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    24-Jul-2014
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2.2.3 Product content

We provide here a general description of the headers and of the data sets stored in the products. We also present in more detail some of the data sets of the Level 1b products and Level 2 products. The exhaustive presentation of the content of the GOMOS products is given in the IODD reference document (see reference in Chapter 4 of this document). A more detailed definition of the terms used in the following sections is available in the Appendix A of this document.

2.2.3.1 Level 0 products

The Level 0 products contain only raw data, which are:

  • UV-visible spectrometer data
  • IR spectrometer data
  • Photometer 1 data
  • Photometer 2 data
  • Occultation recording time
  • Photometer recording time
  • Auxiliary data (e.g. satellite location, mirror position)

They store the data corresponding to a full orbit.

2.2.3.1.1 Level 0 nominal products (GOM NL 0P)

The Level 0 nominal products contain the GOMOS source packets in occultation mode for a full orbit (time ordered Annotated Instrument Source Packets recording the occultation measurements of the GOMOS instrument). They are generated from raw data and they are produced systematically when the instrument is in occultation mode. They are the basis for all higher level processing.

The structure of the Level 0 nominal products is detailed in Table 2.9 .

Field

MPH

SPH

Field

Data set

Data set name

MDS

GOMOS source packets

GOMOS_SOURCE_PACKETS

Table 2.9: Content of the Level 0 monitoring products GOM_NL_0P.

2.2.3.1.2 Level 0 monitoring products (GOM MM 0P)

The Level 0 monitoring products contain the GOMOS source packets in monitoring mode for a full orbit (time ordered AISP which hold data acquired while the instrument is in self-calibration monitoring mode). They are used for the validation and the calibration of the instrument.

There are three modes in which GOMOS is not acquiring stellar occultation data (monitoring mode), but is acquiring data used to establish operating parameters, and to set up look-up tables which are used in subsequent GOMOS data processing. These modes are:

  • Linearity Monitoring Mode,
  • Uniformity Monitoring Mode, and
  • Spatial Spread Monitoring Mode.

The structure of the Level 0 monitoring products is given in Table 2.10 .

Field

MPH

SPH

Field

Data set

Data set name

MDS

GOMOS source packets

GOMOS_SOURCE_PACKETS

Table 2.10: Content of the Level 0 monitoring products GOM_MM_0P.

2.2.3.2 Level 1b products

2.2.3.2.1 Transmission spectra products (GOM TRA 1P)

Those products are the main Level 1b products. They are the basis for further Level 2 processing. The transmission spectra products contain the geolocated and calibrated data, mainly the full transmission (computed as the ratio of the estimated star spectrum to the reference spectrum of the current occultation, without any correction for scintillations and dilution), and the covariance spectra needed by the Level 2 processing. The full transmission spectra show the perturbation of the star spectra due to the presence of the atmospheric constituents. The covariance spectra give an estimation of the errors due to both instrument measurements and Level 1b processing tasks.

Those products also contain a copy or a reference to the auxiliary data, a reference to the algorithms used to generate the product, the datation of the measurements and of the processing, the product confidence indicators at product level and at data level.

Each Level 1b transmission spectra product contains the data corresponding to a whole occultation.

The structure of the geolocated and calibrated transmission spectra products is detailed in Table 2.11 .

 

Field

MPH

SPH

Field

Data set

Data set name

GADS

Summary quality

TRA_SUMMARY_QUALITY

GADS

Occultation data

TRA_OCCULTATION_DATA

GADS

Nominal wavelength assignment

TRA_Nom_wav_assignment

GADS

Reference star spectrum

TRA_Ref_star_spectrum

GADS

Reference atmospheric density profile

TRA_Ref_Atm_Dens_profile

MDS

Transmission

TRA_Transmission

MDS

SATU data and SFA angles

TRA_SATU_and_SFA_DATA

ADS

Auxiliary data

TRA_Auxiliary_data

ADS

Geolocation

TRA_Geolocation

Table 2.11: Content of the transmission spectra products GOM_TRA_1P; list of GADS, MDS and ADS.

Summary quality GADS

         This GADS includes the observation illumination condition "PCD_ILLUM" (see table 3.2 in section 3.3.2.1), as well as other PCD at occultation level.

Occultation data GADS

This GADS stores among other quantities the DS related to the radiometric sensitivity curves (star and background), needed to convert the spectra (star and limb respectively) provided in electrons into physical units.

The radiometric sensitivity curve is given for each occultation as a LUT of conversion factors ("Radiometric sensitivity curve") for a series of wavelength values ("Abscissae of the radiometric sensitivity curve"), which size is given by the DS "Size of the radiometric sensitivity curve". A linear interpolation of the conversion factor is needed to use this curve for any sample of the spectra.

The occultation data GADS is detailed in Table 2.12 .

Occultation data GADS

Unit

Number of points of the spectra

Dl

Number of photometer output data per measurement

Dl

Number of SATU output data per measurement

Dl

Photometers central wavelength

(1.e-1)nm

Spectrometer effective sampling time

S

Effective time shift for ray tracing/geolocation

S

Ref. wavelength for the ray tracing

(1.e-1)nm

Size of the radiometric sensitivity curve (background)

Dl

Abscissae of the radiometric sensitivity curve (background)

(1.e-3)nm

Radiometric sensitivity curve (background)

Lf per e

Size of the radiometric sensitivity curve (star)

S

Abscissae of the radiometric sensitivity curve (star)

(1.e-3)nm

Radiometric sensitivity curve (star)

Sf per e

Thermistor temperature (SP)

(1.e-2)K

Thermistor temperature (FP)

(1.e-2)K

Dark charge used for the spectrometer dark charge correction

E

Mean spectrometer dark charge (3 bands)

E

Mean photometer dark charge

E

Offset between thermistor and CCD arrays temperature

(1.e-2)K

Sun coordinates in the geocentric equatorial inertial system

Dl

Table 2.12: Occultation data GADS in the Level 1b transmission product.

