An important step in filling the gaps in our knowledge of tropospheric constituents and particularly NO₂ has been made by two satellite-based instruments:

• The GOME (Global Ozone Monitoring Experiment) instrument aboard the ERS-2 satellite, launched in April 1995.
  

  Fig 1.3.1: The ERS-2 satellite carrying the GOME instrument.

  Image courtesy of :

• The SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric CHartographY) instrument aboard the ENVISAT satellite, launched in March 2002.
  

  Fig 1.3.2: The ENVISAT satellite carrying the SCIAMACHY instrument.

  Image courtesy of :

Both of these satellites are in similar sun-synchronous polar orbits of about 800 km altitude. The ERS-2 satellite carrying the GOME instrument orbits about 30 minutes behind the ENVISAT satellite with SCIAMACHY.

During their ~100 minute orbits around the Earth they pass through a sunlit, dayside zone (travelling north to south) and a dark, nightside zone (travelling south to north).

The different capabilities of these instruments are discussed in the following sections.