Measurement method and characteristics
What is the measurement principle of GOMOS?
The measurement method of GOMOS is the star occultation technique. This method is detailed in Chapter 1.
What is the geographical coverage for GOMOS measurements?
The geographical coverage of GOMOS measurements is constrained by the planning, specifying the star targets. Until early January 2005, the daily number of measurements was about 400. Since July 2005, it is about 280. See section 1.4 of this document for more details.
What is the spatial resolution of GOMOS measurements?
The spatial resolution of GOMOS measurements is constrained by the planning, specifying the star targets. Until early January 2005, the daily number of measurements was about 400. Since July 2005, it is about 280. See section 1.4 of this document for more details.
What is the vertical sampling and the vertical resolution of GOMOS measurements?
The vertical sampling varies from one occultation to the other, depending on the occultation geometry. During one occultation GOMOS measures the stellar light in 0.5 s integration time intervals. This corresponds in the worst case (occultation in the orbital plane) to an interval of 1.7 km of altitude projected at the limb. The vertical sampling is therefore 1.7 km in the case of an occultation in the orbital plane and better than 1.7 km for oblique occultations.
The limb viewing geometry, the point source nature of stars and the short measurement integration time lead to a good vertical resolution of the profiles retrieved from GOMOS measurements. Figures of the vertical resolution by species and by altitude range are given in Table 1.3 of Section 22.214.171.124 of this document.
What is the accuracy of GOMOS measurements? In what extent does it depend on the star characteristics and the geometry of the measurement?
The accuracy of GOMOS measurements varies from one occultation to the other. The key factors are the characteristics of the occulted star (visual magnitude and effective temperature), and some specificities of the occultation geometry. See section 1.4 of this document for more details.
Are there instrument anomalies impacting the frequency of measurements?
Since the launch of ENVISAT and the beginning of nominal operations for GOMOS, there has been two main periods of perturbations. Between May 2003 and July 2003, the progressive reduction of the instrument Field of View on the nominal side impacted the number of effective measurements which was lower than the planned one. Between January 2005 and August 2005, the nominal operations were stopped, due to the failure of the Elevation voice-coil system. Operations resumed after the implementation of a new mission scenario with a reduced azimuth field of view. With this new mission scenario, the average number of daily occultations has been reduced from about 400 to about 280.
More details are given in the Appendix B of this document.
Processing and generalities on products
What are the main steps of the processing?
The generation of scientific products from GOMOS measurements is obtained from successive processing steps.
The products of lowest level are the Level0 products. The only algorithms applied to generate the Level0 products from the raw data are the determination of the satellite position and the conversion of satellite binary time to UTC. The Level0 products are used as input for the processing of higher level.
The aim of the Level1b processing is to estimate a set of horizontal transmission functions in the UV-visible-near IR between 250 nm and 956 nm using data measured by the GOMOS spectrometers. There are two types of Level1b products: the geolocated and calibrated transmission spectra products (transmission products) and the geolocated and calibrated background spectra limb products (limb products).
The aim of the Level2 processing is to retrieve the vertical profiles of O3, NO2, NO3, O2, H2O and other trace gases profiles, the temperature profiles, the aerosol extinction coefficient and wavelength dependency parameters, and information about atmospheric turbulence, from the full atmospheric transmission spectra. There are three types of Level2 products: the products storing the profiles of temperature and atmospheric constituents; the residual extinction products; the products storing selected profiles processed in NRT for meteo users.
More details are given in sections 2.1.1 and 2.3.3 of this document.
How are organised the product files?
The Level 0 products (in occultation mode) are input products for the Level1 b processing. The Level 1b products are input products of the Level 2 processing. More details are given in section 2.1.1 of this document.
Has there been successive versions of the operational processor?
Details on the current version of the operational processor (IPF 5.00) are given in Appendix B of this document. This version has been activated in August 2006. The previous version was IPF 4.02. Measurements for dates between August 2002 and July 2006 have been reprocessed with the version of the processor currently in operation, so that all measurements since August 2002 are now available from the same updated processor version.
What is the typical size of the GOMOS products?
The typical figures of the size of the products are given in section 2.1.2 of this document.
Which data are available to the users? Where and how to get the GOMOS products?
GOMOS Level1b products are available on request. GOMOS Level2 products are disseminated to the users through dedicated ftp sites. Detailed information about the access to the data is given in section 3.1 of this document.
What is the naming convention of the GOMOS products?
The naming convention for the different level products is explained in section 2.2.1. It follows ESA specifications for ENVISAT products.
Where to get in the products the date and the location of the measurement?
