[mmaimcal]Subarrays

[Al Wootten]

 > Richard Simon wrote to Peter Gray:
 > 
 > > About 2 years ago, it was "decided" that ALMA would support only 4 
 > > subarrays, and not 5.  The reasoning was based on VLA experience, 
 > > where 2 subarrays is routine, 3 is often used, 4 has been rarely used, 
 > > and 5 subarrays were used only for the filming of Contact (!).  
 > > Accordingly, the relevant budget planning numbers were adjusted.  
 > > However, the change was never officially made (to our unofficial 
 > > documents).
 > >
 > > I would like to request that the formal process be completed, so that 
 > > our budget planning (presumably for 4 subarrays) is in agreement with 
 > > the technical specifications.  If if turns out that 5 subarrays are 
 > > required, I will adjust the budget numbers
 > >
 > > While it might be fun if I were to initiate a Change Request, it is 
 > > probably most appropriate if one of you initiates the process.
 > >
Peter responded:
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Richard,
I have checked (with SEARCH)  through the various project documents that 
we have collected
in the almaedm archive and found amongst others the following references 
to the number
of sub-arrays.

- specification ALMA-90.00.00.00-001-A-SPE, old ProjBook Chpter 2, 
current title
" Science - Specifications and Requirements"
p.16 says in system flexibility spec "4 sub-arrays"
p.19 Table 1 says Number of sub-arrays >3 TBD, goal 5
see in almaedm:
http://almaedm.tuc.nrao.edu/forums/alma/dispatch.cgi/docapproval/showFolder/100023/d20020807173924/No/t100023.htm

- spec ALMA-70.00.00.00-001-A-SPE which is the Computing spec
http://almaedm.tuc.nrao.edu/forums/alma/dispatch.cgi/docapproval/docProfile/100019/d20020807165242/No/t100019.htm
says up to 6 as per the correlator spec

- spec ALMA-60.00.00.00-001-A-SPE Correlator spec
http://almaedm.tuc.nrao.edu/forums/alma/dispatch.cgi/docapproval/docProfile/100018/d20020807164617/No/t100018.htm
p.10 says "full support for up to 6 independent correlator sub-arrays"

- spec ALMA-50.03.00.00-001-A-SPE Local Oscillator
http://almaedm.tuc.nrao.edu/forums/alma/dispatch.cgi/docapproval/docProfile/100017/d20020807191726/No/t100017.htm
in Table 1 has the same as ALMA-90.00.00.00-001-A-SPE that is >3 and goal 5.

- in the report from the Systems PDR 200-02-28
http://almaedm.tuc.nrao.edu/forums/alma/dispatch.cgi/systemspdr/docProfile/100004/d20020604122843/No/sysPDRreport.pdf
"The operation of ALMA with maximally 4 Subarrays is recommended."

- in a joint ALMA DH meeting dated 2000-May-02 it says:
"Engineers and scientists have agreed on subarray requirements. Four
simultaneous LOs will be required.  Also all antennas can be controlled
individually. A report defining what type of sub-arrays will be
available and how many will be issued soon and added to the
recommendations of the System Review."

- in the system block diagram which has just been submitted for approval
http://almaedm.tuc.nrao.edu/forums/alma/dispatch.cgi/docapproval/docProfile/100067/d20020909202652/No/t100067.htm 

it is careful not to specify the number of sub-arrays but uses a number N.

I conclude from all the above that the number has been generally agreed to
be 4 sub-arrays, but that this specification has not been clearly defined
and presented in an approved project level specification.
I am not sure of the consequences of the choice of 6 sub-arrays for
correlator and computing, but perhaps this does not have a big cost
impact.

I propose to do the following:

1/ split out the Engineering requirements section of Chapter 2 into a 
seperate document to be called something like
"Top-Level System Engineering Reqirements" and to make
it more visible. This will be for now a simple cut-paste as for the 
other chapters. Then
get it project approved.