Nominal wavelength assignment

This is the nominal wavelength of the centre of each pixel, valid for the whole occultation.

Reference star spectrum

It is obtained by averaging several star spectra measured outside the atmosphere at the beginning of the occultation; the averaging is made to minimise the noise. It is given in electrons and must be converted into physical units (ph/s/cm2/nm) by multiplying the flux values in electrons by the conversion factor inferred from using the radiometric sensitivity curve (star) provided as a LUT in the occultation data GADS (see the description of the Occultation data GADS).

Reference atmospheric density profile

This profile is extracted from a meteorological field analysis (ECMWF) completed by MSIS90 at higher altitudes (at levels of pressure lower than 1hPa). It is used to compute during the Level 1b processing the refraction of the line-of-sight, by full ray-tracing computation.

Transmission

This MDS stores among other quantities the full transmission spectra and the covariance. The transmission spectrum is obtained by dividing each spectrum by the reference star spectrum. It is described as ' full ' because it is the actually measured transmission, not corrected for refraction effects (dilution, scintillation, chromatic refraction) nor for variable PSF. Each spectrum is re-sampled on the wavelength pixel grid of the reference spectrum to get the transmission.

The covariance (which is actually the variance of each pixel signal) is computed from the analysis of S/N ratio.

The scaled estimated central background is the estimated background contribution to the total signal in the central band, which is subtracted to yield the pure stellar signal. Due to the high variation of these spectra with altitude, the coding is dynamic and uses a gain and an offset for each measurement. This gain and this offset are stored in the auxiliary data ADS (see the description of the auxiliary data ADS). The values stored in the MDS must be decoded by applying: background (in electrons) = offset + background code / gain. The decoded spectra are obtained in electrons. They must be converted then into physical units (ph/s/cm2/nm/sr) by multiplying the flux values in electrons by the conversion factor inferred from the radiometric sensitivity curve (background) provided as a LUT in the occultation data GADS (see the description of the Occultation data GADS).

This MDS also stores the photometer data, scintillation data expressed in electrons.

The transmission MDS is detailed in Table 2.13 . It is made of several MDSR, one for each measurement time of 0.5s.

The maximum error bar for the estimated central background is set to 6500%.

Transmission MDS

Unit

Start time of the Data Set Record

mjd

Data Quality Indicator

-

Full transmission spectra

dl

Covariance function of the full transmission

dl

Scaled estimated central background

dl

Error bar for the estimated central background

(1.e-1)%

Photometers engineering data (FP1)

e

Photometers engineering data (FP2)

e

Error bar for the photometers engineering data (FP1)

(1.e-1)%

Error bar for the photometers engineering data (FP2)

(1.e-1)%

PCD at sample level (SP)

dl

PCD at sample level (FP)

dl

Table 2.13: Transmission MDS in the Level 1b transmission product.

SATU data

This is the position of the centroid of star image on the SATU CCD (Stellar Tracking Unit), which enables to know where to find each wavelength in the series of pixel.

Auxiliary data

This ADS contains a copy or a reference to the auxiliary product used during the processing. It stores among other quantities the offset and the gain needed for the decoding of the estimated central background contribution. The scaled estimated central background stored in the transmission MDS must be decoded by applying: background (in electrons) = offset + background code / gain.

The transmission ADS is detailed in Table 2.14 . It is made of several ADS records, one for each measurement time of 0.5s.

ADS

Unit

Start time of the measurement

mjd

Attachment flag

dl

Spectral shift of the star spectra

(1.e-4) nm

Offset for the background spectra coding

e

Gain for the background spectra coding

dl

PCD at measurement level

dl

Table 2.14: Auxiliary data ADS in the Level 1b transmission product.

Geolocation

This includes both the position of ENVISAT spacecraft and the position of the tangent point of the line-of-sight. All the geolocation information is provided for the reference wavelength used for the ray tracing (equal to 500 nm). For example the localisation of the ray nodes and of the tangent point node are provided for this wavelength although at one given time inside the atmosphere, and due to chromatic refraction effects, each wavelength is looking at a different altitude.

This ADS also stores the apparent altitude of the central background, given at half-measurement, for the centre of the band.

2.2.3.2.2 Limb products (GOM LIM 1P)

The limb products are used for the calibration and the validation of the instrument. The limb products contain the geolocated and calibrated background spectra actually measured with the two external bands of CCD spectrometers. Those products also contain a copy or a reference to the auxiliary data, a reference to the algorithms used to generate the product, the datation of the measurements and of the processing, the product confidence indicators at product level and at data level.

Each limb product contains the data corresponding to a whole occultation.

The structure of the background spectra limb products is detailed in Table 2.15 .

Field

MPH

SPH

Field

Data set

Data set name

GADS

Summary quality

LIM_Summary_quality

GADS

Occultation data

LIM_Occultation_data

GADS

Nominal wavelength assignment

LIM_Nom_wav_assignment

MDS

Limb

Lim_MDS

ADS

Auxiliary data and geolocation ADS

Lim_ADS

Table 2.15: Content of the background limb spectra products GOM_LIM_1P; list of GADS, MDS and ADS.

