Introduction to Synthetic Aperature Radar (SAR) Remote Sensing¶
Sarlo includes a series of scripts, tutorials and helper functions for working with Synthetic Aperature Radar.
Radar Remote Sensing¶
SAR is an active Microwave imaging system
| Frequency Band | Wavelength(cm) | Frequency(GHz) |
|---|---|---|
| Ka | 0.8-1.1 | 40 - 26.5 |
| K | 1.1-1.7 | 26.5 - 18 |
| Ku | 1.7-2.4 | 18 - 12.5 |
| X | 2.4-3.8 | 12.5 -8 |
| C | 3.8-7.5 | 8 - 4 |
| S | 7.5-15 | 4 - 2 |
| L | 15 -30 | 2 - 1 |
| P | 30 -100 | 1 - 0.3 |
Advantages Compared to Optical Remote Sensing¶
- all weather capability (small sensitivity of clouds, light rain)
- day and night operation (independence of sun illumination)
- no effects of atmospheric constituents (multitemporal analysis)
- sensitivity to dielectric properties (water content , biomass, ice)
- sensitivity to surface roughness ( ocean wind speed)
- accurate measurements of distance (interferometry)
- sensitivity to man made objects
- sensitivity to target structure (use of polarimetry)
- subsurface penetration
Disadvantages¶
- complex interactions (difficulty in understanding, complex processing)
- speckle effects (difficulty in visual interpretation)
- topograhic effects
- effect of surface roughness
Polarization and Scattering Mechanics¶
Radar can also collect signals in different polarizations, by controlling the analyzed polarization in both the transmit and receive paths. Polarization refers to the orientation of the plane in which the transmitted electromagnetic wave oscillates. While the orientation can occur at any angle, SAR sensors typically transmit linearly polarized. The horizontal polarization is indicated by the letter H, and the vertical polarization is indicated by V.
The advantage of radar sensors is that signal polarization can be precisely controlled on both transmit and receive. Signals emitted in vertical (V) and received in horizontal (H) polarization would be indicated by a VH. Alternatively, a signal that was emitted in horizontal (H) and received in horizontal (H) would be indicated by HH, and so on. Examining the signal strength from these different polarizations carries information about the structure of the imaged surface, based on the following types of scattering: rough surface, volume, and double bounce
- Rough surface scattering, such as that caused by bare soil or water, is most sensitive to VV scattering.
- Volume scattering, for example, caused by the leaves and branches in a forest canopy, is most sensitive to cross-polarized data like VH or HV.
- The last type of scattering, double bounce, is caused by buildings, tree trunks, or inundated vegetation and is most sensitive to an HH polarized signal.
Strong scattering in HH indicates a predominance of double-bounce scattering (e.g., stemmy vegetation, manmade structures), while strong VV relates to rough surface scattering (e.g., bare ground, water), and spatial variations in dual polarization indicate the distribution of volume scatterers (e.g., vegetation and high-penetration soil types such as sand or other dry porous soils). Credit: NASA SAR Handbook.
It is important to note that the amount of signal attributed to different scattering types may change as a function of wavelength, as wavelength changes the penetration depth of the signal. For example, a C-band signal penetrates only into the top layers of the canopy of a forest, and therefore will experience mostly roughness scattering mixed with a limited amount of volume scattering. However a L-band or P-band signal will have much deeper penetration and therefore experience strongly enhanced volume scattering as well as increasing amounts of double-bounce scattering caused by the tree trunk (view canopy penetration figure below).
Interferometry (InSAR)¶
SAR data can also enable an analysis method called interferometry, or InSAR. InSAR uses the phase information recorded by the sensor to measure the distance from the sensor to the target. When at least two observations of the same target are made, the distance, with additional geometric information from the sensor, can be used to measure changes in land surface topography.