L1C File Structure

29AUG23 - v3.3: Add Grd_Typ, Alt_Offset, Alt_Trend, Alt_Quad and extra comment records
Contents

Overview

The MORSE retrieval can work with data from a variety of different instruments. However since each instrument has its own particular L1B file structure, a pre-processing stage, tailored to each L1B product, is required to convert the L1B files to a common format which can be read by MORSE: the L1C file.

Examples: hsdi.l1c, hiros.l1c

Structure

The L1C file has a three-level nested structure, looping over Scans (top level), Sweeps and Microwindows.

Scan
A set of measurements used for a singe retrieval. For Cubemap this will be an occultation event and the L1B/L1C files will contain a single Scan.
Sweep
All measurements within a scan acquired at a single time/viewing geometry. For HIROS this will be the sweep across all the HIROS microwindows. For HSDI this is the HSDI detector outputs at a particular time.
Microwindow
Sweep measurement subdivided into separate spectral domains. For HIROS these correspond directly to the three bands. For HSDI these are the measurements from one the 16 filters arranged as a single vector (of different tangent heights).

File Structure Overview
File_Header_Section Defines NScn, NSwp, Resln
for iScn = 1, NScn Loop over Scans
    Scan_Header_Record
    for iSwp = 1, NSwp Loop over Sweeps
        Sweep_Header_Section Defines NMic
        for iMic = 1, NMic Loop over Microwindows
            HIROS: Microwindow_Data_Section
            or HSDI: Filter_Data_Record

File Header Section
!CMNT Comment record(s) (optional)
Format_ID File format identifier
View_ID Resln Viewing geometry, spectral resolution
Instrument Satellite Instrument, Satellite ID
Nom_Date Julian_Day Nominal date, as yyyymmdd and day# since 1Jan2000
Orbit Time_Start Time_End Orbit#, and Event Start, End as hhmmss
NScn No. of limb scans (1 expected)
NSwp GrdTyp No.sweeps/measurement locations per scan, Grid type
Grd(1) Grd(2) ... Measurement grid altitudes (always high to low)
... Grd(NSwp)
Scan Header Section
iScn Scan# (1 expected)
Sweep Header Section
!CMNT Header text for next record
YMD   HMS   MSC   iScn   iSwp   Lat   Lon   LST   SZA   CldRad   CldIdx   Single record
!CMNT Header text for next record
NMic   Grd(iSwp)   Alt_Adj   Rad_Crv   Single record
!CMNT (HSDI only) Header text Filter Data Records
Microwindow Data Section
!CMNT Header text for next record
Mic_Lab   Mic_Npt   Mic_Min   Mic_Max   Mic_Noi   Alt_Offset   Alt_Trend   Alt_Quad   Single record
Tra(1)   Tra(2)   ...   As many records as required
...   Tra(Mic_Npt)
Filter Data Record
Flt_Lab   Alt_Rel   Tra_Flt   Flt_Noi   Mos_X   Mos_Y   Single record

