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

[Brent Carlson]

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
>

-- 
Brent R. Carlson
Brent.Carlson at nrc-cnrc.gc.ca
Tel/Tél: (250) 493-2277 (ext. 346) |  Fax: (250) 493-7767
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