In other parts of this remote sensing tutorial, we can see how information from instruments that operate in the visible and infrared portions of the spectrum can be used to study the Earth. In this section, we look at the ways that imaging radar can be used.
Radar is an instrument that is called an "active sensor", because it transmits a signal that is then reflected off of a surface before being measured. (Visible and infrared sensors are called "passive sensors" because they just measure the amount of sunlight reflected off of a surface). Radar measurements for the Earth are usually collected at a wavelength between 3 cm and 68 cm. Typically, the data that we show in Virtually Hawaii have been collected at either 6 cm or 24 cm.
Using a radar system to study the Earth (or another planet) has several advantages over passive sensors. Because of the wavelength of the signals, radar can "see" through clouds, and as it is an active system it can operate day or night. It is therefore possible to image the surface whatever the weather conditions are at any time of day. Of course, there are disadvantages, such as the non-unique spectral properties of the returned radar signal. Unlike infrared data that help us to identify different minerals on the ground, radar only shows the difference in the topography (at several scales, the most important of which is close to the wavelength of the radar energy) and moisture content of the ground. Radar and infrared sensors are therefore very complimentary instruments, and are often both used to study the same landscape.
In this tutorial, we will show you some background information on how radar works, some of the applications, and then some example images from the Kilauea Volcano area of the Big Island to show what happens when we change the radar wavelength or polarization.
Go on to Page 1 of this Remote Sensing Tutorial to see how the geometry of radar acquisition differs from other types of remote sensing.
Author: Pete Mouginis-Mark
Copyright by P. Mouginis-Mark
Curator: Lori Glaze