| Description: The PDS3 name for this attribute was CSS:MIN_FOOTPRINT_SAMPLE (with MAP_PROJECTION_TYPE = RING_POLAR), and was defined as "Minimum size of footprints along the sample axis". The meaning of this is unclear, but we believe it means the length along the horizontal direction of the smallest footprint (a CIRS detector) on the pixel grid, and have renamed it in PDS4 accordingly as ringpolar_min_footprint_width. TThe ring polar, also known as ring radius/local time, projection is used for observations of Saturn's rings. The representation of ring data requires a unique projection. The rings are modeled by a thin disk centered on Saturn and in its equatorial plane, with an outer radius of 500,000km. If the field of view falls partially or completely beyond this limit or if it intersects the primary body before intersecting the rings, the data will not be included. If A is the location of the intersection of the CIRS field of view with Saturn’s equatorial plane, the sample coordinate is the distance of A from the center of Saturn (in units of km), and the line coordinate is the local time on Saturn at the intersection with Saturn’s surface of the line between A and Saturn’s center. Local time is expressed in fractional hours, from 0.0 (at midnight) to 12.0 (at noon), to 24.0 (at midnight). We have not generated point perspective projections for the ring observations because few ring observations were taken in a way that would make point perspective images useful. CAVEAT USOR: Several examples of CIRS cubes do not appear to correctly identify their min/max footprint width/heights. For example, some using equirectangular projection list their max footprint width as 360 deg, even when there is no wrapping. |