GetSpots identifies those times, listed in sclkfile, when the boresight of instrumenttype intersects the surface of Bennu less than distance milliradians from the center of individual facets in the OBJ file described in objfile. Needed spice files for this calculation are listed in spicemetakernel.
- At the time a three-dimensional map is made, a designated DSK file will be used to create an OBJ file that act as a framework on which a map will be made. (This is done with the utility mkobj.) The OBJ file identifies a set of vertices (in body centered Cartesian coordinates) and groups these vertices to identify facets. Facet numbers correspond to their ordinal position in the list of facet identifications. An ancillary file locates the center of each facet in latitude (deg), longitude (deg), and range (km) from center of figure.
Mapmakers may wish to assign a value to a facet based on an algorithmic combination of one or more observations that are in the proximity of the facet. In order to do this, they need to understand which observations are candidates for their analysis. Knowing the distance and position angle of the observation relative to the center of the facet is the discriminator for determining which observations are candidates, and GetSpots provides this information.
- For all observations taken by OSIRIS-REx, the unique value of a counter on the spacecraft when the observation was made is known. This is called the sclk ("sklock") value. SPICE files provide spacecraft ephemeris and orientation, Bennu position, orientation, and shape, and instrument boresight and orientations. When combined with the sclk value, the intersection of the boresight on the surface of Bennu can be calculated.
A simple text file of sclk values (one per line) acts as input to getspots. GetSpots can determine which boresight values corresponding to the sclk values do not exceed the maximum distance criteria for each facet, along with the position angle of the spot, and the fraction of the facet covered by the spot. The position angle is valued from 0 to 359 degrees North being 0 with east being 90 degrees. Spots that do not completely intersect the surface of Bennu are flagged. Any additional information on the input line following the sclk value is echoed to the output unchanged, allowing other data useful in the subsequent analysis to be carried along in the output.