intro.lis 2001 March 13 ------------------------------- THE IAU-SOFA SOFTWARE LIBRARIES ------------------------------- SOFA stands for "Standards Of Fundamental Astronomy". The SOFA software libraries are a collection of subprograms, in source- code form, which implement official IAU algorithms for fundamental- astronomy computations. The subprograms at present comprise 27 "astronomy" routines supported by 52 "vector/matrix" routines, all written in Fortran. In the future the number of astronomy routines will increase, and implementations in other languages will be introduced. THE SOFA INITIATIVE The IAU set up the SOFA initiative at the 1994 General Assembly, to promulgate an authoritative set of fundamental-astronomy constants and algorithms. At the subsequent General Assembly, in 1997, the appointment of a SOFA Review Board and the selection of a site for the SOFA Center (the outlet for SOFA products) were announced. The SOFA initiative was originally proposed by the IAU Working Group on Astronomical Standards (WGAS), under the chairmanship of Toshio Fukushima. The proposal was for "...new arrangements to establish and maintain an accessible and authoritative set of constants, algorithms and procedures that implement standard models used in fundamental astronomy". The SOFA Software Libraries implement the "algorithms" part of the SOFA initiative. They were developed under the supervision of an international panel called the SOFA Review Board. The current membership of this panel is listed in an appendix. The SOFA Review Board is now part of Task 5 (Computation Tools) of the IAU Working Group on the International Celestial Reference System (WGICRS), which is part of Division 1 (Fundamental Astronomy). The same person chairs both the SOFA Review Board and WGICRS Task 5. A feature of the original SOFA software proposals was that the products would be self-contained and not depend on other software. This includes basic documentation, which, like the present file, will be plain ASCII text. It should also be noted that there is no assumption that the software will be used on a particular computer and Operating System. Although OS-related facilities may be present (Unix make files for instance, use by the SOFA Center of automatic code management systems, HTML versions of some documentation), the routines themselves will be visible as individual text files and will run on a variety of platforms. ALGORITHMS The SOFA Review Board's initial goal has been to create a set of callable subprograms. Whether "subroutines" or "functions", they are all referred to simply as "routines". They are designed for use by software developers wishing to write complete applications; no runnable, free-standing applications are included in SOFA's present plans. The algorithms are drawn from a variety of sources. Because most of the routines so far developed have either been standard "text-book" operations or implement well-documented standard algorithms, it has not been necessary to invite the whole community to submit algorithms, though consultation with authorities has occurred where necessary. It should also be noted that consistency with the conventions published by the International Earth Rotation Service was a stipulation in the original SOFA proposals, further constraining the software designs. This state of affairs will continue to exist for some time, as there is a large backlog of agreed extensions to work on. However, in the future the Board may decide to call for proposals, and is in the meantime willing to look into any suggestions that are received by the SOFA Center. SCOPE The routines currently available are listed in the next two chapters of this document. The "astronomy" library comprises 27 routines. Coverage is limited in this early release, but the areas addressed include calendars, timescales, ephemerides, precession/nutation, star space-motion, and star catalog conversions. The "vector-matrix" library, comprising 52 routines, contains a collection of simple tools for manipulating the vectors, matrices and angles used by the astronomy routines. Although this library can be used in its own right, it is limited in scope to what the other SOFA routines require. Some users may be better served by the many specialist libraries available elsewhere. There is no explicit commitment by SOFA to support historical models, though as time goes on a legacy of superseded models will naturally accumulate. There is, for example, no support of B1950/FK4 star coordinates, or pre-1976 precession models, though these capabilities will be added if there is a demand. Though the SOFA software libraries are rather limited in scope, and are likely to remain so for a considerable time, they do offer distinct advantages to prospective users. In particular, the routines are: * authoritative: they are IAU-backed and have been constructed with great care; * practical: they are straightforward to use in spite of being precise and rigorous (to some stated degree); * accessible and supported: they are downloadable from an easy-to- find place, they are in an integrated and consistent form, they come with adequate internal documentation, and help for users is available. VERSIONS Once it has been published, an issue will not be revised or updated and will remain accessible indefinitely. Subsequent issues may, however, include corrected versions under the original filename and routine name. However, where a different model is introduced, it will have a different name. The issues will be referred to by the date when they were announced. The frequency of re-issue will be decided by the Board, taking into account the importance of the changes and the impact on the user community. DOCUMENTATION At present there is little or no free-standing documentation about individual routines. However, each routine has preamble comments which specify in detail what the routine does and how it is used. PROGRAMMING STANDARDS The first release is in Fortran 77 only. Work on C equivalents is about to start, and related software in other languages is being considered. The Fortran code conforms to ANSI X3.9-1978 in all but two minor respects: each has an IMPLICIT NONE declaration, and its name has a prefix of "iau_" and may be longer than 6 characters. A global edit to erase both of these will produce ANSI-compliant code with no change in its function. Fortran coding style, and restrictions on the range of language features, have been much debated by the Board, and the results comply with the majority view. There is (at present) no document that sets out the standard, but the code itself offers a wide range of examples of what is acceptable. The routines contain explicit numerical constants (the INCLUDE statement is not part of ANSI Fortran 77). These are drawn from the file consts.lis, which is listed in an appendix. COPYRIGHT ISSUES Copyright for all of the SOFA software and documentation is owned by the IAU SOFA Review Board. The Software is made available free of charge for use by private individuals for non-profit research and by non-profit educational, academic and research institutions. Potential commercial users of the Software should contact the Board. Further details are included in the block of comments which concludes every routine. This block of comments is also given as an appendix to the present document. ACCURACY The SOFA policy is to organize the calculations so that the machine accuracy is fully exploited. The gap between the precision of the underlying model or theory and the computational resolution has to be kept as large as possible, hopefully leaving several orders of magnitude of headroom. The SOFA routines in some cases involve design compromises between rigor and ease of use (and also speed, though nowadays this is seldom a major concern). ACKNOWLEDGEMENTS The Board is indebted to a number of contributors, who are acknowledged in the preamble comments of the routines concerned. The Board's effort is provided by the members' individual institutes. Resources for operating the SOFA Center and for chairing the SOFA Review Board are provided by the UK's Particle Physics and Astronomy Research Council through its various astronomy programs at the Rutherford Appleton Laboratory.