DATA_OBJECT_TYPE = SPICE_I-KERNEL MISSION_ID = 1 SPACECRAFT_NAME = GALILEO SPACECRAFT_NUMBER = 77 INSTRUMENT_ID = 36 PROCESS_TIME = 1990-11-12T12:00:00 SSI I-kernel =========================================================================== This file contains solid state imaging (SSI) subsystem instrument data for Earth I. The purpose of the SSI data contained in this file is to describe the geometric relationship between the SSI coordinate system (``line and sample'' coordinates) and the coordinate system of the GLL spacecraft structure on which the SSI instrument is mounted (scan platform x-y-z coordinates). In addition to this coordinate conversion information, a ``distortion model'' is supplied. This model is required in order to associate features in an image with their actual geometric direction vectors. Although this type of information is not required for Earth I, it is typical of the sort of information that is appropriate for the I-kernel. The data contained in this file, as well as the description of the SSI coordinate system, are from reference [1]. References -------------------------------------------------------- 1. ``SSI Distortion and Twist Models for NAIF,'' JPL IOM, by Ken Klaasen, 7/2/90. 2. ``Formula for SSI Geometric Distortion Correction,'' JPL IOM, by Ken Klaasen, 7/21/88. The two documents [1] and [2] comprise NAIF document number 202.0. 3. ``Preliminary I-kernel Specification,'' (NAIF document number 177.0), by Ian Underwood, 1/19/90. 4. KERNEL ``Required Reading'' file, released by NAIF. 5. Galileo document GLL-1-100. 6. ``Gallileo Attitude and Camera Models,'' JPL IOM 314-323 (NAIF document number 201.0), by W. M. Owen, 11/14/1983. 7. ``SSI Focal Length,'' JPL IOM by Ken Klaasen, 08/04/90. Implementation Notes -------------------------------------------------------- This file is used by the SPICE system as follows: programs that make use of the I-kernel must `load' the kernel, normally during program initialization. Loading the kernel associates data items with their names in a data structure called the `kernel pool'. The SPICELIB routine LDPOOL loads a kernel file into the pool as shown below. CALL LDPOOL ( 'GLL360001.TI' ) In order for a program or subroutine to extract data from the pool, the SPICELIB routine RTPOOL is used. See [4] for more details. Naming Conventions -------------------------------------------------------- All names referencing values in this I kernel start with the characters `INS' followed by the NAIF SSI instrument number ('-77' followed by the instrument subsystem reference number). This naming scheme is standard for all I-kernel files and was instituted to distinguish I-kernel data from other types of data that may also be loaded in the kernel pool. The remainder of the name is an underscore character, followed by the unique name of the data item. For example, the twist offset for the SSI camera is specified by INS-77036_TWIST_OFFSET The upper bound on the length of the name of any data item is 32 characters. If the same item is included in more then one file, or if the same item appears more than once within a single file, the latest value supersedes any earlier values. Definition of the SSI Coordinate System -------------------------------------------------------- The following discussion is from [1]. The SSI boresight is by definition parallel to the scan platform +L vector. The remaining angle needed to specify the SSI CCD raster orientation with respect to the scan platform coordinate system is usually denoted ``twist.'' The AACS group defines twist by a positive rotation about the +L vector. The twist angle is then defined as the angle from the platform +M vector to the SSI image line, where the positive image line direction is toward increasing sample number. The sketch below shows the scan platform coordinate system translated to the center of an SSI image. The +L vector is into the paper [or screen]. SSI image coordinates are defined in terms of line and sample coordinates with the origin (line 1, sample 1) in the upper left corner of an image, line number increasing downward, and sample number increasing on the right. The +L vector is placed at coordinate (400,400). Positive twist then indicates a clockwise rotation of the SSI CCD raster with respect to the scan platform coordinates. The platform +M vector lies nearly in the direction of increasing image sample number and fixed line number while the +N vector completes the platform right-handed coordinate system and lies nearly in the direction of increasing image line number and fixed sample number. [The actual twist angle is much smaller than that indicated in the sketch, in which the magnitude of the angle is exaggerated for clarity.] The current best estimate of SSI twist defined in this way is -0.002797905 rad based on a SCALPS run using data from the December, 1989, minical sequence. line 1, sample 1 line 1, sample 80 ------------------------------------ | | | | | . M | . | | . | | . \ | | line 400, sample 400 . | twist offset angle | . V | (not to scale) | o-----------------| | +L . | | (into the page) | | . | | | | . | | | | . | | | ------------------------------------ line 800, sample 1 N line 800, sample Data -------------------------------------------------------- Mounting alignment twist offset angle, in radians: \begindata INS-77036_TWIST_OFFSET = ( -0.0022797905 ) \begintext Line and sample bounds: \begindata INS-77036_MIN_LINE = ( 1.D0 ) INS-77036_MAX_LINE = ( 800.D0 ) INS-77036_MIN_SAMPLE = ( 1.D0 ) INS-77036_MAX_SAMPLE = ( 800.D0 ) \begintext Field of view center location (note that this item is a vector): \begindata INS-77036_FOV_CENTER = ( 400.D0 400.D0 ) \begintext Definition of the SSI Distortion Model -------------------------------------------------------- We quote from [2]: The theoretical distortion can be modeled to an accuracy of about 0.01 pixel using the following equation: 3 R - r = Ar where R = actual image distance from the center of the field in pixels r = ideal image distance from the center of the field in pixels -9 A = 6.58 x 10 for the theoretical distortion 1 pixel = 0.0006 in = 15.24 um in the horizontal or vertical directio in the focal plane Data -------------------------------------------------------- The pixel size is given in millimeters. \begindata INS-77036_DISTORTION_COEFF = ( 6.58D-9 ) INS-77036_PIXEL_SIZE = ( 15.24D-3 ) \begintext Cameral focal length, in millimeters. This is an estimate taken from reference [7]. The estimate is based data from the December, 1990 minical sequence. The original estimate, from reference [6], was 1500 mm. \begindata INS-77036_FOCAL_LENGTH = ( 1501.039D0 ) \begintext Date on which the above data was entered into this I-kernel: \begindata INS-77036_DATA_ENTRY_DATE = ( @8-AUG-1990 ) \begintext Version The version number is always positive and given in the form V.X. V and X are incremented by following these rules: -- V is incremented by 1 when there is a structural change to an I-kernel. A structural change to an instrument kernel would be adding or deleting data items or changing the number of values associated with an existing data item. A structural change would also be assigning different units to values or changing the name of a data item. -- X is incremented by 1 only if pre-existing values are changed. \begindata INS-77036_VERSION = ( 1.0 )