RFM Files: Irregular Grid

.grd File

16APR22

Type
Input(s), mandatory if GRD Flag enabled without LUT Flag.

Description
Irregular grid for spectral calculations (MIPAS-specific format - the irregular spectral file is probably more convenient for other uses)

Filename
Specified in *GRD section of Driver Table

Structure
The file consists of a header section, then a set of data records containing 50 hexadecimal characters (ie 4 bits each) corresponding 200 points on the regular grid. A bit value 1 means 'use this point' and 0 means 'ignore'.

Thus a grid of NReg points will be represented by
NHex = Ceil ( NReg/4 ) hexadecimal characters, requiring
NRec = Ceil ( NReg/200 ) data records.

The last data record is shortened as required, and the last hex character padded with 0 bits.

Overall File Structure
File_Header_Section defines NReg, hence NRec and NHex
For iRec = 1, NRec
    Data_Section Containing up to 50 hex characters

File Header Section
!CMNT Comment record(s), starting with '!'
FNC Interpolation Function
NReg   NUse   Wno_Min   Wno_Del No.pts in Full, Irr. grids, Spectral axis
Alt_Min   Alt_Max Tangent altitude range for grid
Data Section
HEX(1:50) First 200 encoded grid points
HEX(51:100) Next 200 encoded grid points
...
HEX(*:NHex) Last encoded grid points

Dictionary
Field Type Description Units Range
!CMNT C80 Comment records first character is '!'
Alt_Max R Highest valid tangent altitude for grid km Alt_Min < Alt_Max
Alt_Min R Lowest valid tangent altitude for grid km Alt_Min < Alt_Max
FNC C3 Interpolation function 'lin' expected [1]
HEX CNHex Irregular grid encoded as hexadecimal string '0' : 'F'
NReg I No. pts in full (regular) grid NUseNReg [2]
NUse I No. pts used for irregular grid 1 < NUseNReg
Wno_Del R Wavenumber increment of full grid cm-1 0 ≤ Wno_Del
Wno_Min R Lower Wavenumber of full grid cm-1 0 ≤ Wno_Min
Type: I=Integer; Z=Hexadecimal; R=Real; Cn=character string, length n.

In principle, the FNC field (always lower case) may indicate any of the following interpolation methods (however, see [1])
FNCInterpolation Function
lin y = ax + b
qad y = ax2 + b + c
cub y = ax3 + bx2 +cx + d
1li y = 1 / ( ax + b )
1qa y = 1 / ( ax2 + b + c )
1cu y = 1 / ( ax3 + bx2 +cx + d )
1sq y = 1 / ( ax + b )2
lor y = 1 / ( ax2 + b )
lnl y = exp ( ax + b )
lnc y = exp ( ax3 + bx2 +cx + d )

Notes
  1. FNC: All current irregular grids use the lin function - simple linear interpolation.
  2. A negative value of NReg is interpreted as defining spectral ranges in GHz rather than cm-1

Example
iasi_0.25.grd (Irregular grid for IASI spectral range derived from 0.001 cm-1 regular grid)

Bugs
[none recent]