Description of the Phoebe SPC Shape Model and Assessment Products bundle V1.0 
============================================================

Bundle Generation Date: 2023-09-29 
Peer Review: 2023_Asteroid_Review 
Discipline node: Small Bodies Node


Content description for the Phoebe SPC Shape Model and Assessment Products bundle
==============================================================

We provide the shape model, also known as a global Digital Terrain Model (DTM), of the Saturnian satellite Phoebe, along with assessment data. A Phoebe model was previously generated by Robert Gaskell (Gaskell 2020) using the Stereophotoclinometry (SPC) software suite and submitted to the PDS Small Bodies Node (urn:nasa:pds:gaskell.phoebe.shape-model::1.0). The shape model in this bundle has some minor processing performed by the author, but is relatively unchanged from the Gaskell model. Differences are noted later in this document.

The shape model is provided in a native Implicitly Connected Quadrilateral (ICQ) format, a United States Geological Survey (USGS) Integrated Software for Imagers and Spectrometers (ISIS) cube format, a geoTiff format which can only be read by GIS programs like ArcGIS, a plate vector format, and an Object Vector (OBJ) format readable by software such as the Small Body Mapping Tool (SBMT). To aid the data user, filenames end with a letter to designate the format. These letters are “i”, “c”, “g”, “p”, and “o”, respectively. We also provide quality assessment data for the shape model to include the maximum image resolution, number of images, best maplet resolution, and the sigma value. The quality data is presented in 1 degree bins using the USGS ISIS cube format. The assessment data can be used to understand which locations on the model have a robust radius, and which have a poor radius or even an unconstrained radius. Details about the products are given in document/productdescription.txt, while the assessment and thumbnails of the assessment data are given in document/phoebeshapeassessment.pdf. The shape assessment PDF is particularly important. We annotate the topography and albedo figures to show rough guidelines of where we think these values are reliable.

The model was constructed using the CASSINI Imaging Science Subsystem (ISS) Wide Angle Camera (WAC) and Narrow Angle Camera (NAC) images. Most images had two clear filters, but images were used irrespective of the filter. Non-calibrated images are used, but a correction for geometric distortion is used. ISS WAC and NAC images are mostly 12-bit, and 8-bit images are converted to 12-bit images using a lookup table.

The shape model is provided in two versions. One version is high-resolution with a 300 m Ground Sample Distance (GSD), and the other is low-resolution with a 1.195 km GSD since many software programs cannot read the high-resolution version. Internal checks allow us to estimate the uncertainty of the model, but this value should not be treated as a rigorous solution, rather as a soft solution. A good rule of thumb for the uncertainty is to base it off the best maplet GSD or the shape model GSD, whichever gives the poorer GSD (i.e larger GSD). The uncertainty is then one to two times this poorer GSD. This means that the high-resolution model, with a 300 m GSD, will have a minimum uncertainty of 300 to 600 m. Portions of the the high-resolution model will have a larger uncertainty, depending on the best maplet GSD (see document/phoebeshapeassessment.pdf). The low-resolution model, with a 1.195 km GSD, will have most of the surface with an uncertainty of 1.195 to 2.390 km GSD, with the Northern region having a slightly larger uncertainty due to lower resolution maplets.

The shape model in this bundle has the same images and SPICE kernels as the Gaskell model previously submitted to the PDS Small Bodies Node, but there are some differences. Minor processing was performed by the author to ensure model stability, which may have changed the heights, especially in regions with poor imagery, such as at the poles, that are less constrained. In addition, the coordinate system of the model was shifted 1.03 km so the center of the coordinate system used by SPC was the same as the Center of Figure of the model.


Caveats to the data user 
=================

The radius and relative albedo values of the model are less reliable near the poles. See the shape assessment (document/phoebeshapeassessment.pdf) for details.