All the products issued from the Level 1b processing are accompanied by error estimates. These estimates are calculated assuming that there are no modelling errors and that the error statistics follow a normal Gaussian distribution. The pixel data are assumed to be independent on each other. All correlations between pixels are ignored even if the Level 1b data processing may generate some.

Summary quality GADS

This GADS includes the observation illumination condition "PCD_ILLUM" (see table 3.2 in section 3.3.2.1), as well as other PCD at occultation level similar to the ones of the Level 1b transmission product.

Occultation data GADS

This GADS stores among other quantities the DS related to the radiometric sensitivity curves (background), needed to convert the limb spectra provided in electrons into physical units (ph/s/cm2/nm/sr).

The radiometric sensitivity curve is given for each occultation as a LUT of conversion factors ("Radiometric sensitivity curve") for a series of wavelength values ("Abscissae of the radiometric sensitivity curve"), which size is given by the DS "Size of the radiometric sensitivity curve". A linear interpolation of the conversion factor is needed to use this curve for any sample of the spectra.

The occultation data GADS is detailed in Table 2.16 .

Occultation data GADS

Unit

Number of points of the spectra

dl

Size of the radiometric sensitivity curve (background)

dl

Abscissae of the radiometric sensitivity curve (background)

(1.e-3)nm

Radiometric sensitivity curve (background)

lf per e

Spectrometer effective sampling time

s

Effective time shift for ray tracing/geolocation

s

Sun coordinates in the geocentric equatorial inertial system

dl

Table 2.16: Occultation data GADS in the Level 1b limb product.

Limb MDS

This MDS includes the background spectra from upper and lower bands. The uncorrected spectra and the spectra after straylight and IR-vignetting corrections are stored.

Due to the high variation of these spectra with altitude, the coding is dynamic and uses a gain and an offset for each measurement. This gain and this offset are stored in the auxiliary data ADS (see the description of the auxiliary data ADS). The values stored in the MDS must be decoded by applying: background (in electrons) = offset + background code / gain. The decoded spectra are obtained in electrons. They must be converted then into physical units (ph/s/cm2/nm/sr) by multiplying the flux values in electrons by the conversion factor inferred from the radiometric sensitivity curve (background) provided as a LUT in the occultation data GADS (see the description of the Occultation data GADS).

The MDS is detailed in Table 2.17 . It is made of several MDSR, one for each measurement time of 0.5s.

MDS

Unit

Start time of the Data Set Record

mjd

Data Quality Indicator

-

Scaled upper and lower background spectra before straylight correction

e

Scaled upper and lower background spectra after straylight and IR-vignetting corrections

e

Error bar for the upper and lower background spectra after straylight and IR-vignetting corrections

%

PCD at sample level

dl

Table 2.17: MDS in the Limb product.

The maximum error bar for the upper and lower background spectra is set to 255%.

ADS layout

This ADS stores among other quantities the offset and the gain for the background spectra coding. The scaled upper and lower background spectra stored in the limb MDS must be decoded by applying: background (in electrons) = offset + background code / gain.

The altitude of the apparent tangent point stores actually two values per DSR; the first one is given for the upper band and the second one is given for the lower band. In both cases, it is given at half-measurement for the centre of each band.

The limb ADS is detailed in Table 2.18 . It is made of several ADS records, one for each measurement time of 0.5s.

ADS

Unit

Start time of the Data Set Record

mjd

Attachment flag

dl

Offset for the background spectra coding

e

Gain for the background spectra coding

dl

Latitude of the spacecraft

(1.e-6) deg

Longitude of the spacecraft

(1.e-6) deg

Altitude of the spacecraft

(1.e-2) m

Latitude of the apparent tangent point

(1.e-6) deg

Longitude of the apparent tangent point

(1.e-6) deg

Altitude of the apparent tangent point

(1.e-2) m

Error on the latitude of the apparent tangent point

(1.e-7) deg

Error on the longitude of the apparent tangent point

(1.e-7) deg

Error on the altitude of the apparent tangent point

(1.e-3) m

Sun zenith angle at the spacecraft

deg

Sun zenith angle at the tangent point

deg

Sun azimuth angle at the tangent point

deg

PCD at measurement level

dl

Table 2.18: ADS in the limb product.

2.2.3.3 Level 2 products

2.2.3.3.1 Temperature and atmospheric constituent profiles (GOM NL 2P)

The Level 2 temperature and atmospheric constituent products are generated from Level 1b data. They contain the retrieved vertical profiles, the retrieved tangent line densities, the auxiliary data and reference of the algorithms used to generate the product, datation of the measurement and of the processing, product confidence indicators at product level and at data level.

A set of flags is raised at several stages of the Level 2 processing. These flags give information concerning the validity of the outputs of spectral inversion, vertical inversion, aerosol processing, turbulence processing, and GOMOS atmospheric profiles.

Each Level 2 temperature and atmospheric constituent product contains the data corresponding to a whole occultation.

The structure of the temperature and atmospheric constituent products is detailed in Table 2.19 .

Field

MPH

SPH

Field

Data set

Data set name

GADS

Summary Quality

NL_Summary_quality

MDS

Local density of species

NL_Local_species_density

MDS

Tangent line density of species

NL_Tangent_line_density

MDS

Aerosols

NL_Aerosols

MDS

High Resolution Temperature

NL_HIGH_RES_TEMPERATURE

ADS

Geolocation

NL_Geolocation

ADS

Accuracy estimation

NL_Accuracy_estimation

Table 2.19: Content of the temperature and atmospheric constituent products GOM_NL__2P; list of GADS, MDS and ADS.

