SAR simulation tools are typically used in the design process of SAR systems in the form of parametric studies that consider topics like prediction of image quality parameters, testing of image reconstruction algorithms, motion errors along the synthetic aperture, etc. The recent operational experience of space-borne SAR systems with sub-meter resolution (e.g. SAR-Lupe, COSMOSkyMed, and TerraSAR-X) for reconnaissance purposes demonstrated the importance of the understanding of SAR-specific image effects, especially foreshortening and layover, as well as shadow characteristics for the interpretation of complex targets like airplanes and ships.
SAR simulation could be a key to supply image operators with possibilities that simplify image interpretation and assist in applications like signature analysis or recognition. Therefore, a novel simulation framework has been established. It tries to fulfill all the above demands to the highest possible degree to reflect reality. The modular and flexible structure of the simulator allows adjusting and expanding it for different tasks.
The overall conceptual diagram of the developed SAR simulation framework with its major modules SAREF, SARBIS and RADIAN is shown in Figure 1. The main tool is the SAR effects simulator SAREF. Its specialty is computational efficiency while still retaining high level of details and accuracy. This is possible, because the effects of wave propagation for a SAR image are computed using a fast ray-tracing scheme. Next, the scattered fields are computed by application of geometrical and physical optics for every scattering center and superimposed, in order to generate SAR raw data or finely grained reflectivity maps. The SAR image simulator SARBIS takes these results to reconstruct the simulated SAR image with conventional SAR processors or with novel fast methods using the impulse response of the SAR system.
|Figure 1: Overall conceptual diagram for SAR simulation.|
Signature analysis and target classification
SAREF cannot only be used for the study of image or signal processing effects, but it also provides users with a wide range of functions for signature analysis. Of note is the possibility to overlay simulation results with measured SAR signatures to assist in target recognition and identification. In order to obtain finer grained information about the scattering processes, the different scattering effects can be highlighted and clustered using color coding.
In this way single-bounce, double-bounce, multi-bounce, as well as corner and edge effects can easily be distinguished from each other. Furthermore, SAREF makes it possible to analyze individual scattering centers by visualizing their corresponding propagation ray paths. The physical background of scattering can also be demonstrated for educational tasks, for instance.
|Figure 2: Signature analysis for a 3-D model of a battleship.|
Figure 2 shows a 3-D model of a battleship along with its reflectivity map in ground range. For better distinction of scattering processes the scattering centers are color coded according to their type. A ray path for a triple-bounce scattering center is shown in blue.
|Figure 3: Overlay of a TerraSAR-X signature with the simulated reflectivity map.|
Using the SAR image analysis toolset RADIAN the overlay of simulation results with real signatures is possible for verification with the reality and for target recognition purposes, as shown in Figure 3.