Archived: July 29, 2004
NASA/JPL currently operates an AIRSAR/TOPSAR system from a DC-8 jet. The AIRSAR system is capable of collecting fully polarimetric imagery at three separate frequency bands: C (5.6 cm), L (24 cm), and P (60 cm). Nominal spatial resolution is 10 m for 20 MHz radar data and 5 m for 40Mhz. Multilook post-processing can be applied to the radar imagery to reduce speckle at the expense of decreased spatial resolution.
Measured radar backscatter is a function of surface roughness, surface structure, and electrical properties of the surface including soil moisture and salinity. Generally speaking, the rougher the object the more it will backscatter because a smooth object will just reflect the energy at an angle, equal to the incident angle, away from the radar (Snell's law) . Therefore a rougher object should appear brighter and a smoother object darker. This is true as long as your incident energy isn't perpendicular to the smooth surface, in which case you will get a very bright specular return. The scattering is not only dependent on the angle of incidence and the surface structure, but also upon the frequency and polarization of the radar, as well as the moisture content in the surface.
As the radar's frequency changes so does the scattering. A surface would be considered rough and would strongly backscatter if the surface variations were on the order of the radar's wavelength. As an example, X-Band radars are better for studying snow, sea-ice and vegetation canopies, whereas L-Band and C-Band radars are better for soil moisture, foliage "penetration", and geological structures (L-Band radars also have the capability to penetrate arid soil for the purpose of determining what is below the upper layer of sand.). In each case, the particular measurements to be made determine the frequency of the radar.
Benefits of AIRSAR for wetland mapping
- High spatial resolution improves the identification of wetland communities.
- Fully polarimetric (HH, VV, HV, VH) and multiband coverage (C, L, and P) provide insight into surface properties.
- Radar backscatter is sensitive to salinity and soil moisture which have characteristic zonation patterns in coastal environments.
- Radar is an all-weather sensor, capable of collecting data regardless of weather conditions.
JPL AIRSAR homepage
Sunday, 01-Aug-2004 00:24:38 CDT
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