20 April 2006 1Propagation of Radar Waves: Forward Scattering • Forward scattering (reflection) from the surface of the earth enhances the energy at some elevations and decreases it at other. • Refraction (bending). • Ducting (trapping), a severe form of refraction that can extend radar range. • Diffraction • Attenuation • External Noise • Backscatter, clutter Forward Scattering. Assumptions: Flat-earth model, (almost) total reflection (Γ ~-1), path length almost equal so that differences in amplitude between reflected and direct wave is negligible, horizontal polarization, grazing angle is small. The received power relative to what would have been received in free space is =RhhPtaλπη2sin164 Modified radar equation for point targets (we can do the same for distributed targets): ×=RhhRPGPtatrλππσλ2sin166444322 Maxima (16 times free-space power) and minima (null) occur when (minima) 22(maxima) 124nRhhnRhhtata=+=λλ This leads to “lobing” of the received energy:20 April 2006 2 For small arguments of the sine (region below the peak of the first lobe): )(48242RhhGPPtatrλσπ≈= Notice the R8 dependence.20 April 2006 320 April 2006
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