Summary quality GADS

This GADS includes the observation illumination condition "PCD_ILLUM" (see table 3.2 in section 3.3.2.1), as well as other PCD at occultation level similar to the ones of the Level 1b transmission product. It also provides the "verticality" of the occultation (in °). Values of the "verticality" close to 0 correspond to occultations close to the vertical direction, while high values of the "verticality" correspond to oblique occultations.

Local density of species

The local species density MDS is detailed in Table 2.20 . It is made of several MDSR, one for each processed measurement during the Level 2 processing.

Local species density MDS

Unit

Start time of the Data Set Record

mjd

Data Quality Indicator

-

Local O3 density

cm-3

Standard deviation for the local O3 density

(1.e-1)%

Vertical resolution for the local O3 density

m

Local NO2 density

cm-3

Standard deviation for the local NO2 density

(1.e-1)%

Vertical resolution for the local NO2 density

m

Local NO3 density

cm-3

Standard deviation for the local NO3 density

(1.e-1)%

Vertical resolution for the local NO3 density

m

Local air density

cm-3

Standard deviation for the local air density

(1.e-1)%

Vertical resolution for the local air density

m

Local O2 density

cm-3

Standard deviation for the local O2 density

(1.e-1)%

Vertical resolution for the local O2 density

m

Local H2O density

cm-3

Standard deviation for the local H2O density

(1.e-1)%

Vertical resolution for the local H2O density

m

Local OClO density

cm-3

Standard deviation for the local OClO density

(1.e-1)%

Vertical resolution for the local OClO density

m

PCD summary

dl

Table 2.20: Local species density MDS in the Level 2 temperature and atmospheric constituents product.

The vertical profiles of the local density of O3, NO2, NO3, O2, H2O and air are the main outputs of the vertical inversion of line densities,  assuming local spherical symmetry. OClO is actually not retrieved with the operational processor; the corresponding local density and standard deviation are set to 0.

In the current operational IPF version (IPF5.00), the vertical inversion on air is not activated, and the local density values, the standard deviation and the vertical resolution for this species are set to 0 in the products (as well as the terms related to air in the covariance matrix for local densities after the vertical inversion).

The standard deviation DSR stores the error estimates for the different species. Errors are estimated throughout the processing chain and are propagated along the chain to the final data products. It is assumed that the error statistics follow a normal Gaussian distribution. Values are expressed in % of the local density and correspond to 1σ. The maximum value of the error estimate is set to 6553.5%.

The PCD summary contains flags dedicated to the validity of the outputs of the vertical inversion i.e. the retrieval of the local density profiles. After vertical inversion, local densities are checked for flag setting. There is one flag per acquisition and per species: parameter PCDV(i,j), where index j denotes the acquisition number, and index i denotes the species among the following species list in this order: O3, NO2, NO3, air, O2, H2O, OClO.


Tangent line density of species

The tangent line density of species MDS is detailed in Table 2.21 . It is made of several MDSR, one for each processed measurement during the Level 2 processing.

Tangent line density of species MDS

Unit

Start time of the Data Set Record

mjd

Data Quality Indicator

-

Tangent line density for O3

cm-2

Standard deviation for O3 tangent line density

(1.e-1)%

Tangent line density for NO2

cm-2

Standard deviation for NO2 tangent line density

(1.e-1)%

Tangent line density for NO3

cm-2

Standard deviation for NO3 tangent line density

(1.e-1)%

Tangent line density for air

cm-2

Standard deviation for air tangent line density

(1.e-1)%

Tangent line density for O2

cm-2

Standard deviation for O2 tangent line density

(1.e-1)%

Tangent line density for H2O

cm-2

Standard deviation for H2O tangent line density

(1.e-1)%

Tangent line density for OClO

cm-2

Standard deviation for OClO tangent line density

(1.e-1)%

Number of iterations in the spectral inversion

dl

PCD summary

dl

Table 2.21: Tangent line density of species MDS in the Level 2 temperature and atmospheric constituents product.

Aerosols

The aerosols MDS is detailed in Table 2.22 . It is made of several MDSR, one for each processed measurement during the Level 2 processing.

Aerosols MDS

Unit

Start time of the Data Set Record

mjd

Data Quality Indicator

-

Extinction coefficient at

km-1

Standard deviation of the extinction
coefficient at

(1.e-1)%

Spectral parameters of the extinction coefficients

km-1,

nm-1.km-1, nm-2.km-1, …

Standard deviation of the spectral parameters of the extinction coefficients

(1.e-1)%

Tangent integrated extinction profile at

dl

Standard deviation of the tangent integrated extinction profile at

(1.e-1)%

Spectral parameters of tangent integrated extinction profile

cm-2, cm-2.nm-1, cm-2.nm-2, …

Standard deviation of the spectral parameters of the tangent integrated extinction profile

(1.e-1)%

PCD summary

dl

Table 2.22: Aerosols MDS in the Level 2 temperature and atmospheric constituents product.