The date, the latitude and the longitude of the tangent point are given for the first and the last measurements of the occultation in the SPH of the Level1b products and of the Level2 products (see table 2.8 of section 2.2.2 of this document). The geolocation ADS in the Level1b transmission product and the Level2 atmospheric and constituent product also provides the latitude and the longitude of the tangent point at the beginning and during the measurement, generally at half-measurement (see table 2.20 in section 126.96.36.199.1). The measurement starting date of the first DSR of the product (day, time) is also contained in the product name, following the convention described in section 2.2.1).
What is the structure and the content of the GOMOS products?
All products follow the same structure. They are organised in Product Headers and Datasets. The Main Product Header and the Specific Product Header provide information on the product. The Annotation Data Sets contain auxiliary data relevant to the product. The Measurement Data Sets contain measurements and/or processed data.
More details are given in sections 2.2.2 and 2.2.3 of this document.
How to handle and read the data stored in the products?
Access tools specific to ENVISAT products have been developed. They allow to extract some specific fields stored in the products, and to handle data sets for plotting. Three access tools (BEAT, ENVIVIEW, GOMOS products toolbox) are presented in section 3.4 of this document. An important specificity of BEAT tools and of GOMOS product toolbox should be noted. As described in the IODD reference document, and for some datasets in the Section 2.2.3 of this document, values of some data set records are stored after multiplication by a conversion factor. Both BEAT tools and programs from the GOMOS products toolbox handle those conversion factors and thus return the unconverted data values i.e. the values ready to be used for any analysis purposes. For instance, all standard deviation data set records are stored in (1.e-1)% in the products. The data values returned after ingestion by BEAT tools or from the GOMOS products toolbox programs are given in %.
In which product are stored the transmission spectra?
The full transmission spectra at different tangent heights are stored in the Transmission MDS of the Level 1b transmission products (see 188.8.131.52.1), for each measurement time of 0.5s. They are described as 'full' because it is the actually measured transmission, not corrected for refraction effects (dilution, scintillation, chromatic refraction) nor for variable PSF. They are obtained by dividing each measured spectrum (re-sampled on the wavelength pixel grid of the reference spectrum) by the reference star spectrum.
The transmission spectra corrected for the scintillation and dilution effects are stored in the Residual Extinction MDS of the Level2 residual extinction products (see section 184.108.40.206.2 of this document).
What is the reference star spectrum and in which product is it stored?
The reference star spectrum is computed by averaging several star spectra outside the atmosphere at the beginning of the occultation. The current nominal number of averaged spectra is ten. The two first spectra are not used for pointing instability reason. The reference star spectrum is used to calculate the transmission spectrum which is obtained by dividing each measured spectrum by the reference star spectrum.
The reference star spectrum is stored in the Reference star spectrum GADS of the Level 1b transmission product (see 220.127.116.11.1). It is given in electrons and must be converted into physical units (ph/s/cm2/nm) by multiplying the electron values by the conversion factor inferred from the radiometric sensitivity curve (star) provided as a LUT in the Level 1b transmission product.
How to build up the reference star spectrum from the values stored in the Reference star spectrum GADS of the Level 1b transmission product?
The spectra values stored in electrons in the Level 1b products must be converted into physical units (ph/s/cm2/nm) by using the radiometric sensitivity curve (star). This radiometric sensitivity curve is given for each occultation as a LUT in the occultation data GADS of the Level 1b transmission product (see section 18.104.22.168.2 of this document). The conversion factors are given by the DS Radiometric sensitivity curve, for a series of wavelength values given in the DS Abscissae of the radiometric sensitivity curve. The size of the curve 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 spectra in physical units are then obtained by multiplying the flux values in electrons by this conversion factor.
What are the background limb spectra and in which product are they stored?
The 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. It is computed from the signals measured in the upper and lower bands. It is stored in the Transmission MDS of the Level 1b transmission product (see section 22.214.171.124.1 of this document), for each measurement time of 0.5s.
The background limb spectra in upper and lower bands are stored in the Limb MDS of the Level 1b limb products (see section 126.96.36.199.2 of this document), for each measurement time of 0.5s. Both uncorrected spectra and corrected spectra from straylight and IR-vignetting effects are stored in this MDS.
The quantities stored for the background limb spectra are actually scaled dimensionless values. It is needed first to decode those values to transform them to fluxes in electrons, then to convert the flux values into physical units (ph/s/cm2/nm/sr) by multiplying the electron values by the conversion factor inferred from the radiometric sensitivity curve (background) provided as a LUT in the Level 1b products.
