Imagery from synthetic aperture radar (SAR) satellites is not a familiar data set for most users of geographic information systems (GIS). There are several reasons why radar imagery is not commonly used, primarily because of the nature of the technology and its specialized applications. Another is that radar imagery is not optical, requiring more technical processing and specialized image interpretation skills.
The SAR satellite used to create these images is the Advanced Land Observing Satellite (ALOS). ASF is an archive facility for ALOS data collected over the Americas. The SAR sensor is an L-Band phased array radar capable of imaging in several resolutions and polarizations. Because radar data is ranging data that measures the strength and scatter of the radar pulse, it is not like optical imagery which is visually intuitive. To make the SAR scenes more user friendly, the polarization data was classified as reds, greens, and blues in the image. Another aspect of radar remote sensing is that the ranging data must be terrain-corrected by a process that assigns the ranging values to geographic coordinates by utilizing a digital surface model (DSM). High resolution DSM data are not available for the entire planet and existing data at high latitudes is problematic, especially in areas of very little terrain relief, such as sheet glaciers. To use a consistent DSM for this project and all of the sites being investigated, the DSM data from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) imaging instrument was used for the terrain correction.
Launched on January 24th, 2006 aboard the Advanced Land Observing Satellite (ALOS), the Phased Array Type L-band Synthetic Aperture Radar (PALSAR) instrument has promising applications for natural resource and land applications including parameters applicable to terrestrial nutrient cycle estimates. Research has shown that SAR data by itself, or combined with other optical or active systems, can enhance land characterization with information not otherwise available from passive remote systems. Because Radar emits its own signal, imaging can occur anytime of the day or night independent of sun angle. This is in contrast to passive imaging systems that require the Sun’s illumination. Due to its longer wavelength than visible light, the microwaves used in Radar also have the advantage of not being impeded by cloud cover or other atmospheric contamination. Some examples of PALSAR land applications include estimates and mapping of vegetation above ground biomass, deforestation mapping, wetland (including high latitude) characterization, and cropland monitoring.
Radar Imaging Basics
The PALSAR instrument is a type of Synthetic Aperture Radar (SAR) that emits energy in the long wavelength L-Band (1270 MHz) frequency. SAR radar systems are able to generate high-resolution imagery with a synthetic aperture (or virtual long antenna) by combining signals received by the physically short (real) antenna as it moves along its flight track.
As an imaging radar system moves along a flight path it emits and receives pulses in a single particular microwave wavelength and orientation (waves polarized in a single vertical (V) or horizontal (H) plane). The radar pulse interacts with the Earth’s surface and is scattered in all directions, with some energy reflected back toward the radar’s antenna. Known as backscatter, the returned signal is received by the antenna a fraction of a second later and in a specific polarization (H or V). The brightness, or amplitude, of the backscatter is measured and recorded and the data are used to derive an image. Radar waves interact differently with soil, vegetation, water, ice, and man-made objects such as buildings and roads because the backscatter is affected by the surface properties of objects. For a smooth surface such as water or a road, most of the incident energy is reflected away from the radar system resulting in a very low return signal. In contrast, rough surfaces will scatter the emitted energy in all directions and return a significant portion back to the antenna. In general, vegetation is usually moderately rough with most radar wavelengths.