The wavelength dependence of aerosol extinction may vary, even over one occultation. A polynomial expression is used to describe this wavelength dependence of the aerosol extinction; for a polynomial in degree n:

β(z,λ) = d0(z) + d1(z)(λ - λref) + d2(z)(λ - λref)2 + ... + dn(z)(λ - λref)n

For each measured transmission, it is calculated:

  • the extinction coefficient at the reference wavelength λref, corresponding to d0, given in km-1
  • the other spectral parameters corresponding to the coefficients d1(z) to dn(z) in the polynomial expression, given in nm-1.km-1, …, nm-n.km-1

The reference wavelength value is stored in the SPH of the product ("Reference wavelength used for the ray tracing"); it is equal to 500nm. It is also given in the SPH of the residual extinction product and in the Occultation data GADS of the Level 1b product.

The extinction coefficient at λ=λref is stored twice in the MDS. It is given in the dedicated MDSR "Extinction coefficient at λ=λref" and as the first spectral parameter of the MDS "Spectral parameters of the extinction coefficients". In the current operational processor (IPF 5.00), a polynomial in degree 2 is implemented, so that two other coefficients are given in the MDS "Spectral parameters of the extinction coefficients": d1(z) and d2(z) in nm-1.km-1 and in nm-2.km-1 respectively.

The wavelength dependence of the aerosol optical thickness may be described as:

τ(λ) = σref (r0 + r1(λ - λref) + r2(λ - λref)2 + ... + rn(λ - λref)n)

The MDS Tangent integration extinction profile at λ= λrefn stores the value τ calculated at λ= λref (dimensionless). The MDS "Spectral parameters of tangent integrated extinction profile" stores the coefficients r0 to rn, retrieved by the spectral inversion processing: r0 is the integrated retrieved density column of aerosols given in cm-2; the other parameters r1 to rn are given in cm-2.nm-1, … cm-2.nm-n. In the case of a polynomial in degree 2 as in the current operational processor (IPF 5.00), three coefficients are thus stored: r0, r1 and r2.

 

The standard deviation DSR stores the error estimates. Values are expressed in % and correspond to 1σ. An empirical error estimate has been added after inversion to the aerosol extinction coefficient to account for the effects of turbulence (incomplete scintillation correction).

The PCD summary provides the spectral and vertical PCD (first and seventh values) of the extinction coefficient at the reference wavelength, the other values are set to 0.

High Resolution Temperature Product

The High Resolution Temperature MDS is detailed in Table 2.23 . It is made of several MDSR, one for each processed measurement during the Level 2 processing.

High Resolution Temperature MDS

Unit

Start time of the Data Set Record

mjd

Data Quality Indicator

-

Tangent altitude including fluctuations

m

High resolution temperature profile

(1.e-2)K

High resolution density profile

cm-3

Error bar of the high resolution temperature profile

(1.e-1)%

Error bar of the high resolution density profile

(1.e-1)%

Table 2.23: High Resolution Temperature MDS in the Level 2 temperature and constituents product.

High resolution vertical profiles of temperature and of local density are computed from the analysis of the Fast Photometers signals.

They are derived from the time delay between the peaks of red and blue photometers, if this time delay is significant enough to be analysed.

The maximum error bar for the High Resolution profiles is set to 6500%.

There is no high resolution temperature profile computed in bright limb condition. The output frequency of the HRTP is 40Hz. This corresponds to 20 values for each processed measurement during the Level 2 processing (frequency 2Hz).

Geolocation and Atmospheric profile

Geolocation data include the position of the spacecraft and of the tangent point, and the tangent point pressure and temperature from the external model (ECMWF completed by MSIS90 at pressure levels lower than 1hPa). All the geolocation information is provided for the reference wavelength used for the ray tracing (equal to 500 nm) and stored in the SPH of the product. For example the localisation of the ray nodes and of the tangent point node are provided for this wavelength although at one given time inside the atmosphere, and due to chromatic refraction effects, each wavelength is looking at a different altitude.

The geolocation ADS is detailed in Table 2.24 . It is made of several ADSR, one for each processed measurement during the Level 2 processing.

Geolocation ADS

Unit

Start time of the Data Set Record

mjd

Attachment flag

dl

Latitude of the spacecraft (*)

(1.e-6) deg

Longitude of the spacecraft (*)

(1.e-6) deg

Altitude of the spacecraft (*)

(1.e-2) m

Latitude of the tangent point (*)

(1.e-6) deg

Longitude of the tangent point (*)

(1.e-6) deg

Altitude of the tangent point (*)

(1.e-2) m

Error on the latitude of the tangent point (*)

(1.e-7) deg

Error on the longitude of the tangent point (*)

(1.e-7) deg

Error on the altitude of the tangent point (*)

(1.e-3) m

Instrument pointing direction (azimuth) (*)

(1.e-6) deg

Instrument pointing direction (elevation) (*)

(1.e-6) deg

Tangent point atmospheric pressure (from external model)

Pa

Tangent point temperature (from external model)

K

Tangent point density (from external model)

cm-3

Local air density from GOMOS atmospheric profile

cm-3

Standard deviation for the local air density

(1.e-1)%

Local temperature

K

Standard deviation for the local temperature

(1.e-1)%

PCD summary

dl

Sun zenith angle at the spacecraft (*)

deg

Sun zenith angle at the tangent point (*)

deg

Sun azimuth angle at the tangent point (*)

deg

Table 2.24:  Geolocation ADS in the Level 2 temperature and atmospheric constituents product; (*) variables are provided at the beginning and during the measurements, generally at half measurement (the temporal shift is given by the time shift for ray tracing/geolocation stored in the Summary Quality GADS).

 

The local air density from GOMOS atmospheric profile and the local temperature are not provided in the geolocation ADS products with the current operational version of the processor (IPF 5.00). They are set to 0 and the corresponding standard deviations are set to the maximum value 6553.5.

