[asac] The ALMA site

[Al Wootten]

Dear ASAC Members and others:

Richard Hills sent the following note to the ASAC list but it bounced
to me as a submission from a non-member, so I forward it to you.  It
concerns some of the issues which Leo Bronfman relayed to us from Tony
Readhead concerning his impressions of Chajnantor after a year of
observing there.  As we discussed this at the ASAC meeting, I have
prepended comments, mostly from our discussion at the meeting.

The site is unimproved and access can be difficult.  Anyone visiting
the site should be familiar with the ALMA Safety Rules and follow
them.  Improvements to the site will commence soon after construction
funding is secured.

Site Data Collection

The CBI group report 46% usable time during the Altiplanic Winter and
60% during other seasons.  Time was lost owing to cloud, snow and high
winds.  Richard states that 'there is no doubt that weather conditions
on the site have been considerably worse for the last year or two than
during the period when the original site testing was done.'  Our
monitoring suggests, as I reported at the ASAC meeting, that this year
is average among the years we have monitored the weather at the site.
What is different is that this year we have accounts from astronomers
attempting to observe at a hostile and unimproved site.  The CBI
logbooks convey valuable data which is not available from the suite of
instruments we have monitoring weather, opacity and turbulence at the
site.  We anticipate assessing this data, and that from the Japanese
experience at the ASTE site, as we move toward ALMA construction.  I
must take issue with a point that Richard cites as an explanation for
the site failing to come up to expectations:  that the site test
equipment is solar powered and therefore shuts down if there are
extended periods of cloudiness.  There are three suites of solar
powered equipment running the ALMA/US, the ALMA/EU and the LMSA
instrumentation.  The ALMA/US equipment is adequately powered to
continue through periods of stormy weather and has failed only a very
few times during the past years; this solar power operates the 225 GHz
taumeter and the ALMA/US interferometer.  We obtain and include in the
reductions all data taken during stormy weather.  The ALMA/EU power
supply has been unreliable, perhaps because of inadequate capacity.;
data has been
lost from the equipment operated by that supply including the 183 GHz
radiometers.  Of course, there are periods of time when various pieces
of equipment fail for various reasons and owing to the remoteness of
the site it can take some time to effect repairs; I presented Radford's
assessment of the amount of time each of the fundamental instruments
was working to the ASAC and noted that the taumeter at either the
ALMA/US site or the LMSA site or more often both ran at all times over
the past season. 

Transparency

We do collect cloud cover information from the onsite camera as well
as solar insolation monitors which refer only to the daytime. Infrared
cameras would measure cloud cover at night; however they are costly
(>$20K per unit) and require liquid nitrogen and personal attention, 
an impracticality at the moment.  Cheaper options are available.  
However, we do measure 225 GHz transparency 
and operate the interferometer to measure atmospheric stability 
through all times of day and night 
We have decided that these are the most important quantities to
measure, because these are the quantities that affect the performance
of ALMA.

Access

As Richard points out, access to the Chajnantor site can be a
problem.  This will be a lesser problem for ALMA, as a great deal of
improvement of access is part of the construction plan, as Richard
relates from his conversation with Daniel Hofstadt.  Until that
improvement occurs, however, operations on the Chajnantor site may be
difficult, as they are presently for the CBI group.  

Chajnantor and Pampa la Bola

Richard anecdotally compares Chajnantor with Pampa la Bola.  Data from 
 ALMA Memo 322 which I reported to the ASAC gives useful numbers for a
quantitative  comparison.  The solar insolation data suggest that there is very
little difference in daytime cloud cover between the two sites.  If the CBI
and ASTE operations overlap, visual data will be an important
complement to the insolation data.  The memo details seasonal and
diurnal variations in wind at the two sites.  The median wind velocity 
is in fact somewhat higher at Pampa la Bola.  Pampa la Bola is
now easier to access, but ALMA site development will address this
problem; some control over access may be desirable and this is easier
to accomplish for the Chajnantor site.  Transparency is better
at Chajnantor by the amount expected from the scale height of water.
This suggests that cloudiness variations from one site to another 
don't severly affect transparency.  Furthermore, the reduced data 
from the interferometers
suggest that phase stability is somewhat lower at the Pampa la Bola
site.  Simon Radford has been working on a comparison of transparency at
the two sites to complement Sakamoto's work, which I showed
at the ASAC meeting.  Bryan Butler is working on a similar correlation
between data from the two interferometers over the past year.  We will
review these comparisons at the next ASAC meeting, if the committee wishes.
In the meantime, I hope that people interested in this question will
 review recently posted ALMA Memo 322.

