[time-nuts] Fundamental limits on performance

Hal Murray hmurray at megapathdsl.net
Sun Sep 13 05:49:25 UTC 2009

> In the deep space exploration biz, we've been talking about time and
> frequency transfer between spacecraft. It's pretty easy to figure out
> what the "Shannon limit" for the link between the spacecraft is (given
> power transmitted, antenna gain, etc... And with coding, we get within
> tenths, if not hundredths of a dB of the limit)...

> So, if I have a clock of some performance on Spacecraft A, is there a
> "simple" way to say how well I can do transfering that to Spacecraft
> B? 

Do you want time or frequency?

I don't think it's going to be simple.

As part of demodulating the data stream, you end up with a receive clock.  
That's got noise from the communication channel added to the (probably tiny) 
noise from the transmit clock.  You can filter out the channel noise.  The 
longer you filter (narrower bandwidth) the better the recovered clock will be.

I think what you are looking for is the Allan intercept: the crossover point 
between the noise from your (filtered) received clock and the long term drift 
of the local clock.

Don't forget to correct for Doppler.

These are my opinions, not necessarily my employer's.  I hate spam.

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