How to build up the background spectra from the quantities stored in the MDS of the Level 1b transmission and limb products?
Due to the high variation of the background spectra with altitude, the coding of these quantities in the products is dynamic and it uses a gain and an offset for each measurement. This gain and this offset are stored in the auxiliary data ADS of the Level 1b transmission product for the estimated central background (see section 188.8.131.52.1 of this document) and in the auxiliary data ADS of the Level 1b limb product for the upper and lower background spectra (see 184.108.40.206.2). The values stored must be decoded by applying:
background (in electrons) = offset + background code / gain.
The decoded spectra obtained in electrons must be converted then into physical units (ph/s/cm2/nm/sr) by using the radiometric sensitivity curve (background). This radiometric sensitivity curve is given for each occultation as a LUT in the occultation data GADS of the Level 1b transmission product and of the Level 1b limb product (see 220.127.116.11.1 and 18.104.22.168.2). The conversion factors are given by the DS Radiometric sensitivity curve, for a series of wavelength values given in the DS Abscissae of the radiometric sensitivity curve. The size of the curve 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 spectra in physical units are then obtained by multiplying the flux values in electrons by this conversion factor.
What is the apparent tangent altitude? Where is it stored?
Several altitude definitions are used for the processing of GOMOS measurements. The apparent altitude is the tangent altitude computed with a virtual straight ray directed toward the virtual star direction, while the tangent altitude is the one computed for the real ray path i.e. including refractive effects. The definition of these altitudes, along with the different ray geometries (with no atmospheric refraction effects or with atmospheric refraction effects) is illustrated in Figure A.1 of the Appendix A.
Star spectra are given for tangent altitude heights, while limb spectra are given for apparent altitude heights.
The apparent altitude of the central background is stored in the Geolocation ADS of the Level 1b transmission product (see section 22.214.171.124.1 of this document).
The altitude of the apparent tangent point is stored in the limb ADS of the Level 1b limb product (see section 126.96.36.199.2 of this document). Two values are actually stored for each DS record: the first one corresponds to the apparent altitude of the upper background band and the second one corresponds to the apparent altitude of the lower background band.
In all cases, the apparent altitude is given at half-measurement, for the centre of each band.
What are the main atmospheric product quantities of GOMOS?
The tangent line densities and the vertical profiles of local density are retrieved from the spectrometer measurements for several species: O3, NO2, NO3, air, O2, H2O. With the current operational IPF (IPF5.00), neutral density is actually no more vertically retrieved. The air local density, the error bar on the air local density, the vertical resolution for air, as well as the terms related to air in the covariance matrix for local densities after vertical inversion are all set to 0 in the Level 2 temperature and atmospheric constituent products generated with this IPF version. Also, OClO is not operationally retrieved, though datasets specific to OClO exist in the products (of which values are set to 0). Other atmospheric products include the spectral parameters of the extinction coefficients, and high resolution temperature and density profiles retrieved from the Fast-Photometer signals. Those quantities are all stored in the Level2 temperature and atmospheric constituent products (see section 188.8.131.52.1 of this document). Star light transmissions and limb data are stored in the Level1b transmission and the Level1b limb products respectively (see sections 184.108.40.206.1 and 220.127.116.11.2 ).
What is the altitude range of validity for the vertical profiles of local density?
The altitude range of validity for the vertical profiles of species local density, of aerosol extinction coefficient and of HRTP is given in section 1.4 . The impact of the star characteristics and of the occultation geometry on the data accuracy with respect to the altitude range is detailed in sections 3.3.1 and 3.3.2.
In which product are stored the tangent line densities of O3, NO2, NO3, O2, H2O?
The tangent line densities of O3, NO2, NO3, O2, H2O are stored in the Tangent line density of species MDS of the Level2 temperature and atmospheric constituent products (see section 18.104.22.168.1).
In which product are stored the vertical profiles of local density of O3, NO2, NO3, O2, H2O?
The vertical profiles of local density of O3, NO2, NO3, O2, H2O are stored in the Local species density MDS of the Level2 temperature and atmospheric constituent products (see section 22.214.171.124.1).
In which product are stored the spectral parameters of the aerosol extinction coefficient?
The spectral parameters of the aerosol extinction coefficient are stored in the Aerosols MDS of the Level2 temperature and atmospheric constituent products. The extinction coefficient at the reference wavelength and the other coefficients are given in this MDS. The spectral parameters of the tangent integrated extinction profile are also given. The reference wavelength is given in the Occultation data GADS of the Level 1b product ("Ref. wavelength for the ray tracing"), and in the SPH of the Level 2 temperature and atmospheric constituents product and of the residual extinction product. It is equal to 500 nm.
