***** File GIODAT.TXT Mission: GIOTTO Program: giodat.f Submission: epgiotto.doc text file Original Format The original form of the source code for the FORTRAN subroutine, GIODAT, and the attendant ephemeris data file, POSITION, were received as part of the text file, epgiotto.doc. Reformatting at SBN The following was done to access the subroutine: o The text file epgiotto.doc was edited to separate the parts which contained the ephemeris data file, POSITION, and the source code for the subroutine, GIODAT, into individual files for use. o The trailing control characters at the end of each line were removed so that the position data file could be used normally, and the subroutine, GIODAT could be compiled. o A FORTRAN program, epgiotto.f, was then written to access the position data file via calls to GIODAT. o Numerous tests against known ephemeris data from various sources were then done to test the accuracy of the values returned by GIODAT. Known Anomalies/Problems: o Ephemeris data held in the data file POSITION, as stated in epgiotto.doc, are stored as double precision floating point numbers with 11 significant digits. However, GIODAT will only return values with 8 significant digits. Redefining the size of the variables read from POSITION by GIODAT would solve this anomaly. o GIODAT returns the position and velocity vectors for some specified time, TUTC, in rectangular, barycentric, M.E.S. 1950 coordinates. Care must be taken when comparing to known sources that the proper conversions be made from the coordinates used in the source to those of GIODAT. o The velocity vectors for Comet Halley returned by GIODAT, when compared to known data from the file ephem.tab, were severely in error. It is still unknown why this is so. The following chart is taken from the comparison of the output of GIODAT with the ephemeris values given in the file, ephem.tab, on CDROM HAL_0019. Data From Ephem.tab | From Subroutine Giodat| ERRORS | TIME DELDOT RDOT | TUTC GeoVel HelioVel|Geocentric| Heliocent| (Julian) (KM/S) (KM/S) |(1950 (KM/S) (KM/S) | (KM/S) | (KM/S) | | EME) | | | ----------------------------+-----------------------+----------+----------+ 2446494.5 -39.4056 23.0952 | 13212 44.6541 47.3377 | -84.0597 | -24.2425 | 2446495.5 -40.0354 23.4968 | 13213 45.2591 46.9228 | -85.2945 | -23.4260 | 2446496.5 -40.6201 23.8643 | 13214 45.8436 46.5117 | -86.4637 | -22.6474 | 2446497.5 -41.1600 24.2001 | 13215 46.4080 46.1051 | -87.5680 | -21.9050 | 2446498.5 -41.6551 24.5061 | 13216 46.9526 45.7034 | -88.6077 | -21.1973 | 2446499.5 -42.1054 24.7845 | 13217 47.4779 45.3071 | -89.5833 | -20.5226 | 2446500.5 -42.5104 25.0372 | 13218 47.9843 44.9166 | -90.4947 | -19.8794 | 2446501.5 -42.8694 25.2660 | 13219 48.4725 44.5320 | -91.3419 | -19.2660 | 2446502.5 -43.1814 25.4726 | 13220 48.9427 44.1536 | -92.1241 | -18.6810 | 2446503.5 -43.4450 25.6587 | 13221 49.3957 43.7816 | -92.8407 | -18.1219 | 2446504.5 -43.6585 25.8257 | 13222 49.8318 43.4160 | -93.4903 | -17.5903 | 2446505.5 -43.8195 25.9751 | 13223 50.2519 43.0570 | -94.0714 | -17.0819 | ----------------------------+-----------------------+----------+----------+ where: TIME(julian) = the time according to the Julian calendar DELDOT(KM/S)= the Geocentric velocity magnitude of Comet Halley, as taken from ephem.tab RDOT(KM/S)= the Heliocentric velocity magnitude of Comet Halley, as taken from ephem.tab TUTC(1950 EME)= the same as TIME, converted to days since 1950 for use in the subroutine Giodat GeoVel(KM/S)= the Geocentric velocity magnitude of Comet Halley, as returned by Giodat HelioVel(KM/S)= the Heliocentric velocity magnitude of Comet Halley, as returned by Giodat Giodat uses TUTC instead of TDT (in March 86, TDT was 55.184 seconds greater than TUTC ) The POSITION data file covers data from TUTC 13212 (March 5, 1986) to TUTC 13224 ( March 17,1986)