[alma-config] pad and transporter requirements

[Min Yun]

Attached is a draft version of the antenna pad and transporter
requirements I hope to forward to the antenna group.  Please
read and comment.  I hope to send this off by Monday morning.



					-- Min
					

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ALMA COMPACT CONFIGURATION AND ANTENNA PAD AND TRANSPORTER DESIGN

			  Min S. Yun

		         March 2, 2000 Draft
		 

The interaction between the antenna and the transporter design 
especially with regard to the close packing of antennas has 
been brought up in various discussions lately.  In particular,
Jaap Baars has specifically requested the
ALMA configuration group to consider these issues during the
Tucson meeting.  The question was posed only in the 
context of compact configuration design and its integration with
site infrastructure, but the implications are much wider.  I address
below several related issues and make a set of recommendations 
for the antenna group.


1. Current Compact Configuration Design

While the final ALMA configuration designs are far from being settled, 
most of the ongoing discussions concern only the designs for the
intermediate configurations.  It is already agreed by the configuration
working group that the compact configuration should be a densely
packed design to include a number of short spacings in order to
achieve an excellent brightness sensitivity and an instantaneous 
complete uv-coverage for the mosaic observations.  An example of such 
a configuration is shown as http://www.nrao.edu/~myun/mma/almad.gif 
and detailed justifications for such a design have been discussed 
previously by Helfer & Holdaway (1998, MMA Memo 198) and others.

One certainty for the compact configuration is that a large fraction 
of the pads will be situated with 15 meters of the neighboring pad.  
Often a direct access to any particular antenna may not be possible,
requiring some subset of antennas to be moved to some temporary
pad positions first.  The space to maneuver the transporter will 
also be highly limited.  There are two resulting demands to 
the transporter design: (1) versatility in antenna pick-up and lay-down; 
and (2) high maneuverability.

A traditional transporter design that requires mating of
the transporter with an antenna at one or two specific locations will
face a severe problem during reconfigurations as the approach to
any individual antenna is already extremely limited.  Further,
the same considerations also demand the design and the orientation 
of the individual pads to incorporate the access and the
approach by the transporter.  

Operating the transporters within such limited spaces
also demand that the transporters are highly maneuverable.  Having
a small turning radius, at least at low speed, is essential,
and this is a general requirement for all reconfigurations
(see below).  An ability to move laterally (e.g. the VLA) is
also highly desirable for the close quarters maneuvering.

Lastly, a careful plan needs to be developed for a quick and
efficient reconfiguration in and out of the compact configuration.  
Once the compact configuration, antenna pads, and transporters
designs are finalized, a plan that requires the least amount of
extra work, including the detail orders of the individual antenna
moves, should be drafted.


2. Interaction with Antenna Stations in General

The pad and transporter requirements discussed above are not 
limited just to the compact configuration.  The Chajnantor site
is not sufficiently flat so that preparing an arbitrarily large 
area for an antenna station is not a trivial problem.  During
the drive around Chascon last November for scouting the
potential station locations for the 10+ km array, areas
to the south and southeast of Chascon (also the southern end
of the Chajnantor proper) are found to be very
rugged.  Constructing the access roads will be challenging
and costly enough, and preparing the grounds for an antenna
station with sufficient room to maneuver will be very
difficult in several cases as some loosening of the topographical 
considerations is inevitable in designing the 10+ km configuration.

If the mating of an antenna to the transporter is limited to
a certain position angle only, then the pad construction and 
orientation should take into account the position angle of the 
access road.  (This is not a trivial matter given the SMA 
and other similar experiences.)
If the transporter can approach an antenna at an arbitrary
angle for the pick up and drop off, combined with the high
maneuverability, the demands on the antenna station construction
and pad orientation may be reduced accordingly.


3. Ultracompact Array

If the Japanese contribution of the ultracompact array with 
6 to 8 smaller antennas is realized, it may widely
impact the design of the configurations as well as the
design of the pads and transporters.  Procuring a separate
transporter (and maintaining it) is an obvious and costly
possibility.  An entirely different pad design may also be 
needed unless the smaller antennas are designed with the use of
the existing pads in mind.  

The most important issue is whether these smaller antennas
will be directly incorporated into the existing array designs
or to operate independently as a separate array.
If it were to operate entirely independently of the rest of 
the array and no cross-correlations with the larger antennas
are to be made, then having these small antennas do not
impact the configuration design at least.  If the ultracompact
compact array is placed on fixed pads, this will further
simplify most of the operational concerns listed above.


4. Recommendations

(1) The transporter should be highly maneuverable in order to operate
within the limited spaces allowed by the densely packed compact
configuration.  A capability to move laterally or to rotate
while at stop is probably required.

(2) A particularly desirable capability for the transporter is
the ability to approach and pick up and drop off antennas from
any arbitrary position angle.  This in turn requires an ability
to place an antenna onto a pad also at an arbitrary position angle.
A capability to rotate the telescope base in azimuth direction, 
after the telescope is picked up from the pad, may be an important 
additional requirement if the antenna base and the pad has to
be mated in some specific orientation.

(3) The pad design and the layout should be a critical part of 
the antenna and transporter design so that any limitations
of the transporter can be accommodated.  This is particularly
important if the transporter can pick up and place the
antennas only from a certain position angle.

(4) A careful and detailed reconfiguration plan needs to be 
drafted and executed, at least for the compact configuration.  

(5) The ultracompact array, which is being considered as part 
of the Japanese participation to the project, could have 
wide-reaching implications in the configuration design as well 
as the pad and transporter design.  Most importantly, the mode 
of its operation need to be decided soon in order to avoid 
adding any significant additional cost to the project at a 
later time.