[time-nuts] Fundamental limits on performance

[Lux, Jim (337C)]

Precisely... The GPS satellites don't transfer between each other, but from
the satellite to earth.. But get out past the moon's orbit, and you can't
"see" the GPS signals anymore.

We do a lot of science with sending very quiet (low phase noise) signals
from a probe back to earth: e.g. You can do occultations to look at the
atmospheric structure of moons or the planet, you can do precise gravity
measurements (GRACE and GRAIL, for instance).  The problem is that (as time
nuts are aware) a quiet, stable, low power, reliable frequency reference
isn't easy to come by.  JPL is working on a 1 kilo/1 liter trapped Hg ion
clock, but that's still in the future, it's big and heavy, and consumes
power. 

So, if I have an orbiter with a good clock around Mars, and I want to
discipline a bunch of cheaper oscillators on the surface (or perhaps
orbiters), what is the fundamental limit on how good you can do, given the
radio link available from orbiter to ground station.


On 9/12/09 10:14 PM, "WB6BNQ" <wb6bnq at cox.net> wrote:

> Jim,
> 
> It seems to me the GPS system is the best example to draw from.  Assuming
> these space craft are too far from Earth to be corrected against Earth's
> reference, I would guess your intent is to have them sync'ed with each other ?
> 
> Bill....WB6BNQ
> 
> "Lux, Jim (337C)" wrote:
> 
>> 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?
>> 
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