Data Dictionary

Field Type Description Units Range
!CMNT String*80 Comment record first character is '!'
Alt_Adj Float Adjusted/Corrected altitude of sweep [1] km eg 58.4
Alt_Offset Float Altitude offset for microwindow [9] km −1 : +1 expected
Alt_Quad Float Altitude Quadratic Trend for microwindow [9] km −1 : +1 expected
Alt_Rel Float Altitude of meas. relative to boresight [2] km −15 : +15 expected
Alt_Trend Float Tan.Altitude drift during microwindow [9] km − 1 : +1 expected
CldIdx Float Cloud Index [3] 0.0 (not set)
CldRad Float Cloud Radiance [3] 0.0 (not set)
Flt_Lab String*8 Label of spectral filter channel eg 'HSDI_06'
Flt_Noi Float Filter channel noise estimate [4] eg 0.001
Format_ID Float L1C File Format identifier [5] 3.3 ≤ Format_ID
Grd Float(NSwp) Nominal geometric tangent point altitude of measurement boresight km 0 – 100 expected
GrdTyp String*3 L1C grid type 'GEO' expected
HMS Long Time of measurement in UT hhmmss eg 120001
Instrument String*10 Instrument name eg 'HIROS     '
iScn Integer Scan counter within file [6] 1 : NScn (1 expected)
iSwp Integer Sweep# of measurement 1 : NSwp
Julian_Day Long Nom Date as Day# since 1 Jan 2000 (=0) eg 8401 (1st Jan 2023)
Lat Float Latitude of measurement deg N −90 : +90
Lon Float Longitude of measurement deg E −180 : +180
LST Float Local solar time hr 0.0 (not set)
Mic_Lab String Label identifying HIROS microwindow eg 'HIROS_A'
Mic_Min Double Microwindow lower wavenumber limit cm-1 eg 1135.20
Mic_Max Double Microwindow upper wavenumber limit cm-1 eg 1136.20
Mic_Noi Float Microwindow transmittance noise estimate dimensionless [4] eg 0.01
Mic_Npt Integer Number of spectral points for microwindow eg 1001
Mos_X Integer x-index of mosaic containing measurement [7] eg 1
Mos_Y Integer y-index of mosaic containing measurement [7] eg 1
MSC Long Time of measurement in UT ms eg 43201000
NMic Integer No. Microwindows for sweep or No. filter measurements for image eg 3
Nom_Date Long Nominal observation date yyyymmdd eg 20230101
NScn Integer No. vertical scans within file [6] 1 expected
NSwp Integer No. measurement locations per scan > 0
Orbit Long Satellite orbit number since launch > 0
Rad_Crv Double Earth Radius of Curvature in line-of-sight plane km 6300 – 6400 expected
Resln Float Spectral resolution (0 indicates spectral filters, ie HSDI) cm-1 0.001 expected (HIROS) or 0.0 (HSDI)
Satellite String*10 Satellite name eg 'Cubemap 1  '
SZA Float Solar Zenith Angle of measurement deg 90.0 expected
Time_End Long End of measurement UT hhmmss eg 120000
Time_Start Long Start of measurement UT hhmmss eg 120300
Tra Float(Mic_Npt) Microwindow transmittance spectrum dimensionless [8] eg 0.999955
Tra_Flt Float Filter transmittance measurement dimensionless [8] eg 0.998021
View_ID Integer Viewing geometry/measurement type 2 expected
YMD Long Date of measurement yyyymmdd eg 20230101

Notes

  1. Alt_Adj is the altitude assigned to the microwindows/image within L2. For Cubemap this will be the same as the measurement grid altitude GRD(iSwp) (this parameter allows for instruments which make multiple limb scans at a nominal set of tangent heights, eg MIPAS).

  2. For HSDI filters Alt_Rel is added to Alt_Adj to obtain the geometric tangent altitude of the filter measurement. For a given filter within a given mosaic (ie Flt_Lab, Mos_X, Mos_Y) this number is expected to be constant.

  3. CldIdx and CldRad are undefined here but, for MIPAS (and potentially HIROS), represent cloud-contamination tests.

  4. Flt_Noi, Mic_Noi noise estimates are expressed as uncertainties in transmittance, which has a range 0–1.

  5. Format_ID: the first non-comment field is the format identifier which makes it possible software to handle different versions of the L1C file structure.

  6. iScn, NScn: for Cubemap each file is expected to contain just a single occultation, corresponding to a single limb scan.

  7. Mos_X, Mos_Y: Mosaics within the full HSDI detector array are numbered from bottom (y=1) left (x=1).

  8. Tra, Tra_Flt transmittance measurements are dimensionless nominally in the range 0–1 although may lie outside this range once noise is added.

  9. Alt_Offset, Alt_Trend, Alt_Quad represent parameters of a quadratic fit to the microwindow tangent point motion as a function of time/wavenumber during acquisition. These are taken directly from the corresponding L1B parameters

Document Changes

29AUG23 v3.3: Add Grd_Typ, Alt_Offset, Alt_Trend, Alt_Quad and extra comment records
05OCT22 Original