In summary, the project feels that the case remains strongest for
siting the array at Chajnantor but continues monitoring activities and
study of the data.

Clear skies,
Al
- ----------------------------Letter from Richard Hills-----------------------
Date: Mon, 18 Sep 2000 09:43:45 +0100
From: Richard Hills <richard at mrao.cam.ac.uk>
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To: asac at kochab.cv.nrao.edu, rbrown at nrao.edu, mrafal at nrao.edu, rkurz at eso.org,
        sguillot at eso.org
Subject: Location of centre of the array
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Dear Colleagues,

I was at the ALMA site last week to do some work on the 183 GHz radiometers 
and the 11 GHz radiometer.  I took the opportunity to speak to Tony Readhead, 
Steve Padin, Tim Pearson, et al., about their experiences on the Chajnantor 
site and the comparison with Pampa la Bola.  Here is my interpretation of 
what they told me.  It turns out that Tony had just come back from a meeting 
in the US where he had made a presentation to the NSF people about this (he
mentioned Wayne van Sitters, but I don't know who else was involved). This is
obviously a hot topic at present.  John Richer told me there was a good deal 
of discussion about it at the ASAC so I thought I should circulate this note
to you.

It is clear that the CBI people have had a tough time in the 13 months or
so that they have been working there.  Although Tony was careful to say
that they are still enthusiastic about the place in general, they are
obviously disappointed with the quality of the site and have been impeded
by the practical difficulties of operating in such a hostile environment.

On the site quality, the main issue is the small amount of time that they
have been able to observe.  The causes of lost time include cloud, snow
and high winds.  They quoted only 46% usable nights (they only try to
observe at night) for their first 3 months, which was the 'Bolivian
winter' period, and somewhat less than 60% overall for the last 9 months.  
They did however emphasize that when conditions are such that they can
observe, then the sky is very good indeed (but of course they are only
working at 9 mm wavelength).

There is no doubt that weather conditions on the site have been
considerably worse for the last year or two than during the period when
the original site testing was done.  There was however an additional point
that the CBI people raised as an explanation for it failing to come up to
what they had been led to expect: the site test equipment is solar powered
and therefore shuts down if there are extended periods of cloudiness.  
They wonder whether this fact has been properly accounted for in analysing
the statistics.  They also pointed out that the data we have on cloud
cover comes from measuring insolation and therefore refers only to the
daytime.

A comment they made very emphatically was that there is frequently a
relatively local bank of cloud over the Chajnantor site at times when the
Pampa la Bola site was essentially clear.  I don't think that they have
any statistics on this but they feel that this is a very marked effect and
point out that you can actually see the cloud forming as the air is forced
up over the high ground and dissipating as it flows over onto the la Bola
area.  I realise that this is entirely anecdotal, but it did appear to me
that this was indeed happening to a greater or lesser extent on three of
the six days that I spent at the site on this trip.

As far as logistics are concerned, the main problem has been the frequent
blockages of the road by snow.  They also mentioned the difficulty of
getting any components or services locally.  (They were of course
expecting to have problems with the altitude and my impression is that
they have coped pretty well with this by adding oxygen to the buildings
and being rigorous in using the personal oxygen sets when working
outside.)  I think the practical difficulties caused by the cold and the
wind have also been a factor for them - they certainly have been for me on
both my visits.

The problem of snow on the roads on site is exacerbated by the fact that
the roads have been made with a bulldozer, which throws up ridges on
either side.  The blowing snow then fills in the road up to this level.  
Plowing the road makes the ridges higher so the the next time the wind
blows the situation is even worse.  The high winds (reaching 20 m/s or
greater) in the March-September period cause the snow to be re-distributed
rapidly, so they are sometimes not able to use the same route on the the
up and down journeys even on the same day, which adds to the hazards
significantly.

