Pointing Jacobians are distinct from Jacobians with respect to elements
of the atmospheric profile (JAC Flag),
and are defined
by the tangent heights selected
in the *TAN section of the driver table
The spectra are calculated simply from the difference between the
nominal output spectra: linear difference for the lowest and highest altitudes,
gradient of a quadratic fit for any intermediate altitudes, so accuracy will
depend on the user-defined tangent height spacing
'Perturbation' units (defined by the parameter PTBLOS in
file ptbcon_dat.f90)
are 0.001 of the units in the *TAN
section, i.e., 0.001*km or 0.001*deg (if using *ELE instead
of *TAN)
Each nominal output spectral file,
e.g. rad_a08000.asc will then
have an associated pointing Jacobian file, e.g. rad_a08000_los.asc,
the string "_los" being inserted into the filename immediately
following the tangent height identification.