[time-nuts] Fundamental limits on performance

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Sep 13 22:35:22 UTC 2009


Lux, Jim (337C) wrote:
>
> On 9/13/09 10:03 AM, "Hal Murray" <hmurray at megapathdsl.net> wrote:
>
>   
>> bruce.griffiths at xtra.co.nz said:
>>
>>     
>>> Adding a 210ft diameter antenna to the spacecraft just to detect a
>>> sufficient number of pulsars is probably not an option.
>>>       
>>> Unless a multibeam phased array is used simultaneous observation of
>>> several pulsars isnt possible.
>>>       
>> What sort of frequencies do people use to listen to pulsars?
>>
>> Do I need 210 ft for collecting area or pointing accuracy?  Would a big
>> phased array be useful and lighter?
>>     
>
> Phased arrays raise their own issues.. If you use passive feed networks, you
> have to worry about their losses.  If you have LNAs on each element (or
> subarray), then you have matching issues.  However, phased arrays for space
> applications probably are the future.
>
> (they also use DSN 34m antennas for pulsar detection.. That's only a bit
> more than 100ft.  DSN uses pulsars for Delta DOR time reference.. A sort of
> VLBI to determine the cross range/azimuth of the spacecraft.. Point to
> pulsar, point to s/c, point to pulsar, point to s/c, recording all the
> while)
>
>   
>> Does anybody on Earth used phased arrays to listen to several pulsars
>> simultaneously?
>>
>>
>>     
Whilst the original pulsar observations used a phased array there don't
appear to have been any attempts to do this yet.
However the Embrace demonstrator for the SKA should have this capability.
There have been multibeam pulsar surveys like the Parkes multibeam
survey but these dont use a phase d array.
>>> Kolmolgorov turbulence in the interstellar medium can cause all but
>>> the singals from the most distant pulsars to fade randomly.
>>>       
>> Ugh.  Now I have to figure out what that means.  It looks interesting, but my
>> head hurts already.  Is there an analogy in the electrical world?
>>     
>
> Twinkling of stars?
>
>
>   
Yes, its the same process albeit on a much larger scale at a different
frequency with a lower density medium.
Another phenomenon to consider with pulsars is nulling where 90% or more
of the pulses from a Pulsar disappear.
The exact mechanism doesn't appear to have been established and it may
apply to all Pulsars.
Thus it would be prudent to use more than 4 Pulsars for navigation purposes.
>> --
>> These are my opinions, not necessarily my employer's.  I hate spam.
>>
>>
>>     

Bruce





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