Theory: mmw Imaging

Remote sensing technologies have traditionally been split into two different families : the high resolution ‘optical’ techniques where instrumentation generally focuses and captures incoming photons of light to construct an image (traditional visible cameras, infrared heat imaging, CCDs, lasers, etc) and the longer wavelength ‘electronic’ sensing  technologies that use antennas to measure electric fields (radar, television, radio, mobile phones).

 

 one way atmospheric attenuation millimeter wave

One way atmospheric attenuation though a clear sky. MMW sensors generally operate in the ‘windows’ (marked in red) between molecular absorption lines  in the atmosphere

The main advantage that electronic sensing has over optical lies with the ability to see through weather. Since mm-waves have a shorter wavelength than traditional (microwave) radar they can construct a much higher resolution image of a scene while retaining the ability to see through cloud, drizzle and, to some extent, rain.

MMW_rainattn

 

Attenuation for different weather conditions: MMWs can see significantly further than infrared (or visible) wavelengths. Performance only drops when particle size is similar to the wavelength and the signal is more heavily scattered (heavy rain)

MMWs offer a way by which to montor a volcano regardless of viewing conditions. This can be done in two different ways:

  • PASSIVE: measuring the naturally emitted radiation from a scene (radiometry)

     

  • ACTIVE: Recording scene information by illuminating directly and measuring the returns (radar)