Unlike GOME, which operates in nadir mode, SCIAMACHY can operate in three viewing modes.

• nadir mode, which looks directly down on the Earth's atmosphere,
  

  Fig 1.3.2.1.1: Nadir mode scanning by the SCIAMACHY instrument.

  Image courtesy of :

• limb mode, which looks obliquely through the Earth's atmosphere in the direction of flight,
  

  Fig 1.3.2.1.1: Limb mode scanning by the SCIAMACHY instrument.

  Image courtesy of :

• and occultation mode, which is a similar viewing angle to limb mode, but looking through the Earth's atmosphere at a light source such as the Sun or Moon.
  

  Fig 1.3.2.1.3: Occultation mode scanning by the SCIAMACHY instrument.

  Image courtesy of :

A number of other measurements, mostly for calibration purposes, are possible.

Each of these modes is discussed in detail in the next sections.