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Experimental

The support for GOME-2 is experimental for the following reason

  • The logic for the geolocation need to be documented and validated. The different bands have different numbers of measurements per scan line but the scan line always has 32 points with coordinates.
  • A random test exampled showed weird radiances for band 2a.

GOME-2

The Global Ozone Monitoring Experiment-2 (GOME-2) is a nadir-scanning UV/visible spectrometer on the MetOp satellites. It measures Earth-backscattered radiance in the 240–790 nm wavelength range across six main spectral bands (1a, 1b, 2a, 2b, 3, 4) and four Polarisation Measurement Device (PMD) bands (pp, ps, swpp, swps). The Level 1B product contains calibrated radiance spectra and associated geolocation data.

GOME-2 data is available from the EUMETSAT Data Store.

Two format versions are supported:

  • V13 (NRT products): format_major_version 13, subclass version 6
  • V12 (FDR R3 reprocessed products): format_major_version 12, subclass version 5

The product contains up to four MDR subclasses with different binary layouts: Earthshine (subclass 6), Calibration (7), Sun (8) and Moon (9). Earthshine records carry the nadir-scan measurements and are always present. Sun and Calibration records appear in granules covering the once-per-day solar calibration sequence, and Moon records only during the ~monthly lunar calibration campaigns.

Opening a dataset

Opening a GOME-2 L1B product returns a root dataset where each MDR subclass present in the file is exposed as a group. The root itself only carries the global attributes.

using MetopDatasets
import CommonDataModel as CDM

ds = MetopDataset("GOME_xxx_1B_M01_20260303213859Z_cropped_10.nat")
println(ds.attrib["format_major_version"]) # 13
println(CDM.groupnames(ds)) # subclasses present, e.g. ["earthshine"]

ds_earthshine = CDM.group(ds, "earthshine")
println(ds_earthshine.dim["atrack"]) # number of scan lines

# Sun, Calibration and Moon data are read the same way when present:
if "sun" in CDM.groupnames(ds)
    ds_sun = CDM.group(ds, "sun")
    solar_irradiance = ds_sun["radiance_1a"][:, :, :]
end

close(ds) # closes the root dataset and all its groups

The examples below use the earthshine group:

ds = CDM.group(MetopDataset("GOME_xxx_1B_M01_20260303213859Z_cropped_10.nat"), "earthshine")

Geolocation

Latitude and longitude are extracted from the interleaved CENTRE field. Each scan line has 32 ground pixels. The likely correspond to the second dimension of the band 1b, 2a, 2b, 3 and 4 radiation. The exact use of the geolocation still have to be documented. Note that band 1a and the PMD bands does not have 32 spectra per scan line so there geo location does not match the latitude and longitude variables one to one.

lat = ds["latitude"][:, 1]  # 32 latitudes for first scan
lon = ds["longitude"][:, 1]  # 32 longitudes for first scan

println("Latitude range: ", extrema(lat))
println("Longitude range: ", extrema(lon))

Multiple geolocation fields are available in the dataset centre, corner, scan_centre, scan_corner and sub_satellite_point.

Spectral variables

The main observations in the product is the radiances and their wavelength. The first spectrum from band 1b can be loaded the following way:

wl = ds["wavelength_1b"][:, 1]   # wavelength grid (nm) for first scan
rad = ds["radiance_1b"][:, 1, 1] # radiance for first scan, first readout

Each band provides:

  • wavelength_{band} — wavelength grid in nm
  • radiance_{band} — calibrated or sun-normalized radiance
  • radiance_error_{band} — radiance error estimate
  • stokes_fraction_{band} — Stokes fraction (main bands 1a–4 only)
  • uncorrected_radiance_{band} — uncorrected radiance (PMD bands only)
  • uncorrected_radiance_error_{band} — uncorrected error (PMD bands only)
  • rec_length_{band} — number of spectral elements per record
  • num_recs_{band} — number of readout records per scan

Output selection mode

The radiance units depend on the OUTPUT_SELECTION field in the product. The spectral variables include attributes that report the mode:

rad_var = ds["radiance_1a"]
println(rad_var.attrib["output_selection_mode"])  # "0" or "1"
println(rad_var.attrib["units"])  # "photon s-1 cm-2 nm-1 sr-1" or "1"
  • Mode 0 (abs_rad): calibrated radiance in photon s-1 cm-2 nm-1 sr-1
  • Mode 1 (norm_rad): sun-normalized radiance (dimensionless)

Sun, Moon and Calibration groups

The non-Earthshine subclasses expose the same spectral variables (wavelength_{band}, radiance_{band}, ...) but the physical meaning of the radiance differs:

  • sun: calibrated solar irradiance (photon s-1 cm-2 nm-1)
  • moon: calibrated lunar radiance (photon s-1 cm-2 nm-1 sr-1). Moon records also carry lunar geometry variables such as lunar_azimuth, lunar_elevation, lunar_phase and lunar_fraction.
  • calibration: signal from the internal calibration sources (Dark/LED/WLS/SLS). The output_selection_mode attribute of the spectral variables reports the source derived from the per-record OBSERVATION_MODE field.

The synthesised latitude/longitude variables are only defined for the earthshine group since the other subclasses have no CENTRE field.

Auxiliary variables

The format also contains a range of auxiliary variables. Here are some examples.

sat_zenith = ds["sat_zenith"][:, :, :]  # (32, 3, n_records) — EFG triplets
solar_zenith = ds["solar_zenith"][:, :, :]
scanner_angle = ds["scanner_angle"][:, :]  # (65, n_records)

The EFG triplet dimensions represent points E (before), F (centre), and G (after) along the scan.

close(ds)