deltack can be used to correct most instances of problems with camera attitude. It adjusts camera orientation in the labels of a single Isis cube (resection). In order for deltack to work properly, the input cube must have been initialized with spiceinit using ATTACH="YES".

BUNDLE Method

Like jigsaw, deltack works on both framing cameras and line scanners using the BUNDLE method. The user must supply a latitude/longitude for at least one sample/line coordinate in the image. Angles 1 and 2 of the camera orientation stored in the cube labels will be adjusted to align the coordinate with the entered latitude/longitude. To solve for all three camera angles, select the TWIST option. A second coordinate point and desired latitude/longitude must be entered if the TWIST option is used. deltack will adjust the camera orientation to align both points with their corresponding latitudes/longitudes with minimal residuals in sample and line.

The camera pointing is updated using an iterative least-squares bundle adjustment. The number of iterations (MAXIT)and tolerance (SIGMA0) for convergence are input parameters to deltack. Solving for two angles only is similar to a translation of the image in sample and line. Solving for the twist as well will result in both a translation and a rotation.

deltack creates a set of two output files for analyzing the results. BUNDLEOUT.TXT provides an overall summary of the resection. It lists the user input parameters selected and tables of statistics for both the image and the point(s). RESIDUALS.CSV provides a table of the measured image coordinates and the final sample, line, and overall residuals in both millimeters and pixels.

DIRECT Method

The DIRECT method can be applied to cameras/instruments that are not supported by jigsaw or the viewing geometry conditions are not supported in BUNDLE. The DIRECT method is particularly useful for the Cassini VIMS (and other (spot) spectrometer type instruments) and for cases where the geometry is partially or completely off the target body. The DIRECT method primarily differs from the BUNDLE method in that it computes a rotation matrix directly from the SAMP1/LINE1 to the LAT1/LON1 surface intercept vectors from the spacecraft position reference in the J2000 coordinate system. Essentially, these are the look direction vectors from those camera coordinates in J2000 which eliminates any peculiarities within the camera coordinate system. And, most importantly, it does not require all sample/line coordinates to map to valid geometric coordinates. It does not, however, support the TWIST option or the two-point scenario as does the BUNDLE method.

This approach works in variable acquistion time instrument systems, such as line scan, pushbroom, pushframe and (spot) spectrometers (as well as framing camera instruments) because the rotation matrix is determined independent of spacecraft position. It also works for situations where the geometry is off the target body because the look vector is always properly set in the ISIS camera model even if it does not intersect the surface. Note, however, in the later case, the LAT1/LON1 position must intersect the target body to obtain a valid sample/line coordinate.

The DIRECT method was designed to correct the pointing for the Cassini VIMS spectrometer but it is designed to be generally applicable to all cameras supported in the ISIS system. The BUNDLE method does not work on the VIMS instrument and likely many other complex camera systems with peculiar geometric configurations, such as (spot) spectrometers. The recommended procedure to determine an update to VIMS pointing is to first determine the geometric coordinate of a commmon feature in a reference image, such as a Cassini ISS observation for example, where the same latitude/longitude coordinate maps to a line/sample in the VIMS image. The VIMS geometric coordinate is expected not match the feature in the ISS image but qview and/or campt must report a valid line/sample coordinate from the latitude/longitude coordinate. The next step is to choose the best sample/line coordinate in the VIMS image that best matches the feature in the ISS image. To correctly update the pointing in the VIMS image, provide the latitude/longitude coordinate from the ISS image in the LAT1/LON1 parameters and the line/sample coordinate of the feature in the VIMS image in the SAMP1/LINE1 parameters to deltack. You may make a copy of the original image prior to running deltack if desired.

A similar procedure to the one described for VIMS will also work to update the pointing of an image where the SPICE ephemeris data does not provide a valid latitude/longitude on the target body. The distinction is that the SAMP1/LINE1 coordinate need not intersect the body. Indeed, this is a common problem encountered typically with older data sets and generally with flyby images where the target is partially or fully in the field of view of the camera. There is an alternative technique for flyby images of this type that does not require a second reference image. The case where this technique is most effective is typically used for framing cameras where the center of the target body is in view and there is at least 1 pixel in the image that maps to the surface where geometric data can be computed. Using campt, provide the sample/line coordinate of any valid geometric point in the image - it does not have to visibly intersect the target body - any sample/line coordinate will suffice. campt reports the sub-spacecraft latitude/longitude coordinate for every valid geometric coordinate - it will always be the same coordinate for framing cameras. If the full disk of the target body is viewable and the image is relative free of noise/artifacts, the center program computes the centroid sample/line coordinate of the target body. If this condition is not met, use qview to manually approximate the center pixel coordinate of the body. Provide the centroid sample/line in the SAMP1/LINE1 coordinate and the SubSpacecraftLatitude/SubSpacecraftLongitude values computed by campt in the LAT1/LON1 parameter to deltack.