22
3 Radar Targets and Its Reflecting Properties
RCS calculation in most cases narrows to solution of corresponding diffraction
problems, determination of necessary polarization components of scattered field and
further calculation of RCS according to abovementioned formulas. Let us make a
brief review of some presently used calculation methods of RCS.
It is commonly known that to find a scattered one in designated space point, it
is necessary to solve Maxwell equations with respect to corresponding boundary
conditions. This way leads to so-called rough methods of diffraction theory. It is
not recommended of course to think that solution obtained via rough method gives
a more precise answer for real targets than obtained by the approximation method.
The point is that in all cases during RCS calculation, the radar target is changed with
necessity by the approximate idealized model based on such models for which it is
possible to obtain a rigorous solution. Unfortunately, it has a limited number: sphere,
spheroid, disk, thin wire and a number of some others. An error arises due to this
change on frequent occasions which happens to be so big that knocks the bottom out
of all advantages of rigorous solutions. In this connection, the approximate methods
can give a more precise solution as at modeling stage, it permits to consider more
factors determining a scattering of a field. Nevertheless, at rigorous methods, it is
always a possibility to rigorously estimate an error of obtained solutions at all stages
of problem solution, while use of approximate methods there is not such a possibility
as it is mathematical difficulties make due using one or another physical hypothesis.
Approximate methods have become widespread in radiolocation for targets RCS
calculation: geometrical optics method and aperture-field method. Let us examine it
in detail.
The basis for geometrical optics method lies in the following physical assumptions:
• electromagnetic field wavelength is considerably less than characteristic dimension of bodies with which a field is interacted.
• boundary lines (interface) of interacted with electromagnetic field bodies and
medium are completely smooth, and interface curvature is insignificant; then,
within small areas, the boundaries refract and reflect electromagnetic waves in
accordance with Fresnel’s equations for planar interface.
• energy propagation of electromagnetic field happens along the rays.
• in case of electrically non-uniformity medium, the wavelength of electromagnetic
field in medium is considerably less than a distance at which medium parameters
are notably changed.
As for the most radar targets mentioned above, the conditions are quite accepted,
and therefore, solutions obtained within geometrical optics are quite satisfactory.
In fact, this method is applicable to large highly conductive bodies and gives an
opportunity to calculate the RCS without use of any other laws of electromagnetic
besides Snell law of reflection and refraction.
Let us illustrate the abovementioned example of RCS calculation of flat and
convex bodies, the sizes of which considerably exceed the wavelength. Let the flat
electromagnetic wave incidents on such body (Fig. 3.7).
3 Radar Targets and Its Reflecting Properties
RCS calculation in most cases narrows to solution of corresponding diffraction
problems, determination of necessary polarization components of scattered field and
further calculation of RCS according to abovementioned formulas. Let us make a
brief review of some presently used calculation methods of RCS.
It is commonly known that to find a scattered one in designated space point, it
is necessary to solve Maxwell equations with respect to corresponding boundary
conditions. This way leads to so-called rough methods of diffraction theory. It is
not recommended of course to think that solution obtained via rough method gives
a more precise answer for real targets than obtained by the approximation method.
The point is that in all cases during RCS calculation, the radar target is changed with
necessity by the approximate idealized model based on such models for which it is
possible to obtain a rigorous solution. Unfortunately, it has a limited number: sphere,
spheroid, disk, thin wire and a number of some others. An error arises due to this
change on frequent occasions which happens to be so big that knocks the bottom out
of all advantages of rigorous solutions. In this connection, the approximate methods
can give a more precise solution as at modeling stage, it permits to consider more
factors determining a scattering of a field. Nevertheless, at rigorous methods, it is
always a possibility to rigorously estimate an error of obtained solutions at all stages
of problem solution, while use of approximate methods there is not such a possibility
as it is mathematical difficulties make due using one or another physical hypothesis.
Approximate methods have become widespread in radiolocation for targets RCS
calculation: geometrical optics method and aperture-field method. Let us examine it
in detail.
The basis for geometrical optics method lies in the following physical assumptions:
• electromagnetic field wavelength is considerably less than characteristic dimension of bodies with which a field is interacted.
• boundary lines (interface) of interacted with electromagnetic field bodies and
medium are completely smooth, and interface curvature is insignificant; then,
within small areas, the boundaries refract and reflect electromagnetic waves in
accordance with Fresnel’s equations for planar interface.
• energy propagation of electromagnetic field happens along the rays.
• in case of electrically non-uniformity medium, the wavelength of electromagnetic
field in medium is considerably less than a distance at which medium parameters
are notably changed.
As for the most radar targets mentioned above, the conditions are quite accepted,
and therefore, solutions obtained within geometrical optics are quite satisfactory.
In fact, this method is applicable to large highly conductive bodies and gives an
opportunity to calculate the RCS without use of any other laws of electromagnetic
besides Snell law of reflection and refraction.
Let us illustrate the abovementioned example of RCS calculation of flat and
convex bodies, the sizes of which considerably exceed the wavelength. Let the flat
electromagnetic wave incidents on such body (Fig. 3.7).