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1.2.2 Measurement technique

The stellar occultation technique relies on the measurements of the series of spectra emitted by one star during its setting while the platform is moving along its orbit (Figure 1.3 ). The comparison between these spectra modified by the atmospheric constituents present along the line-of-sight between the source and the instrument and a reference star spectrum measured outside the atmosphere allows to infer the density of atmospheric constituents.

Figure 1.3: Geometry of the stellar occultation measurement by GOMOS.

First, the spectrum of this star is measured when the line of sight star-to-spacecraft is well above the atmosphere (typically 120 km), without absorption (reference stellar spectrum). Then, due to the orbiting motion of the spacecraft, as the line-of-sight goes deeper and deeper in the atmosphere, the star spectra measured by the instrument from the spacecraft contain absorption features depending on the composition of the atmosphere along the line-of-sight. The atmospheric transmission is obtained by dividing the star spectra inside of the atmosphere by the star spectrum outside of the atmosphere.

The atmospheric constituent densities are then obtained by retrieval of these transmissions. The line density or integrated quantity of ozone (and other constituents) along the line-of-sight is obtained at various altitudes z. The vertical distribution of the ozone local density (and other ozone constituents) is then retrieved from this series, assuming that the atmosphere is locally spherically symmetric.

Assuming that the change in the instrument response function can be considered as constant during the relatively short time of a single occultation (typically 40 seconds), nearly calibration-free horizontal transmission spectra are obtained. Even if the spectral sensitivity of the instrument is changing with time, this method is inherently self-calibrated.

The main key features of the GOMOS measurements are summarized in Table 1.2 .

Measurement parameters

geographical coverage


number of measurements

about 400 occultations/day before 01/2005

about 280 occultations/day since 07/2005

altitude range

< 20 km  -  > 100 km

sampling vertical resolution

1.7 km or better


varying from one occultation to another

calibration issues


Table 1.2: Main key features of the GOMOS measurements.