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Chapter 5

5.1 Conventions and definitions

5.1.1 Instrument components and processing definitions


Wavelength assignment
This is the wavelength assignment of the centre of each pixel, for each transmission measurement (it may change during one occultation due to imperfect tracking of the star). The algorithms to retrieve the species concentration from the GOMOS measurements need a good accuracy of the spectral assignment of the star spectra samples.

Reference star spectrum
Several star spectra measured outside the atmosphere are used to computed an averaged spectrum (diminution of the noise). Inside the atmosphere, the star spectra are divided by this reference spectrum to obtain the transmission.

Reference atmospheric profile
A reference atmospheric profile is used to complete the GOMOS atmospheric profile (under and above the extreme altitudes covered by the GOMOS measurements) in the Level 2 processing.

Full transmission spectra and covariance
Main product of the Level 1b processing, the transmission and covariance spectra are computed from the star spectra measured by the GOMOS instrument, after several stages of corrections and conversions. The Level 2 processing will use these transmissions to retrieve the species concentrations.

Central background estimate and error
GOMOS measures the signal coming from both star and partly illuminated atmosphere. Two of the GOMOS bands (upper and lower bands) are used to compute an estimation of the background signal included in the central band. This quantity is subtracted from the band samples to get only the signal due to the star. The maximum error bar for the central background estimate is forced to 6500%.

Photometers data and error
Fast photometers are used to estimate the high frequency variations of the amplitude of the star photonic flux, especially in the low layers of the atmosphere. These variations may also be used in the level 1b processing during the flat-field correction of the star signal. They are also needed to correct the transmission from scintillation at the beginning of the level 2 processing. The time delay between the signal of the two photometers is also used to derive the High Resolution Temperature Profile.

SATU data
Signal coming from the SATU at a frequency of 100 Hz. This information is used to locate the star spectra on the CCD arrays during each spectrometer measurement. This information is crucial for the flat-field correction of the star spectra.

The SATU information is provided as deviation angles (deg) in both X and Y directions.

SFA angle measurements
These measurements provide information related to the pointing direction of the GOMOS telescope during the spectrometer measurements (at 10 Hz, i.e. 5 times per measurement). They are expressed in degrees.

Wavelength assignment of the spectra
Actually, this is the spectra shift between the effective spectral assignment of the transmission and covariance spectra written in the product and the nominal wavelength assignment. If the transmission spectra have been resampled on the nominal spectral grid, then all these vectors are equal to 0 (this is the default processing mode).

Geolocation and error
A good location of the tangent point is important to affect the species concentrations on the Earth. A specific algorithm included in the Level 1b processing is dedicated to this task. Longitude, latitude and altitude are provided.

Upper and Lower Background Spectra and error
Spectra measured by GOMOS at the same time as it measures the star spectra. These additional information could be used to estimate the dark charge at the beginning of the occultation (as this is only possible in full dark limb conditions, this function is not activated in the processor) and to estimate the contribution of the limb signal in the central/target band if the observation has been performed in straylight, twilight or bright limb conditions. The maximum error bar for the upper and lower background spectra is forced to 255%.

Bad pixel
A list of bad pixels is included in the Calibration auxiliary product. These pixels present a very high dark charge or a very poor quantum efficiency. In both cases, their S/N is poor. Any CCD column flagged for containing one bad pixel is not used in the GOMOS ground processing.

Dark charge
Electrons are generated in the GOMOS spectrometer CCD arrays due to thermal effects. These electrons must be estimated in order to correct the measured spectra. Several options are available in the ground processing, based on on-ground characterisation dark charge maps or on an estimation based on the first measurements of the occultations (see the dark charge correction step in the Level 1b processing). Since the Cal/Val phase, one specific occultation is performed every orbit, in full dark limb condition, the instrument looking at a so-called “dark sky area” (or DSA), where no input flux is expected at the entrance of the instrument. The measurements of this occultation are used to estimate a dark charge map, valid for all occultations of the same orbit.

Non uniformity
Each CCD pixel has its own radiometric sensitivity, due to physical variations from one pixel to the other (material, size...). As the star spectrum is moving in front of the CCD due to pointing instabilities, the response of the instrument depends on which pixels the star flux is falling. The estimation of the star movement combined with the knowledge of the pixel response non uniformity (PRNU) allows to correct the star spectra from this effect.

Illumination condition PCD
This is the actual illumination of the observation, computed from geometrical configuration between the Sun, the satellite and the GOMOS instrument pointing direction.

0: full dark limb condition

1: bright limb condition

2: pure twilight condition

3: straylight condition

4: twilight+straylight condition

This flag is not dependent on the dark/bright limb flag that is read in the Level 0 product; as this latter comes from the mission scenario and may not fully reflect the actual observation conditions.

Occultation obliquity
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. The obliquity varies in the same direction as the azimuth direction of the line-of-sight. For a purely vertical occultation with a field-of-view inside the orbital plane (azimuth value equal to 0°), the obliquity is equal to 0°. For an occultation with a field-of-view outside the orbital plane (larger azimuth values), the obliquity takes larger values. For a tangent occultation, the altitude of 35 km may be never reached, and in that case, the obliquity is arbitrarily fixed to 90°.

This quantity is actually called "verticality" in the products.

Apparent altitude
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 5.1 .

Star spectra are given for tangent altitude heights, while limb spectra are given for apparent altitude heights.


Depending on the final use of the data, different ray geometries, star directions and altitudes are defined for the geometry and the processing of GOMOS measurements.

The nominal star direction is the direction as it is outside the atmosphere,

The virtual star direction is affected by the refraction effects (so a few mdeg above the previous one),

The apparent altitude (see the Note on apparent altitude below), hb, is the tangent altitude computed with a virtual straight ray directed toward the virtual star direction,

The direct altitude is the tangent altitude of a straight ray directed toward the nominal star direction.

Except when specified, the tangent altitude, h0, is the one computed for the real ray path i.e. including refractive effects.

Figure 5.1: Ray geometries and illustration of the different altitudes.

Note on apparent altitude: In order to determine the apparent geolocation, (including in particular the apparent altitude hb (b=L,C,U) used in the central background estimation algorithms, and written in the limb product), the star displacement in the SATU and its relation with the spectrometers reference frames have to be taken into account in order to avoid wrong allocation of measurements.