[daip] Re: Question about DELZN

[Leonia Kogan]

Hi Richard,

I have carried out the test using multisource data of Mark Reid. (the 
same data I have used always for the DELZN's test..
I selected only one source at this test to see if the DELZN works with 
only one calibrator.

The result of the test says that DELZN WORKS WITH ONLY ONE CALIBRATOR!!

But the coefficients of the fitted polynomial are very different in 
comparison of multy calibrator test.

There are two reason of this.

1.the solution can be differentely redistributed between the antanna, 
reference antenna and between atmosphere and clock residual.

2. The argument (time) of the fitted polynomial is the current time 
minus the center of  of the observation. of the
     calibrators. This center can be very different depending on the 
selected set of calibrators.
     But when I used the different found polinomials for the case of all 
calibrators and the one calibrator to restore
     ATM_ant - ATM_ref (for different elevation)  (which are the actual 
data, I got an excelent coincident.
     I used DELZN to fit only ATM or to fit ATM+CLOCK and always 
obtained the good coincident  of
     the fitted polinomyal presentation of the data and the data themself..

So  DELZN works correctely with one and many calibrators.

You concern about 'stability' of DELZN. I am sure DELZN (and the test 
confirms that) shows the 'stability'.

I hope the items 1, 2 clarify the issue.

I have looked at your plots and see that the elevation differences are 
less than 15 degrees. May be it is too small.

LK
   
    

Richard Dodson wrote:

>Hi Lenoia
>
>On Fri, 2006-03-03 at 18:29, Leonia Kogan wrote:
>  
>
>>Hi Richard,
>>
>>1.Can DELZN work with only one calibrator ?????
>>2. If it can why it fails with your data
>>
>>I have put the plot of  one of the DELZN try into my web side 
>>www.aoc.nrao.edu/~lkogan
>>The file name is DELZN.PS
>>    
>>
>
>I have that. Nice. This is what I would have hoped to see with Maria's
>data. And this would appear to be 4 antennae as well(?).
>
>  
>
>>This is the plot of the polynomial (three term) into the zenith 
>>atmosphere for the three antennas together with the data
>>(given antenna -ref antenna.)
>>You see a rather good fitting.
>>    
>>
>
>It is good, but we get a `good' fit to the data in our plots as well.
>They are just not stable. Can I ask you to do something for me? Try it
>with a linear fit to the atmosphere. If it is a stable solution the
>range of the derived atmosphere would stay the same, but with less (no)
>curvature for LA. Well, I should say that is what I would expect. Is it
>what you'd expect? 
>
>  
>
>>>If one alters the number
>>>of parameters fitted for ATM one gets wildly different answers.
>>>      
>>>
>>Of cource. You can fit the linear polynomial to these data with different parameters.
>>The fitting will be a little bit worse but still reasonable.
>>    
>>
>
>If you look at the plots I sent you of the atmospheres it was not a
>little bit worse, but wildly different. This is why I think there is a
>problem.
>
>  
>
>>>Therefore I am sure that the minimisation for ATM is misbehaving. 
>>>      
>>>
>>Looking into the data points at the plot, you can see some order.
>>Look at your data. If they look as a random points you can not expect any good fitting.
>>    
>>
>
>The data looks like it should solve. I think I will have to extract it
>and bash it with directly. Am I correct in believing there is no SVD
>subroutine to be called on in the AIPS source tree? 
>
>  
>
>>By the way the least square subroutine DLESQR can give a warning about the bad fitting
>>
>>Now you main question:
>>
>>1.Can DELZN work with only one calibrator ?????
>>
>>I have always considered the answer is NO. Now I am not sure. I need to 
>>think.
>>A problem can appear for a small array when all antennas have almoust 
>>the same elevation at the given time.
>>    
>>
>
>Yes - VERA is not small, but is not the VLBA. I have attached the
>elevation plots, and the elevation difference from the first antenna.
>Does this give you any insights into whether we have sufficient range?
>
>  
>
>>The atmosphere contribution into the given antenn and the reference one 
>>are almoust identical at this case and therefore
>>the expected atmosphere difference VAL_ANT-VAL_REF is close to zero for 
>>any time!!
>>
>>And finally, Mark Read as I know have used succesfully DELZN!!
>>    
>>
>
>I can see this from your file name! Can I confirm, however, that this is
>not his phase referencing project? (Or is it?). This particular project
>is very close to the VERA mode of use and so can be used as a model for
>the VERA analysis. 
>
>For the Mark Reid project one would want (I believe -- please tell me if
>you agree) to use a mapping function which is the differential of the
>usual one. I.e. tan(z)sec(z)\Delta z (ApJ 524 816). I have being playing
>around with the code. (By the way what is mode 'BP'? ICODE=3). I am not
>sure how to transfer the position of the phase reference into the
>subroutines but once I have a neat way of doing that (suggestions? I'd
>like to use a source number -- but there would only be the single source
>in the source table.) I would like to have any changes I make -- if they
>meet your approval and are useful -- added to the default DELZN.
>
>
>		All the best and thanks for your help
>
>			Richard
>