Inverse Synthetic Aperture Radar Imaging With MATLAB Algorithms. Caner Ozdemir

Inverse Synthetic Aperture Radar Imaging With MATLAB Algorithms - Caner Ozdemir


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      (2.5)equation

      This is the scattered electric field in the far field using the PO approximation. As will be explored in Chapter 4, this formula will constitute a basis for the derivation of inverse synthetic aperture radar (ISAR) imaging algorithm.

      

      Radar cross section (RCS) can be regarded as the measure of the EM energy intercepted and reradiated by an object (or target). The unit of RCS is square‐meters (m2). RCS is generally used to categorize the object's EM reflectivity or ability to scatter the EM energy for a particular direction and at a particular frequency.

      RCS is the main parameter in detection of airplanes, ships, tanks, or, more commonly, military targets. The low observable (or stealth) aircrafts/ships are designed to give very low RCS values so that they cannot be detected by the opponent's radar. These special design features for low‐observable platforms include planar body surfaces that reflect the incoming wave to another direction and/or using special radar‐absorbing material (RAM)‐based coating or paint. Passenger airplanes, on the other hand, usually provide relatively much higher RCS values due to bare metallic surfaces that reflect almost all of the incoming energy, rounded shape design that scatters the EM wave almost in all directions, and canonical shapes and cavities (such as engine ducts) that scatter most of the incident EM wave in the backward direction. Furthermore, they are also big in size compared to fighters. Therefore, their RCS values are much higher than specially designed stealth aircrafts.

      2.3.1 Definition of RCS

      A more formal definition of the RCS (σ) of an object can be made as the following: “It is the equivalent area intercepting the amount of power that, when scattered isotropically, produces at the radar receiver a power density Ws that is equal to the density scattered by the actual object” (Chu et al. 1991).

Schematic illustration of EM energy scatters in all directions when it hits a target.

      (2.6)equation

      According to the above definition of RCS, this reflected power will reradiate in all directions (isotropically). Therefore, the power density Ws of the reflected wave at the radar receiver is

      (2.7)equation

      Therefore, σ in the above equation can be left alone to give

      The formal equation of RCS can be easily obtained as the following:

      In an alternative definition of RCS (σ), it is the measure of the ratio of backscatter power per unit solid angle (steradian) along the radar direction to the power density that is intercepted by the object (or target)

      (2.10)equation

      (2.11)equation

      The RCS area of a target does not necessarily bear a direct link with the physical cross‐sectional area of that target but depends on other parameters as well. Besides the aperture of the target seen by the radar (or projected cross section), the EM reflectivity characteristics of the target's surface and the directivity of the radar reflection caused by the target's geometric shape are the key parameters that affect the RCS value. Therefore, RCS can be approximated as the multiplication of (i) projected cross section, S;


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