[Difx-users] New to DiFX - issues with test dataset rdv70

[Phillips, Chris (S&A, Marsfield)]

Hi Geoff,

Actually the exact statement you make is not true.

	- Atmosphere  correction can be turned off in Calc, so obviously a specific array may use Calc but have no atmosphere correction (OK this is an unlikely setup)
	- ATCA uses Calc and definitely DOES need atmosphere corrections turned in as per usual with running DIFX

**For the ATCA *specific* case, Calc is run for each antenna wrt a specific reference location (fixed and not associated with any antenna). This includes atmosphere. 
In tied array mode, the phase rate and delay values of the nominated reference antenna (within the array) is subtracted from each antenna. This essentially forces the
Referene antenna to zero phase/delay AND removed the bulk atmosphere from each antenna. So even if we recorded a single antenna only, it would be fringe rotated to
the location of the reference antenna but with only the “differential” atmosphere correction applied. 

For the ALMA case I have no idea, but the inclusion or not of atmosphere is going to be very obsevator implementaton specific.

(** I’m typing this from memory - there is a chance I have got the exact specifics wrong, but the outcome is right)

Cheers
Chris


> On Aug 12, 2021, at 10:08 PM, Geoff Crew via Difx-users <difx-users at listmgr.nrao.edu> wrote:
> 
> For completeness:  an array using Calc (e.g. ALMA) for its delay model ends up having made these corrections; these must then be uncorrected in the VLBI correlation.
> 
> On 8/11/21 11:02 PM, Adam Deller via Difx-users wrote:
>>  * DRY and WET refer to specific sub-components of the delay model (the dry and wet troposphere, respectively.) You don't need to worry about those - they are just there in case for some reason a user wants to undo the default calculations of these contributions to the delay model in post-processing and re-apply their own. (Mostly used by geodesists.) 
> 
> -- 
> Geoff Crew
> gbc at haystack.mit.edu
> 
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