Microwave radiometers are passive remote sensing instruments. They measure the thermal radiation of a pressure broadened ozone emission line. The line width depends on pressure and temperature and is used to determine the altitude of the emitting gas. The strength of the emission line contains the information of the ozone concentration. Microwave radiometers work at all weather conditions, their operation is only limited by very high tropospheric water vapor concentrations.

• Microwave altitude range:              ~20km - 70km
• Microwave vertical reolution:         ~10km               (20km - 30km altitude)
                                                           ~14km                (30km - 40km altitude)
                                                           ~20km                (> 40km altitude)
• Microwave accuracy:                      ~3%                    (all altitudes)

Co-location: Only co-located satellite and microwave ozone profiles are used in the validation process. The criteria for temporal and geographical coincidence are defined as follows:

• Temporal coincidence:
  Microwave profiles must be measured within 2 hours of the satellite overpass. A mximum time difference of 8 hours between both profiles might be allowed for ground stations with only a few measurements.
• Geographical coincidence:
  Satellite pixel centres must be located within +-2° in latitudinal and longitudinal distance from the reference station.