General information about SPICE C-kernels ========================================= In the SPICE system, pointing data for an instrument platform, or for a specific instrument, are stored C-kernels. The pointing of a platform or an instrument is often expressed in terms of a transformation matrix from some standard inertial reference frame to a instrument-fixed frame. In the past, the instrument was often a camera, and the transformation was thus dubbed the ``C-matrix''; hence the choice of C as the name for the pointing kernel. CKs can be accessed by subroutines that are part of the SPICE library, the major component of the SPICE Toolkit. These subroutines can be integrated into user application programs. For information on the SPICE Toolkit and/or how to obtain it, please consult SPICE_INST.CAT and SOFT.CAT. To use C-kernels, a Spacecraft Clock Kernel (SCLK) is required. Also, for C-Kernels containing orientation of a project-defined frame, a Frame Definitions File (FK) is required. Refer to the CK.REQ and FRAMES.REQ documents provided with the Toolkit or available from the NAIF Node of PDS to get more information on how to access C-kernel data. Venus Express C-Kernel Files ============================ The following file describes the contents of the DATA/CK directory of the Venus Express SPICE data set. Sources of kernels and naming convention: ---------------------------------------- - Kernels that are produced from the ESOC Flight Dynamics attitude products. These kernels are generated by an automated system located at ESAC, and operated by the ESA SPICE Service. The naming convention for these kernels is based on the naming convention of the attitude products used to create them; their file names start by ATNV_ (for predicted attitude) and by ATNM_ (for measured attitude). - Kernels with the orientation of the spacecraft solar arrays. They are created from the telemetry, by ESA SPICE Service. The file names for each of these kernels start by VEX_SA_. - Kernels that represent the orientation of the moving parts of an instrument (if the instrument has moving parts). These kernels are produced by the instrument Team, possibly with the support of ESA and NAIF. Their file name start by VEX_[INSTRUMENT_NAME], where [INSTRUMENT_NAME] is the name of the instrument. The contents and time span covered by any CK file may be easily determined using the 'ckbrief' utility program available in all copies of the SPICE Toolkit. Usage: %ckbrief This utility requires additionally a SCLK file and a LSK file; use the latest versions of these files available in this data set. Descriptive information about how/why/when an CK file was created is usually available in the 'comment area' of the file. This may be viewed using the 'commnt' utility program available in all the SPICE toolkits. Use the '-r' option to read the comments. Example: %commnt -r All binary CK files (*.BC) contained in this directory are little-endian (LTL-IEEE) binary files. Venus Express C-kernels contained on this data set ================================================== The following Venus Express C-kernels are provided in this data set: ATNV_T6_PYYMMDDhhmmss_xxxxx.BC Contains Venus Express predicted attitude information covering the time span starting at the date YYMMDDhhmmss. This product is a Type 6 CK kernel and requires at least the SPICE Toolkit N0065 to be used. This attitude information corresponds to the ESOC attitude file named: ATNV_FDLMMA_DAPYYMMDDhhmmss_xxxxx.VEX where xxxxx designates the version number. ATNV_MEASURED_YYYY_Vvv.BC Contains Venus Express measured attitude information covering the time span starting at the date YYMMDD and finishing at the date yymmdd. vv indicates the file version. VEX_SA_YYYY_Vvvvv.BC Provides measured orientation of the spacecraft Solar Arrays. Each file of this type stores available information for one year of the mission (indicated in the file YYYY of the file name). It can be that the file corresponding to the last year in the archive is not complete, if that file was archived before the year ended. VEX_ASPERA_SAF_YYMMDD_VV.BC Kernel describing the ASPERA scanner motion from the telemetry. It is provided by the ASPERA Team. VEX_BOOM_Vvv.BC kernel with the orientation of the MAG boom. VEX_VIRTIS_YYYY_Vvv.BC Contains the optical angles of the VIRTIS scanning mirror orientation, which correspond to the actual direction of pointing of each acquisition with respect to the instrument's boresight. Note that all the labels for the CK files in this data set have SPACECRAFT_CLOCK_START_COUNT and SPACECRAFT_CLOCK_STOP_COUNT set to N/A. The authors of the data set think that it is not appropriate to put a clock count value for a product that was generated on ground (from the Flight Dynamics products provided by ESOC). Note that several versions of the same kernel can be provided. In that case, the file with the highest version number is the most up to date, and the one that should be used. Kernel File Details =================== The most detailed description of the data in a binary CK file is provided in metadata included inside the comment area of the file. This information can be viewed using utility programs 'commnt' and 'spacit' included into the SPICE Toolkit. At least a basic knowledge of the SPICE system is needed in order to use these kernels. The SPICE Toolkit provides versions in Fortran (SPICELIB), C (CSPICE), IDL (Icy), Matlab (Mice), and Java (JNISpice) and the user can choose any one that suits him/her. The SPICE routine FURNSH can be used to load a kernel file into a SPICE-based application to make kernel data usable with SPICE APIs. In the case when two or more binary CK files contain data overlapping in time for a given object the file loaded last takes precedence. Please note that CKs that contain 'T6' in their filename indicate that they are Type 6 CKs as opposed to type 5. T6 CKs contain the same data as their non-T6 counterparts but they have precedence due to computational efficiency (although they require Toolkit version N0065 or higher).