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This document outlines the requirements for Very Long Baseline interferometry (VLBI) to enable data from the world's major arrays (EVN, VLBA, APT, CMVA, JNET) to be reduced within AIPS++. The primary instrument models for software development in AIPS++ have been connected-element (CE) arrays like the VLA, WSRT and ATCA, although much care has been taken to adopt a formalism that is not tied to any specific instrument or type of data (the Measurement Equation). There are, however, specific requirements for (VLBI) that should be identified and incorporated into the software development at an early stage, including special measures to allow for astrometric and geodetic VLBI data processing as well as Space VLBI. The purpose of this document is to outline those requirements. No attempt has been made to separate the specific VLBI requirements from those that are already defined for other interferometer data processing. However, it is clear that the requirements for VLBI presented in this document do not form a complete set of functionality either; for many detailed and non VLBI specific functions we have assumed that the equivalent of AIPS tasks will be available. We think it is understood that these things will be required if AIPS++ is to replace AIPS eventually.
In discussing VLBI-specific requirements, parallels to existing or developing AIPS routines will be drawn. Of course, some fraction of the requested functionality is not available in AIPS, although in some cases (notably for Space VLBI) it is under construction.
The final list of possibly interesting functions for AIPS++ has become quite lengthy. It is clear that some of these tasks will not emerge without specific effort from the interested parties, but we feel it is important to list all of them, in order to prevent that the future implementation of some of these is made difficult (or impossible) in design decisions.
This document has been produced after discussion with NRAO, EVN, JIVE and ATNF staff, and draws on previous documents such as AIPS++ Note 185, various AIPS++ Consortium User Specifications, a draft document by Tony Beasley (NRAO) on VLBI requirements, and a draft of Post-Correlation Data Processing Requirements for the Enhanced EVN by Dave Shone (Jodrell Bank). A draft version was discussed at a Workshop on VLBI Requirements for AIPS++ held in Alcala de Henares (Spain) on 22 October 1996. Participants included, besides most of the authors, Francisco Colomer, Carla Fanti, Mike Garrett, Leonid Gurvits, Kari Leppänen, Maria Massi, Jan Noordam, Richard Strom and Peter Wilkinson.
Our approach is to follow the data path from correlator to image (or other final product) in AIPS++ (Figure 1), and point out the specific VLBI requirements, including those for Space VLBI, along the way.