*TAN Section

RFM Driver Table Sections: Ray Path
25APR25
Type
Primary Section#6

Description
Definition of ray-path, which itself depends on the viewing geometry. The original RFM was for limb-viewing so the section was used to specify TANgent height, but now used as a generic section header for specifying some aspect of ray path which depends on viewing geometry.
For the default circular geometry (usually limb-viewing) the section label itself defines the contents of the section.
*TAN Refracted tangent height (as below)
*GEO Geometric tangent height
*ELE Elevation angle
For other geometries, defined by various flags, the user can use *TAN for this section marker (since it is always section#6 in the driver table) but the recommendation is to use the following geometry-specific headers for clarity:
*HGT Height levels for outputs Flags: FLX
*LEN Homogeneous path length Flags: HOM
*SEC Plane-parallel atmosphere airmass factor Flags: NAD or ZEN
*DIM Look-Up Table dimensions Flags: TAB

Format
Multiple fields, any order
Field Type Description Units Range
TAN R Refracted tangent height [2] km BOA:TOA[3]
TANFIL C200 Name of a Data File [1]
Type: R=Real; Cn=character string, length n.
BOA = Bottom of the Atmosphere; TOA = Top of the Atmosphere

Notes
  1. TANFIL The type of field is identified by first using the Fortran INQUIRE statement to see if it is a file (assumed to be a .dat file), in which case the file is opened and the numbers read from the file as if they were directly inserted within the section at that point. This can be used in combination with actual numerical fields.

  2. TAN Tangent height allowing for refraction, ie minimum altitude of the ray path. This altitude has to be within the atmosphere, with min/max allowed values set by the BOA (Bottom of the Atmosphere) and TOA (Top of the Atmosphere), representing the min/max values of the height profile specified in the *ATM section.

  3. With the observer within the atmosphere (OBS Flag) the highest tangent height is the observer altitude (HGTOBS), corresponding to a horizontal path (use *ELE if upward-viewing paths are also required)

  4. While refracted tangent paths (by definition) cannot intersect the surface, if FOV convolution is included (FOV Flag) it is is possible that some of the additional ray paths will intersect the surface, in which case SFC Flag will be required (you can always included the SFC flag anyway, just in case).

  5. Tangent heights must generally be distinct to 5 significant figures (1m below 100km, 1 km above 100km) so that they give distinct output filenames otherwise a 'Repeated Tan.Hgt' fatal error message results.

Example
Normal use as list of actual tangent heights for limb path calculations
*TAN paths.tan ! list of tangent heights from a standard scan pattern. 40 41 42 ! additional paths at 40, 41, 42 km,

Bugs
[None recent]