*TAN Section  
RFM Driver Table Sections: Ray Path  31JUL23 
Field  Type  Section  Flags  Description  Units  Ext. Range  Int. Range 

 C200  (any)  (any)  Name of a Data File ^{[1]}  
Circular Geometry (Limbviewing)  
R  Actual tangent height ^{[2]}  km  BOA : TOA  BOA : HgtObs^{[5]}  
R  Projected tangent height ^{[3]}  km  −RadCrv^{[4]} : TOA  −RadCrv^{[4]} : HgtObs^{[5]}  
 R  OBS^{[5]}  Elevation angle^{[6]}  deg  −90 : TOA  −90 : +90  
Plane Parallel Geometry  
 R   NAD/ZEN  sec(zenith angle)^{[7]}^{[8]}  ≥1.0  ≥1.0  
 R    Elevation angle^{[6]}^{[8]}  deg  −0.1 : −90  −0.1 : −90 
 Elevation angle^{[6]}^{[8]}  deg  +0.1 : +90  +0.1 : +90  
 R   FLX  Output level^{[9]}  km  BOA : TOA  n/a 
Homogeneous Paths  
 R   HOM  Path length^{[11]}^{[13]}  km  >0  n/a

UNITS=  C2   HOM  Units for path length^{[12]}  ∈ {'km', 'm', 'cm','mm'} 
χ = k ρ swhere ρ is the absorber density and s is the path length. The aim is to set ρs =1 mole/m^{2}, so that, numerically, χ=k.
Expanding ρ= vp/(RT), at temperature T=296 K and standard atmospheric pressure p, assuming a volume mixing ratio of v=1 ppmv (keep the concentration low to return airbroadened k rather than selfbroadened)
s = (1 mole m^{2} ) ( R T )/( v p )
= (1 mole m^{2} ) ( 8.3145 J mole^{1} K^{1} × 296 K) / ( 10^{6} × 1013.25 Pa )
= 2.4289 × 10^{6} m = 2428.9 km
General Comments
*TAN paths.tan ! list of tangent heights from a standard scan pattern. 40 41 42 ! additional paths at 40, 41, 42 km,
*SEC 1 ! Direct nadir view only
*ELE 90, 1.5, 0.5, 0.0, 0.5, 1.5 ! [deg] nadir, plus views towards limb
*LEN UNITS=cm 50.3 ! Output filenames will be, eg tra_50300.asc