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3.3 Data selection

3.3.1 Illumination conditions

Information on the illumination condition is provided for each occultation, basing on the sun zenith and azimuth angles at both instrument and tangent point locations. Those angles are computed by the geolocation process, in the Level1b chain. The illumination condition is then determined according to the requirements given in Table 3.1.

Illumination condition

Flag index value ("PCD_ILLUM")


full dark


not in bright limb nor twilight nor straylight conditions



SZA at tangent point < 97° for at least one measurement with ALT at tangent point < 50 km

pure twilight


not in bright limb and SZA at tangent point < 110° for at least one measurement with ALT at tangent point < 100 km



not in bright limb and SZA of ENVISAT < 120° for at least one measurement



not in bright limb and both conditions for twilight and for stray light are verified.

Table 3.1: Requirements on the occultation geometry for the definition of illumination condition; SZA stands for solar zenith angle

The illumination condition is stored in the flag index "PCD_ILLUM" in the Level1b transmission and limb products and in the Level2 temperature and atmospheric constituent products of each occultation. It is not stored in the Level2 meteo product. 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 of the Level2 meteo product, and by applying the requirements detailed in Table 3.1.

The full dark measurements ("PCD_ILLUM" = 0) may be used with no restriction.

Occultations measured in bright limb illumination conditions ("PCD_ILLUM" = 1) are of quite bad quality. The bright limb is an extended source competing with the stellar signal and the accuracy of these measurements is strongly degraded because of their large noise.

Occultations measured in twilight illumination conditions ("PCD_ILLUM" = 2) are in a lesser extent of quite bad quality. They correspond to measurements made when the Sun is just below the horizon, and they are contaminated by solar light.

Occultations are measured in straylight illumination conditions ("PCD_ILLUM" = 3) if the instrument optics are illuminated by light (solar light scattered by the grating or by ENVISAT hardware, or coming from the sun-illuminated limb or nadir). Straylight measurements are actually considered as of good quality.

Some measurements may be characterized as in both straylight and twilight illumination condition ("PCD_ILLUM" = 4). It is not recommended to use them because of the solar light contamination.