******************************************************************************** Orientation of the Voyager 1 Scan Platform during the Saturn Encounter ====================================================================== Version 5: Continuous, C-smithed Pointing ========================================= This is a Type 3 (linear interpolation) C kernel containing pointing information for the Voyager 1 scan platform during the Saturn encounter. Note that the scan platform's coordinate system is defined as coincident with that of the narrow-angle camera. This C kernel describes the pointing relative to the J2000 coordinate frame almost continuously during the encounter. Data Summary ------------ This C kernel was generated using the pointing information encapsulated in two "Supplemental Experiment Data Record" (SEDR) files from the Voyager project: (1) The Imaging Science Subsystem (ISS) SEDR file; (2) The Ultraviolet Spectrometer (UVS) SEDR file. These files were originally generated shortly after the Voyager flyby in Vax binary format. The ISS file was provided by the Multimission Image Processing Laboratory (MIPL) at JPL. The UVS file was provided by Jay Holberg at the University of Arizona's Lunar and Planetary Laboratory. The pointing information was "C-smithed" using two additional sources of data: (1) Mark Showalter's personal collection of pointing corrections for 834 Voyager 1 images of Saturn and the rings. (2) A collection of SEDR updates obtained from JPL/MIPL for 259 images of Saturn's satellites. These udpates date back to the early 1990s. Pedigree -------- A variety of corrections were applied to the pointing information found in the original ISS SEDR file. (1) The SEDR file provides pointing information for both the wide- and narrow-angle cameras. These cameras have a small offset, which is recorded in the Voyager frames kernel. However, it became apparent that the SEDR file was generated using an incorrect value for this offset. The (incorrect) value used in the SEDR file was revealed by a constant offset between the optic axes of wide- and narrow-angle images shuttered simultaneously. By removing this offset, it was possible to determine the orientation of the scan platform in a consistent manner, regardless of whether the SEDR described a wide- or a narrow-angle image. Note that this information is preserved in C kernel V1SAT_VERSION1_TYPE1_ISS_SEDR.ck (2) The collection of SEDR updates from JPL/MIPL was based on the satellite ephemeris information that was also stored in the SEDR file. Unfortunately, this ephemeris information was extremely inaccurate. Using an up-to-date ephemeris for Saturn's satellites, the correct J2000 right ascension and declination of each satellite's apparent position relative to Voyager was determined. Using the SEDR file, the (inaccurate) J2000 right ascension and declination coordinates that had been used to update the SEDR were determined. The difference was applied to each of the 259 satellite images. The result was a generally reliable set of new updates. (3) The two sets of pointing updates (834 for the planet and rings, 259 for the satellites) were combined. Most of these corrections were at the level of ~ 100 narrow-angle pixels, which is consistent with prior experience with the SEDR. A few outlying updates were eliminated from the set, after confirming that they were inaccurate. (4) A sequence of continuous, smooth models were fitted to the complete set of pointing corrections. In cases where there were relatively long gaps between sets of corrections, linear interpolation was applied. The result was a continuous model for how to correct the pointing of the Voyager images. (5) This model was then applied to every Voyager image in the SEDR file. The resulting C kernel has been preserved as file V1SAT_VERSION4_TYPE1_ISS_CSMITHED.ck The UVS SEDR shows many incompatibilities with the ISS SEDR at the level of ~ 1 degree. However, the ISS SEDR file only describes the scan platform orientation at the time when images were taken, leaving long intervals of the Voyager encounter with no pointing information. The key advantage of the UVS SEDR file is its continuity, because it records the UVS instrument's orientation every 48 seconds (except for one gap post-encounter). The information from the UVS SEDR file was incorporated as follows. (1) At the times of Voyager images, the scan platform orientation derived above was deemed accurate. (2) At each of these moments, a pointing offset in narrow-angle camera coordinates was determined to align the UVS SEDR boresight with the narrow-angle camera's boresight. (3) At intervening times where only UVS pointing information was available, linear interpolation was performed between the times of the nearest, adjacent images. (4) The C kernel was generated using SPICE program MSOPCK. Accuracy -------- In general, ISS pointing information should be accurate to the level of a few tens of narrow-angle pixels, i.e., ~ 0.01 degrees or ~0.2 milliradians. However, some errors may be larger, particularly around the closest approach period when the scan platform was moving rapidly. It is reasonable to assume that this C kernel has comparable accuracy, except perhaps during long periods of time during which no images were taken. However, it is not possible to test this assumption. Contact Information ------------------- Mark R. Showalter Manager, PDS Rings Node SETI Institute mshowalter@seti.org ******************************************************************************** MSOPCK SETUP FILE: V1SAT_VERSION5_TYPE3_MERGED_CSMITHED.ker ******************************************************************************** \begindata INTERNAL_FILE_NAME = 'V1SAT_VERSION5_TYPE3_MERGED_CSMITHED.ck' COMMENTS_FILE_NAME = 'V1SAT_VERSION5_TYPE3_MERGED_CSMITHED_comments.txt' LSK_FILE_NAME = 'NAIF0007.tls' SCLK_FILE_NAME = 'VG100008.tsc' FRAMES_FILE_NAME = 'VG1_V02.tf' CK_TYPE = 3 INSTRUMENT_ID = -31100 REFERENCE_FRAME_NAME = 'J2000' ANGULAR_RATE_PRESENT = 'MAKE UP/NO AVERAGING' DOWN_SAMPLE_TOLERANCE = 0. MAXIMUM_VALID_INTERVAL = 2000 INPUT_DATA_TYPE = 'MATRICES' INPUT_TIME_TYPE = 'SCLK' PRODUCER_ID = 'Mark R. Showalter/PDS Rings Node' \begintext ******************************************************************************** RUN-TIME OBTAINED META INFORMATION: ******************************************************************************** PRODUCT_CREATION_TIME = 2007-01-02T22:17:22 START_TIME = 1980-08-22T23:10:10.455 STOP_TIME = 1980-12-16T02:08:35.137 ******************************************************************************** INTERPOLATION INTERVALS IN THE FILE SEGMENTS: ******************************************************************************** SEG.SUMMARY: ID -31100, COVERG: 1980-08-22T23:10:10.455 1980-10-17T13:10:58.781 -------------------------------------------------------------------------------- 1980-08-22T23:10:10.455 1980-10-17T13:10:58.781 SEG.SUMMARY: ID -31100, COVERG: 1980-10-17T13:10:58.781 1980-12-14T02:13:23.137 -------------------------------------------------------------------------------- 1980-10-17T13:10:58.781 1980-11-20T23:59:47.066 1980-11-21T05:31:47.066 1980-11-21T19:20:35.066 1980-11-24T18:10:59.066 1980-11-24T22:52:35.066 1980-11-25T00:00:35.066 1980-12-14T02:13:23.137 SEG.SUMMARY: ID -31100, COVERG: 1980-12-14T02:13:23.137 1980-12-16T02:08:35.137 -------------------------------------------------------------------------------- 1980-12-14T02:13:23.137 1980-12-16T02:08:35.137 ********************************************************************************