the best strategy. Say you clock the bits out at
10.23 MHz, and you could use the 154th harmonic of the same oscillator
as your L1 carrier and all that.
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