Sensors: Image Processing Library
This information was extracted from several JPL AIRSAR pages and was developed as an internal resource for CSR remote sensing personnel.
The NASA/JPL AIRSAR is a side-looking imaging radar system which utilizes the Synthetic Aperture principle to obtain high resolution images which represent the radar backscatter of the imaged surface at different frequencies and polarizations. The image coordinates are usually slant or ground range and azimuth. Slant range is a measure of the radial distance of the image object from the radar. Ground range is a measure of the distance of the object from a point directly below the aircraft. Near range in the images is closest to the aircraft and far range is furthest away. The azimuth dimension in the images is parallel to the line described by the track of the aircraft (see figure). The spatial resolution of the AIRSAR images, which determines how far apart two objects have to be in order to separate them in the image, is on the order of 12 x 12 meters. Note that this is not the same as the range or azimuth pixel spacing which is contained in the variable format header of the AIRSAR data.
The information contained in each pixel of the AIRSAR data represents the radar backscatter for all possible combinations of horizontal and vertical transmit and receive polarizations (i.e. HH, HV, VH, and VV). The data is in compressed Stokes matrix format, which has 10 bytes per pixel. From this data format it is possible to synthesize a number of different radar backscatter measurements. Each pixel contains information about the radar backscatter for a single frequency only. Other image files may be available which contain information about the radar backscatter at different frequencies. The radar backscatter measurement is a strong function of incidence angle, and usually has a tendency to decrease as the incidence angle increases. This effect can often be seen in AIRSAR images, where the incidence angle is typically in the range of 0 to 70 degrees. The smaller incidence angles correspond to near range in the AIRSAR data, while the larger incidence angles correspond to far range.
Much of the data produced by the AIRSAR is now calibrated, so that the radar backscatter measurements are in normalized radar cross section format (m2/m2) or [[sigma]]0. [[sigma]]0 is the radar cross section (measured in m2) normalized by the area of the measurement, which in this case is the pixel area in square meters. Results obtained using uncalibrated AIRSAR data should be treated with caution. If a trihedral corner reflector is deployed within the imaged scene and a few simple measurements about that reflector are known.
The direct measurements of radar backscatter which are made by the AIRSAR systems are the elements of the scattering matrix: Shh, Shv, Svh and Svv, which represent the backscatter for each pixel in response to the 4 different combinations of H and V transmit and receive polarizations. Shv is the scattering matrix term for H transmit, V receive, for example. The scattering matrix measurements are complex numbers, with amplitude and phase.
Last Modified: Wed Apr 14, 1999