ASAC Recommendation on the Long Baselines Array geometries
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 At its June 5 2002 teleconf, the ASAC was requested to `summarize the
 scientific and technical issues associated with the long baseline
 array geometries currently under consideration and to advise the
 ACC/ad hoc ALMA Board as to possible cost, land use, and land access
 impacts of which it should be aware'.
 
 In the current discussions on the Exclusive Use area on Chajnantor two
 options are under consideration regarding the long baseline (14 km)
 geometry: the `14-km Ring Configuration' and the `Y+ Configuration'.
 
 The major argument in favor of the Y+ Configuration is that it offers
 more continous reconfiguration, providing higher flexibility and less
 time lost in non-optimal hybrid configurations, which translates
 directly into operational advantages. Other arguments are: shorter
 fibers, so better phase stability; better expandability towards even
 longer baselines; more flexibility in re-weighting (tapering and/or
 uniform) data to adjust angular resolution; possibly less pads,
 because no complex hybrid between the 5 km Gaussian and the 14 km
 array is needed.
 
 The main argument in favor of the 14-km Ring Configuration is that
 this configuration would reach slightly better angular resolution than
 the Y+ Configuration. However, this argument is not decisive since a
 proper choice of the Y+ Configuration can result in similar angular
 resolutions than those obtained with the 14-km Ring Configuration.  In
 addition, the possibility to expand the Y+ Configuration towards even
 longer baselines opens up the possibility to achieve even better
 angular resolution in the future. Theoretically, the 14-km Ring
 Configuration would provide a slightly better astrometry than the
 Y+ Configuration, but this improvement would only be marginal under 
 realistic observing conditions.

 Based on the above arguments, the ASAC recommend to abandon the
 project of the 14-km Ring Configuration, and to work on optimizing an
 Y+ Configuration with similar angular resolution.