Advances in satellite instrumentation have resulted in a general trend towards multifunctional capabilities in many types of sensors, resulting in instruments with the capability to operate using different viewing modes and angles, as well as multiple polarisations. The latest SAR instruments demonstrate this trend. The category of ‘multiple direction/polarisation instruments’ is used here, however, to describe instruments which are custom built for observing the directional or polarisational characteristics of the target’s signature (either visible/IR or microwave), as a means of deriving geophysical information.
Multi-directional radiometers can make observations from more than one incidence angle of the diffused or emitted radiation emitted by a particular element of the Earth’s surface or clouds. In this way, information on anisotropies in the radiation may be identified. The emphasis in these instruments is on spectral (rather than spatial) information, with the result that the detection channels, which typically span the visible to the IR, are precisely calibrated and the spatial resolution is usually about 1 km.
Polarimetric radiometers are used for applications in which radiative information is embedded in the polarisation state of the transmitted, reflected or scattered wave. Some polarimetric radiometers also have a multi-directional capability, so that directional information can be determined or used during retrievals of geophysical parameters.
Using IR channels, multiple-angle viewing capabilities are used to achieve accurate corrections for the effects of (variable) atmospheric absorption, making it possible to infer precise temperature values, for example, of sea and land surfaces. Multi-directional radiometers are also capable of measuring cloud cover and cloud top temperatures, together with atmospheric water vapour and liquid water content.