Monitoring vegetation in the future: radar

HOLMES, M. G., 1992. Monitoring vegetation in the future: radar. The use of radar for remote sensing of the biosphere has several advantages over conventional methods using the visible and infra-red wavebands. These advantages include independence from solar energy, an all-weather capability, and the ability to select an appropriate wavelength to study either the surface layers of vegetation or to penetrate deeper into the canopy to study the properties of undergrowth or the soil substrate. Despite these advantages, radar remote sensing is still in the developmental stage. The mode of operation of the radar instrument, and the physical laws governing the propagation of microwaves in scattering media, introduce some unique aspects for remote sensing of the biosphere.
Increased interest in radar has arisen primarily from the need to make multi-temporal studies of vegetation in areas which are susceptible to cloud cover, such as temperate and equatorial regions. Radar penetrates all but the densest cloud, and therefore overcomes this problem. At the same time, however, many of these wavelengths also penetrate deeply into the crop. As a result, analysis of the principles controlling the interaction of radar wavelengths with vegetation and soil are complex. These principles are discussed in terms of current microwave backscatter models.
Vegetation and soil factors influencing radar backscatter are discussed, and the significance of instrument characteristics such as frequency, polarization and incidence angle are considered. The advantages and disadvantages of radar are reviewed, and particular reference is made to the potential of combining optical and radar studies.