[time-nuts] Different Thunderbolt versions

Lux, James P james.p.lux at jpl.nasa.gov
Sun Feb 22 01:45:54 UTC 2009




On 2/21/09 5:20 PM, "Bruce Griffiths" <bruce.griffiths at xtra.co.nz> wrote:

> Tom Van Baak wrote:
>>> For observations at 1420MHz, and short integration times (~ 100sec) the
>>> phase error needs to be 100ps or less.
>>> For longer integration times (10,000 sec is desirable) and higher
>>> frequencies the timing constraints are far more stringent.
>>> Whilst clock modeling can be used to improve performance it can only
>>> correct systematic effects.
>>>    
>> 
>> Bruce,
>> 
>> Approximately what range of tau and ADEV(tau) would this
>> VLBI requirement correspond to? A couple of the TBolts I've
>> tested are at or just under 1e-12 at 1 second and stay almost
>> flat out to 100 or even 1000 seconds.
>> 
>> /tvb
>> 
>> 
>>  
> Tom
> 
> Really should use something like TDEV but roughly (within a factor of 2
> or so at 1420MHz):
> 
> ADEV < 1E-10 or better @ tau = 1sec for 1 second integration time.
> (integration time to short for anything but very bright objects)
> ADEV < 1E-11 or better @ tau = 1sec for 10 second integration time.
> (integration time to short for anything but very bright objects)
> ADEV < 1E-12 or better @ tau = 1sec for 100 second integration time.
> (integration time somewhat marginal for most objects)
> ADEV < 1E-13 or better @ tau = 1sec for 1000 second integration time.
> ADEV < 1E-14 or better @ tau = 1sec for 10,000 second integration time.
> 
> 
> 
> Also need to take into account that a similar LO would presumably be
> used at each antenna.
> 
> For integration times of up to a few hundred seconds VLBI has been done
> with a hydrogen maser at one antenna and a PRS10A at the other.
> However the performance isnt Earth shattering and there are some issues
> with long term drift and transport induced frequency shifts of the PRS10A.
>

In the Deep Space Network, we do Delta DOR, which is a form of VLBI, using a
convenient quasar as the common view timing reference. (and, of course, they
have hydrogen masers at the stations as well)

http://trs-new.jpl.nasa.gov/dspace/handle/2014/37674





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