SOFTWARE Overview There are several elements to NRAO's current software strategy for supporting data processing and analysis. First, AIPS supports the reduction and analysis of (primarily) radio interferometric data. AIPS has extremely broad and flexible capabilities and is certainly used in many other areas of image processing and analysis outside of ordinary interferometric radio astronomy. Second, UniPOPS still supports the reduction and analysis of single-dish data, with a strong emphasis on NRAO's two current single dish instruments, the 140 Foot Telescope in Green Bank and the 12 Meter Telescope on Kitt Peak. These will be gradually phased-out as the third element in NRAO software, AIPS++, becomes operational over the next few years. In addition, specialized tasks for single-dish on-the-fly (OTF) mapping are currently supported in an experimental version of AIPS; as these tasks stabilize they will migrate to the standard distributed version of AIPS. The current distribution of personnel between these three efforts is roughly: - AIPS: One full-time position in Charlottesville, four full-time positions in Socorro (including effort dedicated to Orbiting VLBI for JPL) plus part-time support from four other people (two in Socorro, two in Charlottesville). The goal of this AIPS support is primarily to keep AIPS stable and to provide support as needed for Space VLBI, the VLBA, and the Very Large Array (VLA) efforts. In March 1996, a new position was added to the AIPS group to increase the manpower committed to Orbiting VLBI specific software. Eric Greisen left the group in order to continue software development within his own, more experimental system. In August 1996, a support scientist left the AIPS group; his position was not replaced. Thus the AIPS group currently consists of approximately six full-time employees (less roughly 25 percent of that time for scientific research that staff members are allowed). This staff shares the load of developing new software, maintaining existing software, supporting the world-wide community of users, and maintaining the various AIPS installations at both sites. This number has seen a gradual decrease over the last five years from nine in 1992-93 to the current six. AIPS RELEASES: AIPS had a new release in October 1996. As usual, an AIPS letter describing the changes in the new release was distributed. The next release is planned for April 1997. Though several important general purpose applications were added, the emphasis of both releases was clearly on new developments in the area of Space VLBI software. The new copyright, introduced in 1995, has led to a surge in requests for copies of AIPS. A total of 225 copies of the January 1996 version of AIPS have been delivered, almost 90 percent electronically, the remainder have been shipped by tape. MIDNIGHT JOBS: Due to rapid developments, especially in Very Long Baseline Interferometry (VLBI) related applications, an increasing number of non-NRAO institutes run the Midnight Job, which guarantees that their local AIPS installation is synchronous with the test version of AIPS at the various NRAO sites. A total of five non-NRAO sites are currently running the Midnight Job; four other sites are expected to follow soon. AIPS APPLICATION SOFTWARE - GENERAL: New bandpass calibration modes were added, specifically bandpass interpolation, with correct treatment of flagged bandpass entries, and allowing weighted and unweighted two-point interpolation. The error analysis in the image fitting routines was completely overhauled; these tasks now yield a far better error estimate. Two new tasks were written that copy GPS ionospheric data from an ASCII file to an AIPS table, which is used by a second task that fits a simple, local model of the ionosphere and uses this model to correct phases for the excess path in the ionosphere and to calculate ionospheric Faraday rotation. A new task was introduced that solves both for the gain dependence on elevation, and interpolates the required corrections into a gain table. This will especially benefit observations at high frequencies, where there is a strong dependence of antenna gain on elevation. Another new task offers improved model fitting with self-calibration. Other new developments in the general area concentrated on increasing the overall robustness of AIPS, and implementing changes and improvements as suggested by the AIPS user community. APPLICATION SOFTWARE - SPACE VLBI AND VLBA SPECIFIC: Development of VLBI software has included significant work for the VSOP/HALCA orbiting VLBI mission, but new VLBA support has been added. This includes support for subarrays, and IF-selection and weight-based flagging in the data loading task FITLD. A new task has been added to pre-process VLBA external calibration files into a form directly readable by AIPS, and the option of second-order VLBI polarization correction has been added to the calibration system. The testing of code correctness using simulated data has expanded. Developments of relevance to the VSOP mission include enhancements to the baseline-based fringe-fitting tasks, and the implementation of a new task to phase-up the ground array. In addition, the standard fringe-fitting and self-calibration tasks have been improved to be of greater usefulness for OVLBI data reduction. The data loading task FITLD has been modified to support the new spacecraft orbit ephemeris table, and the time-variable use of OVLBI filters in the VLBA correlator through the implementation of a new correlation id. Random parameter. A memory-based sorting task has been introduced to deal with the special case of mis-sorted OVLBI data, and a new task has been provided for orbit table manipulation. The data simulator has been expanded to generate fake data for OVLBI antennas. In addition, plotting tasks have been modified to plot models for orbiting antennas. DOCUMENTATION AND ONLINE HELP: The experimental pseudoverb XHELP was added to AIPS. It provides direct access to HTML versions of cookbook-level help. When invoked, it will automatically start a Netscape process (if not already running) and loads the specified HTML file. A draft Cookbook chapter on the reduction of OVLBI data has been written. SYSTEM: The AIPS DDT was used extensively to test new architectures considered for purchase by NRAO. At first, a DEC Alpha machine reached the highest AIPS mark, namely 12.0, only to be surpassed by an SGI machine within AIPS mark of 13.7 (for comparison, a Sparc IPX has an AIPS mark of 1). It was decided to purchase seven Sparc Ultra-2 dual processor machines for the Array Operations Center (AOC) in Socorro. These machines are capable of an AIPS mark of 8 per processor. AIPS benchmarks of multiprocessor systems at the National Center for Supercomputing Applications (NCSA) were less successful, partly because AIPS code is not parallelized enough to make use of many processors at once. COORDINATION WITH CVX: We assembled a list of tasks and utilities currently in the experimental software system CVX that we considered deserved porting to AIPS TST, and ranked them according to desirability and impact on AIPS TST. As a result, two tasks allowing interactive editing of calibration tables and individual baselines, were successfully ported. In addition, an improved version of the self-calibration and imaging task SCMAP was introduced in AIPS TST. The impact of introducing other selected software from CVX will be greater, and we will defer porting it until after the Space VLBI-critical 15APR97 release. USER SUPPORT: The focus of AIPS user support is the Designated AIP program, in which five staff scientists rotate in handling requests for support from AIPS users around the world. We have found this to be the most effective way of handling requests for support.