From its orbit around the Earth a satellite-borne spectrometer can take two types of measurements:

• A spectrum of the earthshine radiance, that is, the light coming from an observed pixel of the Earth. This light consists of solar radiation that has passed into the Earth's atmosphere and has undergone one or more interactions with some part of the atmosphere (and probably also with the surface of the Earth) before returning through the atmosphere to the spectrometer on the satellite.
• A spectrum of the solar irradiance reaching the satellite. This spectrum, of course, has not undergone any interactions with the Earth or its atmosphere. It can therefore be used as a reference spectrum with which to compare the earthshine spectrum, allowing the interactions and particularly the absorptions within the earthshine lightpath to be identified.

Fig 3.1.1.1: Measurement of earthshine and direct solar radiation by satellite-borne spectrometers.

Image: AT2-ELS

As described in previous sections of the module, the earthshine is scanned pixel by pixel across the track of the satellite during its dayside orbit. This scanning is effected by mirrors in the spectrometer.

In contrast, the spectrum of the solar irradiance is measured once during each dayside orbit (in GOME and SCIAMACHY, for example). The spectral measurement of solar irradiance is not carried out using the normal scanning lightpath into the spectrometer via the scanning mirrors, but is carried out using a diffuser plate. The diffuser plate integrates the radiance coming from all directions, thus giving a value for the solar irradiance. (For the difference between radiance and irradiance see the section 'Radiation basics'.)