Accuracy estimation

The accuracy estimation ADS is detailed in Table 2.25 . It is made of several ADSR, one for each processed measurement during the Level 2 processing.

Accuracy estimation ADS

Unit

Start time of the Data Set Record

mjd

Attachment flag

dl

c2 final value

dl

Scale factor for the elements of the covariance matrix
(spectral inversion)

dl

Covariance matrix for local densities after spectral inversion

cm-4

Scale factor for the elements of the covariance matrix
(vertical inversion)

dl

Covariance matrix for local densities after vertical inversion

cm-6

Table 2.25: Accuracy estimation ADS in the Level 2 temperature and atmospheric constituents product.

For the spectral inversion, the χ2 (final best fit) is given.

The covariance matrix for the spectral inversion provides information on the influence of each gas on the others. It is a 12 x 12 matrix whose terms are in the following order: O3, NO2, NO3, air, OClO, aerosols (a maximum of 6 parameters) and a spare gas. This matrix is symmetrical and only the relevant part (78 terms) is written in the product.

The covariance matrix for the vertical inversion provides information on the influence of each altitude on the other. It is symmetrical. 12 species are given in the following order: O3, NO2, NO3, air, O2, H2O, OClO, aerosols and 4 spare gases. Only the diagonal terms and 6 off-diagonal terms are given in the product, as a 12 x 7 matrix for each acquisition. The scale factor for the elements of the covariance matrix DS gives the power of 10 to be applied to the covariance matrix elements read in the product in order to interpret them (same value for all species):

V = Vp x 10fact

where:

V: the covariance matrix element calculated in the Level 2 processing

Vp: the scaled covariance matrix element read in the product

fact: the scale factor read in the product.

More details on the storage of the elements of the covariance matrix are given in Appendix A of this document.

2.2.3.3.2 Residual extinction products (GOM EXT 2P)

The residual extinction products are used for instrument calibration and validation purposes.

They also contain information needed to build a high resolution aerosol product.

The residual extinction products are generated by the GOMOS Level 2 processing. They contain the spectral transmission corrected for scintillation and dilution effects.

Each residual extinction product contains the data corresponding to a whole occultation.

The structure of the residual extinction products is detailed in Table 2.26 .

Field

MPH

SPH

Field

Data set

Data set name

GADS

Summary quality

EXT_Summary_quality

GADS

Nominal wavelength assignment

EXT_Nom_wav_assignment

MDS

Residual extinction MDS

EXT_MDS

ADS

Residual extinction ADS

EXT_ADS

Table 2.26: Content of the residual extinction products GOM_EXT_2P; list of GADS, MDS and ADS.

Summary quality GADS

This GADS includes the observation illumination condition "PCD_ILLUM" (see table 3.2 in section 3.3.2.1), as well as other PCD at occultation level similar to the ones of the Level 1b transmission product. It also provides the "verticality" of the occultation (in °). Values of the verticality close to 0 correspond to occultations close to the vertical direction, while high values of the "verticality" correspond to oblique occultations.

 

Residual extinction MDS

The residual extinction MDS is detailed in Table 2.27 . It is made of several MDSR, one for each processed measurement during the Level 2 processing.

Residual extinction MDS

Unit

Start time of the Data Set Record

mjd

Attachment flag

-

Transmission corrected for scintillation and dilution effects

dl

Covariance function of the transmission after scintillation
 and dilution corrections

dl

Transmission model function

dl

Flags for transmission model

dl

Table 2.27: Residual extinction MDS in the residual extinction product.

They contain the spectral transmission corrected for scintillation and dilution effects, along with the result of a forward model of the transmission with the "best fit" values of the parameters. This may allow to verify that the difference Tmes - Tmod is really randomly distributed in wavelength (non-random features may contain some information). These products could be used to compare the outputs of GOMOS processing with the outputs of other off-line processing.

2.2.3.3.3 Meteo products (GOM RR 2P)

These products contain selected vertical profiles processed in NRT. They are distributed mainly to meteo users. The primary application is NRT global atmospheric modelling and monitoring.

The processing is identical to the standard processing, but the atmospheric model used is based on meteorological forecast for pressure levels up to 1 hPa (if available) and MSIS90 model above. If the meteorological forecast is not available, data from the MSIS90 model are used from the bottom of the atmosphere. For the processing of off-line products, the atmospheric model is based on the meteorological analysis up to 1 hPa, and on MSIS90 above. Thus, the available orbit data may be less precise than for off-line processing, and some external data may be sub-optimal. But in all cases, the most accurate orbit at the time of processing is used.

Table 2.28 indicates the source of external atmospheric data depending on the end product and on the availability of ECMWF data.

End product

Meteorological data

Climatological model

NRT

ECMWF 24h forecast if available

   

MSIS90

 

 if not available:

most recent ECMWF data

 
   

if none available: MSIS90

Off-line processing

ECMWF analysis if available

   

MSIS90

 

 if not available:

most recent ECMWF data

 
   

if none available: MSIS90

Table 2.28: Possible sources of external atmospheric data for the processing of NRT and off-line products.

Each meteo product contains the extracted profiles corresponding to a whole occultation, with the structure detailed in Table 2.29 .

Field

MPH

SPH

Field

Data set

Data set name

MDS

Local species density

RR_Local_species_density

ADS

Geolocation

RR_GEOLOCATION

MDS

High Resolution Temperature

RR_HIGH_RES_TEMPERATURE

Table 2.29: Content of the meteo products GOM_RR__2P; list of GADS, MDS and ADS.

