RFM Driver Table Sections

*TAN Section: Tangent Heights (or other ray path definitions)

02JAN08
Type
Primary Section#6

Description
Contents of section defined according to section label and flags: (NB: 'limb-viewing' here refers to any mode using a full ray-trace calculation for a curved atmosphere, ie any mode not using the HOM, FLX, NAD, ZEN or TAB Flags).
*TAN can also be used as the default section label for all options unless *GEO or *ELE are specifically required since in the other cases the definition of this section's contents are determined by the Flags.

Format
Generally: Multiple fields, arbitrary order, except for
*DIM Section/TAB Flag: 6 fields, specific order for p,T axes for tabulated absorption coefficients
SectionFlagsFieldTypeDescriptionRange
(any) (any) FILTAN C*80 Name of a .tan file
*TAN TAN R*4 Actual (refracted) tangent heights [km] (Lowest:Highest) *HGT levels in first .atm file
*GEO GEO R*4 Projected (geometric) tangent heights [km] -(Radius of Curvature):(top *HGT level after refraction)
*ELE ELE R*4 Elevation angles [deg] relative to observer horizontal,
usually negative=viewing below the horizontal 		Observer outside atmosphere: -90:(top *HGT level after refraction)
Observer within atmosphere: -90:+90
ZEN Flag: 0.1:+90
NAD Flag: -90:-0.1
*SEC NAD or
ZEN 		SEC R*4 Airmass factors (=sec(zenith angle)) [dimensionless], for plane-parallel atmospheres >= 1.0 (1=vertical path)
*LEV FLX LEV R*4 Altitude levels [km] for flux calculation outputs. (Lowest:Highest) *HGT levels in first .atm file
*LEN HOM LEN R*4 Path lengths [km] for homogeneous path calculations. >0
*DIM TAB PRE1 R*4 Max/Min pressure [mb] for tabulation >0
" " PRE2 R*4 Min/Max pressure [mb] for tabulation >0
" " NPRE I No. points on tabulation pressure axis >0
" " TEM1 R*4 Max/Min temperature [K] for tabulation >0
" " TEM2 R*4 Min/Max temperature [K] for tabulation >0
" " NTEM I No. points on tabulation temperature axis >0

Notes
  1. The type of field is identified by first attempting to read it as a real number (eg TAN) then, if failing (or if first character is one of T,t,F,f), attempting to open it as a .tan file (FILTAN).
  2. For limb paths which intersect the surface (defined as the lowest altitude specified in the first *HGT profile in the *ATM Section rather than 0km), it is also necessary to enable the SFC Flag so that the surface parameters are specified. This is always mandatory for NAD and FLX (although not FLX+ZEN) geometries which, by definition, include surface terms . Although TAN values themselves must lie within the atmosphere it is possible that the FOV convolution causes the `surface' to be seen, in which case a fatal error results unless the SFC Flag is enabled.
  3. The *GEO Section label can be used to specify rays which intersect the surface (in which case the `refracted' tangent height is meaningless). The minimum geometric tangent height, given by to -(Radius of Curvature), corresponds to a nadir view.
  4. The *ELE Section label requires the OBS flag to be enabled since an observer altitude has to be specified in order to calculate the projected path. However, this is the only way to specify an `upward-viewing' path in a spherical atmosphere since tangent height, refracted or otherwise, has no meaning in this case.
  5. Tangent heights, elevation angles and path lengths must generally be distinct to 5 significant figures so that they give distinct output filenames (eg 1m below 100km, 0.001 deg elevation, but note: 1 km above 100km) otherwise a 'Repeated Tan.Hgt' fatal error message results (doesn't apply to *DIM/TAB Flag option).
  6. The use of *TAN, *ELE or *GEO section labels also has an impact on the interpretation of the data in the .fov file.
  7. With the GRA Flag enabled (limb-viewing with horizontal gradients), the location of the tangent point is either determined by the observer location PSI=PSIOBS (OBS Flag enabled) or taken at the reference profile, PSI=0.0 as defined in the *ATM section.
  8. Note that there are two different types of *LEV sections, the one here is a primary section and requires the FLX Flag to be enabled. The other definition of the *LEV section is a secondary section linked to the LEV Flag which generates intermediate output levels for a ray path calculation. The *FLX and LEV Flags are mutually exclusive so there should be no ambiguity in the interpretation of the *LEV section.
  9. With the *LEV/FLX option if LEV does not correspond to an existing atmospheric profile level a new level is inserted. This may not be desirable when also using the MTX option so a warning is issued in that case.
  10. For the *DIM/TAB option: in the generated .tab file, the starting point P1 for the -lnp axis is determined by -ln(PRE1/mb), whether this is the maximum or minimum pressure, but the sign of the -lnp increment DP will change. Similarly, T1 is set to TEM1 and the sign of the temperature increment DT is +ve if TEM2 > TEM1, otherwise negative.
  11. If only one point is selected for the pressure or temperature axes, then the corresponding PRE1,PRE2 or TEM1,TEM2 values must be equal.
  12. If either TEM1 or TEM2 (but not both) is negative this actually defines the temperature axes of the .tab file as relative to the atmospheric profile in the *ATM section. In this case, the PRE1 and PRE2 must also lie within the pressure range of the profile.

Examples
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,
Used as tabulation axes for generating a .tab file (TAB Flag) 
*DIM
       1000.0  0.01  31  ! 6 points for every factor 10 in pressure
       180 320 15        ! Increments of 10K in temperature
Nadir-viewing calculations for plane-parallel atmosphere (NAD Flag). 
*SEC
    1  ! Direct nadir view only
Set of elevation angles for a balloon observations (OBS Flag) 
*ELE
     -90,  -1.5, -0.5, 0.0, 0.5, 1.5  ! [deg] nadir, plus views towards limb

Bugs
Bug#2 (Fixed v4.02)
Bug#69 (Fixed v4.28)