More details are presented in section 126.96.36.199.1 and Appendix A.
In which product is stored the HRTP (High Resolution Temperature Profile)?
The HRTP is stored in the High Resolution Temperature MDS of the Level2 temperature and atmospheric constituent product (see section 188.8.131.52.1). In the same MDS is also stored the High Resolution density profile. The output frequency of the High Resolution profiles is 40Hz, corresponding to 20 values for each spectrometer measurement of 0.5s. No High Resolution profile is calculated in bright limb condition.
What is the meaning of the quality flags and how to use them?
For several quantities, flag values are stored in Product Confidence Data (PCD). They provide indications on the configuration and the performance at several stages of the retrieval. The meaning of the possible PCD values for the different datasets of the products is detailed in Appendix A.
For the local density species MDS and the tangent line density species MDS stored in the Level2 products, the PCD provides information about the validity of the outputs of the spectral and the vertical inversion, for the following species and in the listed order: O3, NO2, NO3, air, O2, H2O, OClO. Only values of data sets with PCD equal to 0 (non-flagged values) should be used (see also section 3.3.3).
Why is it possible to get negative non-flagged values of local density and of tangent line density in the products?
In the current operational version of the processor (IPF 5.00), negative column densities and local densities are not systematically flagged anymore (i.e. their PCD value may be equal to 0). The quality flag provided in the PCD summary of products with this processor version is actually a processor flag, indicating if the retrieval has been successful or not. Negative values of line densities after the spectral inversion may be kept, and non-flagged negative values (i.e. with PCD value equal to 0) may be included in the vertical profiles. Filtering out all the negative non-flagged values may yield an artificial positive bias in statistical averages computed from a specific dataset. It is recommended instead to apply complementary selection criteria based on the "standard deviation" values stored in the products.
Where are stored the error bars on the species local density and tangent line density?
The error estimates of the density values are stored in the local species density MDS and in the tangent line species density MDS, in the Level2 temperature and atmospheric constituent products. They are given in % of the density value and correspond to 1 . The maximum error bar for the species local density and tangent line density is set to 6553.5. An empirical error estimate has been added after inversion to the error bar of O3 line densities, of NO3 line densities and of aerosol extinction coefficient to account for the effects of turbulence (uncorrected scintillation). No modelling error is included in the overall error budget.
Which criteria should be applied to select the best quality data?
The accuracy of the local density vertical profiles depends on the star characteristics and on the occultation geometry. Depending on the retrieved species and on the altitude range, it is recommended to apply criteria on the star effective temperature, on the star visual magnitude, on the occultation obliquity, and on the illumination condition. Those criteria are detailed in sections 3.3.1, 3.3.2, and 3.3.3 of this document.
Where are stored the star ID and its characteristics?
The star ID, along with the star visual magnitude and the star effective temperature, is stored in the SPH of the Level1b and of the Level 2 products.
In which products is stored the illumination condition of the occultation?
The illumination condition of the occultation is stored in the Summary Quality GADS of the Level 1b products and of the Level 2 atmospheric constituents products and residual extinction products (see sections 184.108.40.206 and 220.127.116.11). The meaning of the illumination condition is stated in table Table 3.1 . The illumination condition is not given in the meteo products; however, it may be determined by using the solar zenith angle of the tangent point, the solar zenith angle of the satellite, and the altitude of the tangent point given in the geolocation ADS, and by applying the requirements detailed in table Table 3.1
How is the occultation obliquity defined and where is it stored?
The occultation obliquity is defined as the angle between the motion of the line of sight (with respect to the atmosphere) and the direction of the Earth's centre. The altitude chosen to calculate the obliquity is fixed to 35 km for any occultation. For a purely vertical occultation (field-of-view inside the orbital plane), the obliquity is equal to 0°. For an occultation with a field-of-view outside the orbital plane, the obliquity takes larger values. The obliquity quantity is actually called "verticality" in the products: "verticality" values close to 0 correspond to occultations close to the vertical, while high values of the "verticality" correspond to oblique occultations
The verticality of occultation is stored in the Summary Quality GADS of the Level2 temperature and atmospheric constituent product and of the Level2 residual extinction product (see sections 18.104.22.168.1 and 22.214.171.124.2).
Where to get the orbit in the products?
The orbit of the satellite corresponding to the measurement is given in the product name, as described in the presentation of the product name convention (see section 2.2.1). It is also given in the MPH of the products (see section 2.2.2).