VEX Orbit Number Files ====================== This "aareadme.txt" file describes the contents of the kernels/orbnum directory of the VEX SPICE data server. It also provides the file naming conventions used for the VEX Orbit Number files, and it provides identification of the most current version of each kind of ORBNUM file. These are created from the SPK files converted from the ESOC's DDID Orbit Files. Orbit Number files (*.ORB) contained in this directory are unix text files. These may be used in a non-compatible text environment by using the ASCII mode of FTP to port the files. Contact ESA SPICE Service esa_spice@sciops.esa.int , or Jose Luis Vazquez jlvazquez@sciops.esa.int if you have any questions. References ========== 1. Data Delivery Interface Document (DDID) Appendix H - FD products Issue 3.0 (VEX-ESC-IF-5003) Important Note Regarding Orbit Number Files =========================================== In the Venus Express orbit number files produced by NAIF's ORBNUM utility program each of the records contains the time of THAT orbit's pericenter and the time of the NEXT orbit's apocenter. For example, the line for orbit '3' provide UTC and SCLK of orbit '3' pericenter and UTC of orbit '4' apocenter. Since the project accepted orbit numbering scheme dictates the orbit number change to occur at apocenter, the time boundaries for a given orbit can be determined by taking the apocenter time from the PREVIOUS orbit record and the apocenter time from THIS orbit record. Orbit number 1 corresponds to the first mapping orbit, 2006 APR 20. All derived quantities provided on each record -- sub-solar lon/lat, sub-spacecraft lon/lat/alt, and orbital elements -- are computed for the time of pericenter. Example ------- This is an example of VEX orbit number file (spacing was altered to fit the page): No. Event UTC PERI Event SCLK PERI OP-Event UTC APO ... ===== ==================== ==================== ==================== ... 1 2006 APR 20 08:07:37 1/0035885257.24893 2006 APR 21 04:19:46 ... 2 2006 APR 22 00:32:13 1/0036030733.35058 2006 APR 22 20:44:33 ... 3 2006 APR 23 16:57:00 1/0036176220.25438 2006 APR 24 05:48:44 ... 4 2006 APR 24 18:40:36 1/0036268836.20944 2006 APR 25 07:32:26 ... Given this example one can determine that: -- orbit 3 pericenter time is ``2006 APR 23 16:57:00'' -- orbit 3 time boundaries are from ``2006 APR 22 20:44:33'' to ``2006 APR 24 05:48:44'' Venus Express Orbit Number Directory Structure ============================================== aareadme.txt This file. former_versions Subdirectory that contains obsolete versions of the ORBNUM files that have been updated in the Current ORBNUM File Set. ORVF_______________xxxxx.ORB Contains Venus Express orbit pericenter and apocenter times, orbital elements, and additional geometry parameters computed from the SPK long term file named: ORVF_______________xxxxx.BSP where xxxxx designates the version number. This is the predicted file that should be used for planning. ORVV_MERGED_xxxxx.ORB Contains Venus Express orbit pericenter and apocenter times, orbital elements, and additional geometry parameters computed from the latest set of ORVV SPK files available in the FTP server. xxxxx designates the version number. This file corresponds to the reconstructed orbit of the spacecraft, and therefore should be used for data analysis, but not for planning. ------------------- This file was last modified on March 17th, 2009. (J. Vazquez)