## .grd File | ||

## RFM Files: Irregular Spectral Grid | ## 13FEB24 |

**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 or
*SPC sections of the 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.

File Structure !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 For iRec = 1, **NRec**HEX(1:50) First 200 encoded grid points HEX(51:100) Next 200 encoded grid points ... etc. HEX(*:NHex) Last encoded grid points [eof] -
**Fields** -
Field Type Description Units Range !CMNT C80 Comment records (first character is '!') FNC C3 Interpolation function ^{[1]}`'lin'`NREG I No. pts in full (regular) grid ^{[2]}≥ NUSE NUSE I No. pts used for irregular grid 1 < : ≤ NREG WNO_MIN R Lower Wavenumber of full grid ^{[2]}cm ^{-1 }≥ 0 WNO_DEL R Wavenumber increment of full grid ^{[2]}cm ^{-1}≥ 0 ALT_MIN R Lowest valid tangent altitude for grid km ≤ ALT_MAX ALT_MAX R Highest valid tangent altitude for grid km ≥ ALT_MIN HEX(:) Z Irregular grid encoded as hexadecimal string ∈{ `0`,`1`...`F`}**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]})FNC Interpolation Function lin y = ax + b qad y = ax ^{2}+ b + ccub y = ax ^{3}+ bx^{2}+cx + d1li y = 1 / ( ax + b ) 1qa y = 1 / ( ax ^{2}+ b + c )1cu y = 1 / ( ax ^{3}+ bx^{2}+cx + d )1sq y = 1 / ( ax + b ) ^{2}lor y = 1 / ( ax ^{2}+ b )lnl y = exp ( ax + b ) lnc y = exp ( ax ^{3}+ bx^{2}+cx + d ) **Notes**- FNC:
All current irregular grids use the
lin function, i.e., simple linear interpolation. - A negative value of
NREG is interpreted
as defining spectral ranges in GHz rather than cm
^{-1}

- FNC:
All current irregular grids use the
**Example**- iasi_0.25.grd (Irregular grid
for IASI spectral range derived from 0.001 cm
^{-1}regular grid) **Python Reader**- read_grd.py
**Bugs**- [none recent]