[evla-sw-discuss] [Fwd: Re: EVLA WIDAR: Delay Model - format and content]

[John Benson]

If the correlator has sufficiently large delay buffers, the antenna delays
should be with respect to the earth geocenter. This is of course what VLBI 
correlators do that use CALC, and it seems likely that the WIDAR will 
correlate VLBA data in the future. It's much simpler to use the geocenter. 
Let the wavefront arrival times at the geocenter define the wave front that 
is simultaneously correlated. The geocenter is less of an non-inertial point
that the center of the VLA...

John

Brent Carlson wrote:
> Dave Fort just pointed out to me that some clarification is required.  I 
> meant to indicate that the delay model sent to the correlator does not 
> have to have any artificial offsets to make up for its buffer sizes, 
> however the delay model sent to the correlator for each antenna must 
> include the following:
>
> 1. The geometric delay from CALC.  No specific array phase center needs 
> to be chosen to make these all positive or all negative.  I think its 
> important that the array phase center not be any specific antenna, to 
> ensure the residual baseline delay error (i.e. due to +/-1/32 sample 
> delay tracking) is the same on all baselines.
> 2. The "outbound" fiber delay from the central "array timing 
> reference"/GPS unit to the antenna.  As pointed out in the Programmer's 
> Guide to the correlator (A25290N0000), page 17, the accuracy of when 
> models are applied, and when data gets timestamped, depends on the 
> accuracy of one of these (the Reference Antenna)...the rest being 
> determined relative to the Reference Antenna via calibration, and 
> subsequently included in the models sent to the correlator.
> 3. The time-variable delay from the round-trip fiber 
> measurement--effectively time variable refinements of 2., and can be 
> positive or negative depending on if the fiber delay has increased or 
> decreased relative to 2. respectively.
>
> Fiber delays always have a positive sign, following the convention 
> established in the Programmer's Guide.
>
> Hopefully this clarifies my comments.
>
> Brent
>
>
>   
>>     
>>> 4.  Delays.  Who organizes the delay center?  Do I send numbers which 
>>> might
>>> be negative, or should I add something to them to make them always 
>>> positive?
>>> If so, what?
>>>  
>>>
>>>       
>> I think it is most unambiguous if the pure delay, without any offset, 
>> is sent to the correlator.  If the s/w engs follow the 
>> discussion/implementation on delay tracking in the Programmer's Guide 
>> to the correlator, then any offsets required will be incorporated in 
>> CMIB s/w.
>>
>> Brent
>>
>>     
>
>