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• passive technologies sense natural radiation emitted by the earth and atmosphere or solar radiation reflected, refracted or retransmitted by the earth and atmosphere.
  
  • Contamination of atmospheric signals over land and over coastlines can be a problem, although increasingly new ways are being found to utilise such data. For example with cycle 45r1 introduced in June 2018 ECMWF began assimilating non-surface-sensitive infra-red channel data over land, and all sky micro-wave sounding data over coasts.
    
• active technologies emit radiation and sense how much is transmitted, reflected or scattered back.  For example:
  • the GPS radio occultation satellite-to-satellite signal is very sensitive to the temperature and humidity structure of the atmosphere particularly to the sharp moisture and temperature gradients beneath the boundary layer inversion,
  • Scatterometers derive surface-wind vectors from back-scattered radar signals from sea-surface ripples,
  • the Advanced Scatterometer (ASCAT) also enables soil moisture pseudo-observations that observe subsurface/subcanopy sub-surface and sub-canopy climate-related features such as water content of sub-canopy and continental surfaces.
    

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Satellite data is vital for an effective analysis and the use satellite observations is increasing rapidly.

Fig2.4.-1: Pie chart showing the proportion of data types used by the IFC assimilation.  ATMS predominate. Ground-based observations constitute a relatively small proportion.

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