Local species density

The local species density MDS is detailed in Table 2.30 . It is made of several MDSR, one for each processed measurement during the Level 2 processing.

Local species density MDS

Unit

Start time of the Data Set Record

mjd

Data Quality Indicator

-

Local O3 density

cm-3

Standard deviation for the local O3 density

(1.e-1)%

Vertical resolution for the local O3 density

m

Local NO2 density

cm-3

Standard deviation for the local NO2 density

(1.e-1)%

Vertical resolution for the local NO2 density

m

Local NO3 density

cm-3

Standard deviation for the local NO3 density

(1.e-1)%

Vertical resolution for the local NO3 density

m

Local air density

cm-3

Standard deviation for the local air density

(1.e-1)%

Vertical resolution for the local air density

m

Local O2 density

cm-3

Standard deviation for the local O2 density

(1.e-1)%

Vertical resolution for the local O2 density

m

Local H2O density

cm-3

Standard deviation for the local H2O density

(1.e-1)%

Vertical resolution for the local H2O density

m

Local OClO density

cm-3

Standard deviation for the local OClO density

(1.e-1)%

Vertical resolution for the local OClO density

m

PCD summary

dl

Table 2.30: Local species density MDS in the meteo product.

OClO is actually not retrieved with the operational processor; the corresponding local density and standard deviation are set to 0.

In the current operational IPF version (IPF5.00), the vertical inversion on air is not activated, and the local density values, the standard deviation and the vertical resolution for this species are set to 0 in the products (as well as the terms related to air in the covariance matrix for local densities after the vertical inversion).

The standard deviation stores the error estimates for the different species. Errors are estimated throughout the processing chain and are propagated along the chain to the final data products. It is assumed that the error statistics follow a normal Gaussian distribution. Values are expressed in % of the local density and correspond to 1σ. The maximum value of the error estimate is set to 6553.5%.

The PCD summary contains flags dedicated to the validity of the outputs of the vertical inversion i.e. the retrieval of the local density profiles. After vertical inversion, local densities are checked for flag setting. There is one flag per acquisition and per species: parameter PCDV(i,j), where index j denotes the acquisition number, and index i denotes the species among the following species list in this order: O3, NO2, NO3, air, O2, H2O, OClO.

Geolocation

The geolocation ADS is detailed in Table 2.31 . It is made of several MDSR, one for each processed measurement during the Level 2 processing. All the geolocation information is provided for the reference wavelength used for the ray tracing (equal to 500 nm). For example the localisation of the ray nodes and of the tangent point node are provided for this wavelength although at one given time inside the atmosphere, and due to chromatic refraction effects, each wavelength is looking at a different altitude.

 

Geolocation ADS

Unit

Start time of the Data Set Record

mjd

Attachment flag

dl

Latitude of the spacecraft (*)

(1.e-6) deg

Longitude of the spacecraft (*)

(1.e-6) deg

Altitude of the spacecraft (*)

(1.e-2) m

Latitude of the tangent point (*)

(1.e-6) deg

Longitude of the tangent point (*)

(1.e-6) deg

Altitude of the tangent point (*)

(1.e-2) m

Error on the latitude of the tangent point (*)

(1.e-7) deg

Error on the longitude of the tangent point (*)

(1.e-7) deg

Error on the altitude of the tangent point (*)

(1.e-3) m

Instrument pointing direction (azimuth) (*)

(1.e-6) deg

Instrument pointing direction (elevation) (*)

(1.e-6) deg

Tangent point atmospheric pressure (from external model)

Pa

Tangent point temperature (from external model)

K

Tangent point density (from external model)

cm-3

Local air density from GOMOS atmospheric profile

cm-3

Standard deviation for the local air density

(1.e-1)%

Local temperature

K

Standard deviation for the local temperature

(1.e-1)%

PCD summary

dl

Sun zenith angle at the spacecraft (*)

deg

Sun zenith angle at the tangent point (*)

deg

Sun azimuth angle at the tangent point (*)

deg

Table 2.31: Geolocation ADS in the meteo product;  (*) variables are provided at the beginning and during the measurements, generally at half measurement.

The local air density from GOMOS atmospheric profile and the local temperature are not provided in the geolocation ADS products with the current operational version of the processor (IPF 5.00). They are set to 0 and the corresponding standard deviations are set to the maximum value 6553.5.

The illumination condition is not provided in the meteo product; however, it may be determined from the altitude of the tangent point, the sun zenith angle at the spacecraft, and the sun zenith angle at the tangent point stored in the geolocation ADS, by applying requirements for the different illumination conditions as defined in Table 3.2 in section 3.3.2.1.

High Resolution Temperature

The High Resolution Temperature is detailed in Table 2.32 . It is made of several MDSR, one for each processed measurement during the Level 2 processing.

High Resolution Temperature MDS

Unit

Start time of the Data Set Record

mjd

Data Quality Indicator

-

Tangent altitude including fluctuations

m

High resolution temperature profile

(1.e-2)K

High resolution density profile

cm-3

Error bar of the high resolution temperature profile

(1.e-1)%

Error bar of the high resolution density profile

(1.e-1)%

Table 2.32: High Resolution Temperature MDS in the meteo product.

The maximum error bar for the High Resolution profiles is set to 6500%.

There is no high resolution temperature profile computed in bright limb condition.

The output frequency of the HRTP is 40Hz. This corresponds to 20 values for each processed measurement during the Level 2 processing (frequency 2Hz).

