This section is an introduction to the use of satellite-borne spectrometers for the measurement of concentrations of NO₂ in the troposphere and stratosphere. It covers three main topics:

• The importance of NO_x (that is, NO₂ and NO).
  This topic explains the important role that NO_x compounds play in the chemistry of tropospheric pollution and explains the need to measure the spatially and temporally very variable local concentrations of these species on a global scale.
• The techniques of satellite measurements.
  This topic explains the techniques of satellite-based measurement, including the characteristics of satellite platforms used for the spectroscopic measurement of trace gas species.
• The characteristics of the satellite-borne instruments that are used to retrieve the concentrations of trace gas species with particular reference to NO₂ and the two instruments GOME and SCIAMACHY.