Troubleshooting Issues with ISDs

This document is a non-exhaustive collection of identified issues/concerns with the creation of a new ISD.

Missing Kernels/Information

There are a number of missions that fail to include certain information/corrections to their data within their kernels. ISIS handles this with the inclusion of a number of "instrument addendum kernels" (iak) within it's data. ALE, however, does not make use of such kernels, and so certain values may need to be provided. (Much of this is included as hard-coded values within the code itself until a better solution is found.) This may account for some discrepancy between sensor models created in USGSCSM and ISIS.

Ephemeris Time

ALE pulls the ephemeris times from the image labels. This does not, however, take light time into consideration, and therefore does not make correction for it.

Light Time Correction: When an image is taken, the sensor takes note of the time. As the speed of light is not instantaneous, it takes time for the light from the surface of the target to reach the sensor. This means that the time that the sensor marks as the beginning of the image is actually a small time after the light reflected off of the surface, essentially taking an image of the target's past and misrepresenting it's position and rotation. There is a default flag that exists in ISIS that takes light time correction into consideration when calculating positions and rotations. USGSCSM, however, does not yet have this capacity built-in (and may never?). This will account for a minor delta in position and rotation calculations between the two sensor models.