THz Radar Cross Sections

 

Measurement of the radar cross section (RCS) is a standard technique, particularly important for military and defense-related purposes including detection and identification of aircrafts, ships and other targets as well as for countermeasures such as RCS reduction and stealth. The necessity of operating on massive objects, such as full-size airplanes or ships, can make those measurements complicated, time consuming, and expensive.

RCS of F16 at FMT

Figure 1: RCS measurements of F-16 fighter at Danish military

At our group we combine time-domain based THz systems and RCS techniques to obtain accurate RCS values at THz frequencies. By employing scaling laws we are able to scale values measured in the low THz range (0.1-2 THz) into values that would be measured at radar frequencies. Typical radar systems operate at frequency range from hundreds of MHz up to a few tens of GHz. This defines the scaling factor between THz waves and radar waves from tens to few hundreds, forcing scale objects to be in size range of few centimeters and larger. Object of such dimensions is easy to handle and manipulate, and allows for iterative design and testing procedure, where the test object is manufactured by a rapid prototyping system such as a computer numerically controlled 3D milling machine or a 3D printer.

Figure 2: (a) Schematic diagram of the THz RCS setup. (b) 10 cm-long 1:150-scale metal model of aircraft fighter F-16

Figure 3(a) shows the logarithm of the instantaneous amplitude of the THz waveforms vs. azimuthal angle and range (calculated from the time-of-flight of the reflected THz pulse and the speed of light), recorded on the F-16 scale model. In such high range resolution maps the single point scatterers are seen as sine functions (sinograms) of the rotation angle. On the azimuthal map, the orientation 0° corresponds to the situation at which the airplane is exposed to THz radiation directly from above. Scattering from wing surfaces and edges, tail, fuselage and even plastic missiles is easily distinguishable.

Figure 3: Logarithm of the instantaneous amplitude of THz waveforms scattered from the F-16 scale model shown in Fig. 2(b) for different (a) azimuthal angles and (b) its frequency-averaged RCS. Letter marks indicate positions of different scattering parts of the airplane model: nose (N), wing tips (WT), wing surface (WS), vertical tail surface (VTS), vertical tail tip (VTT), fuselage (F), missiles (M1, M2).

Our THz radar system is similar to a reflection tomography setup. Each sinogram contains enough information about the target to transform it into a two-dimensional image of the target. The filtered back projection algorithm (FBP)  is a possible method to retrieve the spatial distribution of scatterers. We have applied the FBP algorithm with windowing on the data fromthe model aircraft sinogram in Fig. 3(a), and the results are shown in Fig. 4. The outline shapeof the airplane can be easily recognized. Also particular scatterers such as fuselage, tail,wings, end of the wings and even missiles are distinguishable.

Figure 4: Cross section of the scale model of the F-16 aircraft reconstructed using the filtered back projection algorithm on data from Fig. 3(a) with windowing out flat surface reflections. Letter marks indicate positions of different scattering parts of the airplane model: wing tips (WT), wing (W), tail (T), fuselage (F) and missiles (M1, M2).

You can read more about our RCS experiments with scale models of airplanes here.

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© 2012 Terahertz.dk Our research is carried out at DTU Fotonik - Department of Photonics Engineering, Technical University of Denmark. Suffusion theme by Sayontan Sinha