The value is calculated by first transforming the rest frequency (given by the RestFreq attribute) from the standard of rest of the source (given by the SourceVel and SourceVRF attributes) to the standard of rest of the observer (i.e. the topocentric standard of rest). The resulting topocentric frequency is assumed to be in the same sideband as the value given for the DSBCentre attribute (the "observed" sideband), and is transformed to the other sideband (the "image" sideband). The new frequency is converted back to the standard of rest of the source, and the resulting value is returned as the attribute value, in units of GHz.
KAPPA --- Kernel Application Package