2/ after approval of the above, ask you to submit a change request CR to 
change in the
above doc the ">3 and goal 5 " reference to a fixed 4 spec
sub-arrays, and to modify any other references in this document to suit. 
This CR can then be reviewed
and commented and can be used as a way of informing the project properly 
of this fixed requirement.

3/ start the process of getting this Enginering Requirements document 
revised and reformatted to make it
clearer and referenceable (ie number and clearly define each requirement 
in the tables). Then make sure
cross-referencing inside this and other documents specifies the 
requirement reference, not the requirement.

Peter

PS A reminder for those on the CC list for this email, it would help the 
above process if you could
also complete asap the review of the above documents so that we can 
complete the approval process
and move on to try to organise ourselves to avoid future confusions like 
this "number of sub-array" question.
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I responded:

Dear Peter and Richard

This decision had nothing whatever to do with Contact nor with the VLA;
I don't know where this silly impression arose.  In my recollection, this was 
discussed at the System PDR and revisited many times since for not very
good reason.  A subarray is a slippery beast, and the first thing that
must be understood is that it means different things to different people.
In its loosest sense, it is a collection of antennas whose signal may be
correlated independently of other collections of antennas.  This is a
correlator subarray, and I understand that the current correlator supports
16 of this variety of subarray.  This sort of subarray should make little
difference to costing.  

Another variety of subarray (which I think you consider) consists of 'groups
of antennas with independently tuned LOs', which is a different item.
Yet another variety mixes these two; during Early Science for example,
I envision an Early Science array operating at 3mm and 1.3mm while the
Commissioning Array operates at perhaps 3mm and Band 7 or 9 while a total
power antenna operates in yet another 'array' at one of these frequencies.
This might constitute four LO subarrays, but five correlator subarrays for
example.

This is in accord with the System Review, endorsed by the Division Heads on
 2000-May-02 as stated by Peter.

The Science Group has a number of similar examples which may easily be found
by using the search engine at:
http://www.cv.nrao.edu/~awootten/mmaimcal/

Steve Myers wrote a report proposing eight subarrays 29 Feb 2000, available as
http://www.aoc.nrao.edu/~smyers/alma/subarrays.txt

In the report
on the first meeting of the ASAC, held in Leiden, The Netherlands, on 
March 10-11, 2000, the ASAC said:

System 

The ALMA system deals with many aspects of ALMA. We expect to revisit many 
of these areas in the future. We summarize below our recommendations on 
the issues addressed at this meeting. 

1. The main array should consist of a number of 4 to 6 sub-arrays, but the 
number of frequencies operating simultaneously will not exceed 3 or 4. At 
present we could envision 4+1 subarrays. Namely: 
       (a) The main interferometric subarray 
       (b) Antennas for reconfiguration and baseline determination 
       (c) Two subarrays to simultaneously carry out two of the following 
functions: 
                    Secondary subarray at second frequency band 
                    Transient event monitoring 
                    mm-wave VLBI 
                    Testing, repair, receiver warm-up or cool-down, etc. 
       (d) The single-dish subarray or an ultra-compact array (if included 
in the final project). 
2. The prototype antennas should be equipped with nutators and stable 
receivers. The number of ALMA antennas equipped for total power measurements 
(nutators) should be 4, but this number will be reconsidered
after the tests with the prototype antennas. If feasible, the rest 
of the array antennas should be equipped with receivers of good gain 
stability ( in 1 second). (See also section 3 and 6). 
...

The ALMA Correlator handles 16 correlator subarrays; the group working on the
next generation correlator asked the ASAC how many subarrays should be
functional in that instrument.  At their meeting in Santiago, they replied:
* Capability for being a spectrometer for up to 5 subarrays (the ALMA 
Correlator handles 16).

I confirmed with Ray Escoffier that the first quarter of the ALMA correlator
offers the full capabilities of the final correlator, though with some
wiring and 'personality' differences to achieve this.  It will have full
subarray capability.

In my opinion no change request is needed, only a clarification to the
unclear documentation. 

Clear skies,
Al
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 However, since the documents are now controlled and
are internally inconsistent, a change request does seem mandated.
          
Al