The main Chile-Argentina road has also been closed quite often this year
due to snow.  The main problem has again been wind-driven snow forming
drifts in places where the road is cut down into lumps in the terrain.  
There were still remnants of these drifts many feet deep to be seen when I
was there, although it had been several weeks since the last storm.  The
CBI guys reported rescuing five people whose car had got stuck in one of
these places during a snow-storm.  They also said that there was often a
lot of very slippery black ice on the road, presumably as a result of the
big day-night temperature cycle of the black tarmac.  There have been
several accidents as a result of this ice, including, I believe, one fatal
one.

Regarding the comparison between Chajnantor and Pampa la Bola, the CBI
group are completely clear that for their purposes la Bola would have been
a much better choice.  The factors which favour Pampa la Bola are:
 less cloud
 less build-up of snow
 somewhat less wind
 easier access.

The last point applies both under good conditions, when it is 10 to 15
minutes less driving time from San Pedro to la Bola than it is to
Chajnantor, and much more strongly under bad conditions, when the pass
between the two sites is the place that is most often blocked.  (The dirt
road onto Chajnantor direct from the west is no longer being used.  It is
now hard to get onto it from the highway, because of new crash barriers,
and it also becomes impassable in snow.)

Taken together these points represent a strong case for placing the centre
of the ALMA array on Pampa la Bola.  An additional consideration for the
array, which was not relevant for the CBI, is that the terrain appears a
good deal easier on la Bola than on Chajnantor.  I feel that if we have to
make a decision right now it would be extremely hard to justify going to
Chajnantor.  (I am of course saying this in the belief that the advantages
of lower water vapour and phase fluctuations on Chajnantor are marginal.)

If we can afford to keep the options open for one more year, then I
believe the project should do the following:

1) Purchase IR cameras and install them on both sites so that we can get
both day- and night-time data on cloud cover.  We must figure out a way of
keeping these operating during bad weather.  (In principle a wind-powered
generator added to the solar panels would be a good option, but I don't
know whether we can purchase wind-mills that can stand up to the
conditions on this site.  Alternatively the cameras should have their own
power system with a long battery life.)  We will have to try to relate the
cloud data to the opacity and phase variations to try to understand
whether the somewhat better values that have been obtained for Chajnantor
outweigh the deleterious effects of cloud.  (Remember that millimetre-wave
and even sub-millimetre observations are not ruled out by thin cloud,
especially if it consists of ice particles, although calibration and phase
correction are likely to be harder.)

2) try to determine from long-term weather data (plus perhaps satellite
images?) whether the conditions for this past year or so have been truly
exceptional or whether snowfall and cloud cover and levels approaching
these can be expected fairly frequently.  (Lars-Ake and Guillermo told me
that meteorological data is already being looked at, but I am not clear
whether these parameters can be obtained in that way.)  This will indicate
whether we need to revise our planning to allow for the regular occurrence
of such harsh conditions.

3) if it is felt that there is still a strong case for Chajnantor, then
the project should look very hard at providing high-quality access direct
from the western side.  The idea here would be to keep the length of the
journey that is above 4000m to an absolute minimum.  Again, I learnt in
Santiago (from Daniel Hofstadt) that this is already being studied and
that even the idea of a (cog?) railway was being discussed.  This sounds
very interesting to me.

4) roads on the site must be designed to minimize build-up of blown snow.  
Presumably this implies raising them slightly above the surrounding
ground, perhaps creating ditches on either side.

Unless this is already a well-understood art we should probably test the
practicalities of this on the site.  A few days work with a bulldozer and
a roller ought to be enough to provide suitable some test pieces on the
road that is presently being used every day by the CBI people.

Any tarmac roads should be constructed with a coarse and if possible
porous surface and a steep camber to minimise the build-up of black ice.

5) we need to think about the problem of snow build-up around the
antennas, particularly in the close-packed configuration.  What are the
pros and cons of having the surfaces of the antenna mounting pads above
ground level?  IRAM's experience on Plateau de Bure is obviously highly
relevant here.

REH  16th Sept 2000


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