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Synthetic Aperture Radar

The Center for Space Research is currently involved in several aspects of Synthetic Aperture Radar (SAR) imagery including processing, modelling, and application. The use of radar for remote sensing applications has become quite popular over the last decade. It has been used to cover topics as diverse as sea ice extent, soil moisture content, vegetation canopy morphology, ocean wave dynamics and soil erosion. High resolution, unique scattering properties and low atmospheric attenuation are a few of the reasons behind its popularity.

In many ways, the methods for remote sensing via SAR are not as intuitive as those used for the optical (visible and near infrared) portion of the spectrum. This section contains background information and several common methods of analysis for interpreting SAR data. Microwave remote-sensing systems distinguish between different subjects primarily by the differences in the signal strength received by the radar [Ulaby, Moore, and Fung 1986b]. Therefore, the received signal strength is the most important measurement made by a radar used for remote sensing. To distinguish among different targets, measurements of angle and distance to a target are made by recording the arrival times of the received signals.

Imaging Geometry

In order to properly interpret radar data, it is necessary to account for the geometry of the acquisition. Radars used for remote sensing of terrain are almost always side-looking systems, while most optical instruments are nadir-looking. This difference exists because optical instruments are able to distinguish among targets based upon their angular distance from the sensor. However, a radar can only distinguish the returns from various targets based upon the arrival time of the received signals. A nadir-looking radar could not distinguish between two scatterers a and b (see Figure 2.2) that are equal distances from the sensor because a single incident wave front illuminates both points at the same instant, so the scattered returns from both points arrive at the receiving antenna simultaneously. This leads to an ambiguity in range for any right/left-symmetric, equidistant points. If the radar illumination is restricted to one side of the platform, the wave front illuminates the same two points at different times. Their scattered returns arrive at the sensor separated in time and are thereby distinguishable from each other.

Polarimetric SAR

With the advent of synthetic aperture techniques, imaging radars have become well suited for many remote sensing applications. The increased spatial resolution has made more precise mapping possible. However, accurate discrimination between similar scattering returns is still difficult for most systems which operate at only one transmit/receive polarization combination. More recently, fully polarimetric SAR systems have been used to determine scattering properties (magnitude and relative phase of the scattered electric field) at any polarization combination. Both vertical and horizontal linear polarizations are used to transmit and receive, thus four transmit/receive combinations are possible, hh , vv , hv, and vh.

Knowledge of an area's scattering properties as a function of polarization can yield information about the roughness of surfaces and orientations of the structural components of vegetation.

Analysis Methods for SAR Data

Some information can be gained by visually analyzing SAR images and relating the responses to physical phenomena qualitatively. These images can be simple gray-scale or color images based on combinations of the backscatter, or color bitmaps resulting from a formal data-classification scheme. Both the visual and classification techniques break up the data into homogeneous groups based upon various backscatter combinations.

Another, more physically-based, approach involves deriving the theoretical relations that govern the electromagnetic scattering and using them to relate the observed scattering to physical characteristics of the target. An intermediate approach is to empirically derive relations between the backscattering coefficient and physical parameters of interest. This approach is not as rigorously based on the physics of the scattering mechanisms as the theoretical approach, but it can be simpler to understand and implement.

The following sections provide in-depth information about SAR processing as well as applications that CSR is involved with.


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Last Modified: Wed Apr 14, 1999
CSR/TSGC Team Web