2.2.3.4 Auxiliary products

2.2.3.4.1 Calibration database (GOM CAL AX)

The content of this auxiliary product is given in Table 2.33 .

Field

MPH

SPH

Field

Data set

Data set name

GADS

General

CAL_General

GADS

Bad pixel map

CAL_Bad_pixel

GADS

Non-linearity functions

CAL_Non_linearity

GADS

Fast photometer dark charge maps

CAL_FP_Dark_charge

GADS

Fast photometer non-uniformity maps

CAL_FP_PRNU

GADS

Fast photometer straylight

CAL_Sun_straylight

GADS

Instrument noise

CAL_Instrument_noise

MDS

Spectrometer dark charge

CAL_SP_Dark_charge

MDS

Spectrometer non-uniformity maps

CAL_Sp_PRNU

MDS

External sun straylight maps

CAL_Sun_straylight

MDS

External Earth straylight maps

CAL_Earth_straylight

Table 2.33: Content of the calibration database auxiliary product GOM_CAL_AX; list of GADS, MDS and ADS.

2.2.3.4.2 Star catalogue (GOM CAT AX)

The content of this auxiliary product is given in Table 2.34 .

Field

MPH

SPH

Field

Data set

Data set name

GADS

General

CAT_General

GADS

Annotation

CAT_Annotation

MDS

Star catalogue file

CAT_STAR_INFORMATION

Table 2.34: Content of the star catalogue auxiliary product GOM_CAT_AX; list of GADS, MDS and ADS.

2.2.3.4.3 Cross section database (GOM CRS AX)

The content of this auxiliary product is given in Table 2.35 .

Field

MPH

SPH

Field

Data set

Data set name

GADS

O3 cross-sections (SPA)

CRS_O3_cross_sect_SPA_GADS

GADS

O3 cross-sections (SPB)

CRS_O3_cross_sect_SPB_GADS

GADS

NO2 cross-sections

CRS_NO2_cross_secT_GADS

GADS

NO3 cross-sections

CRS_NO3_cross_secT_GADS

GADS

OClO cross-sections

CRS_OCLO_cross_secT_GADS

GADS

O2 cross-sections

CRS_O2_cross_secT_GADS

GADS

H2O cross-sections

CRS_H2O_cross_secT_GADS

MDS

O3 cross-sections (SPA)

CRS_O3_cross_sect_SPA_MDS_

MDS

O3 cross-sections (SPB)

CRS_O3_cross_sect_SPB_MDS

MDS

NO2 cross-sections

CRS_NO2_cross_secT_MDS

MDS

NO3 cross-sections

CRS_NO3_cross_secT_MDS

MDS

OClO cross-sections

CRS_OCLO_cross_secT_MDS

MDS

O2 cross-sections

CRS_O2_cross_secT_MDS

MDS

H2O cross-sections

CRS_H2O_cross_secT_MDS

Table 2.35: Content of the cross-section database product GOM_CRS_AX; list of GADS, MDS and ADS.

2.2.3.4.4 Instrument physical characteristics data (GOM INS AX)

The content of this auxiliary product is given in Table 2.36 .

Field

MPH

SPH

Field

Data set

Data set name

GADS

General

INS_General

GADS

Static spectral PSF GADS for spectrometer A

INS_SPA_spectral_PSF

GADS

Static spectral PSF GADS for spectrometer B

INS_SPB_spectral_PSF

GADS

Static spatial PSF GADS for spectrometer A

INS_SPA_spatial_PSF

GADS

Static spatial PSF GADS for spectrometer B

INS_SPB_spatial_PSF

Table 2.36: Content of the instrumental physical characteristics data product GOM_INS_AX; list of GADS, MDS and ADS.

2.2.3.4.5 Level 1b processing configuration database (GOM PR1 AX)

The content of this auxiliary product is given in Table 2.37 .

Field

MPH

SPH

Field

Data set

Data set name

GADS

General

PR1_General

GADS

Atmosphere

PR1_Atmosphere

Table 2.37: Content of the Level 1b processing configuration database auxiliary product GOM_PR1_AX; list of GADS, MDS and ADS.

2.2.3.4.6 Level 2 processing configuration database (GOM PR2 AX)

The content of this auxiliary product is given in Table 2.38 .

Field

MPH

SPH

Field

Data set

Data set name

GADS

Atmosphere

PR2_Atmosphere

GADS

Parameters

PR2_General

GADS

Convergence criteria

PR2_Conv_criteria_GADS

MDS

Convergence criteria

PR2_Conv_criteria_MDS

MDS

Reference line densities

PR2_Ref_line_densities

MDS

Group of species (initialisation phase)

PR2_Group_of_species_init

MDS

Group of species

PR2_Group_of_species

MDS

Spectral windows (initialisation phase)

PR2_Spectral_windows_init

MDS

Spectral windows

PR2_Spectral_windows

Table 2.38: Content of the Level 2 processing configuration database auxiliary product GOM_PR2_AX; list of GADS, MDS and ADS.

2.2.3.4.7 Stellar Spectra databank (GOM STS AX)

The content of this auxiliary product is given in Table 2.39 .

Field

MPH

SPH

Field

Data set

Data set name

GADS

General

STS_General

GADS

Annotation

STS_Annotation

GADS

Wavelength assignment

STS_Wav_assignment

MDS

Star information

STS_Star_information

MDS

Star spectrum

STS_Star_spectrum

Table 2.39: Content of the stellar spectra auxiliary product GOM_STS_AX; list of GADS, MDS and ADS.