Basics on Radar Cross Section Reduction Measurements of Simple and Complex Targets Using Microwave Absorbers
- Citations (1)
- Cited In (0)
- Antenna theory, analysis and design, Harper & Row, ISBN 0-47160-352-X Radar cross section analysis and control, Artech House, ISBN 0-89006-371-0 RCS measurements, transformations and comparisons under cylindrical and plane wave illumination. C A Balanis, A K Bhattacaryya, D L Sengupta . 329-333.
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Basics on Radar Cross Section Reduction
Measurements of Simple and Complex Targets
Using Microwave Absorbers
Marcelo A. S. Miacci1 and Mirabel C. Rezende2
1National Institute for Space Research (INPE),
2Institute of Aeronautics and Space, Department of Aerospace Science and Technology,
Brazil
1. Introduction
The research of Radar Cross Section (RCS) of simple and complex objects is decisively
important to identify targets such as aircraft, missiles, rockets, ships and other objects, with
the purpose of improving or rendering difficult their radar visibility in various frequency
ranges. The use of RCS measurements of targets have expanded to more than solely military
applications in the identification and control processes of defense systems (Burgess, 1988).
Higher the RCS value is, easier it becomes to detect and identify an object. However, when
these targets present different geometrical forms and different types of electromagnetic
radiation absorber materials (ERAM) on their surfaces, they can become stealthy and
practically invisible to radars at determined frequency ranges.
In order to make target identification more precise, it is indispensable to analyze and
understand the RCS patterns generated by the targets. These patterns represent the
reflection mechanisms in the interaction process of the wave with the target, i.e., the
interaction of the wave with the aspects of the target’s geometry and the material’s physical-
chemical characteristics of its surface (Dybdal, 1987).
RCS measurements aim to determine the equivalent effective area of the target when it is
impinged by a radar wave. In other words, it is the ratio between the electromagnetic
energy irradiated by radar over a target and the energy scattered by it. The scattering
measurements can be performed in monostatic condition, whereby the electromagnetic
waves reflected by the target are measured in the same direction as the emitting source
(radar), or in bistatic condition, when the reflected waves are detected in other directions.
The present work describes experimental studies of RCS measurements of targets with
simple and complex shapes and the RCS reduction by using ERAM in the microwave
frequency range of 5 – 12 GHz (C and X bands). This chapter intends also to give some
highlights about the theory involved and some measurement topics concerning target
reflectivity, calibration techniques, enhancement methods and some experimental results
achieved in this research performed at Materials Division of the Institute of Aeronautics and
Space from Brazil.
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2. Main definitions
When an electromagnetic wave focuses on an object the energy is spread in all directions.
The spatial distribution of energy depends on the target geometry, material composition, the
operating frequency and polarization of the incident wave. This distribution of energy is
called scattering and the material is often called target (Blake, 1986).
Based on this principle, we can define the radar cross section, or RCS, as a measure of the power
that returns to source or reflects in a given direction, normalized in relation to an incident wave
power density. The purpose of normalization is to remove the effect of distance and facilitate
the description of the cross section independent of the distance between the target and radar
(R). The RCS is defined as shown in equation 1 (Bhattacharyya & Sengupta, 1991).
? ? 4? lim
?→?????????
?
??????
?? 4? lim
?→?????? ????
?
??? ????
?
(1)
where:
σ: radar cross section of the target (m2);
????: reflected or scattered electric field (V/m);
?? ???: reflected or scattered magnetic field (A/m);
????: incident electric field (V/m);
?? ???: incident magnetic field (A/m).
The scattered electric and magnetic fields are due to the presence of a target, so, the total
field is the sum of the incident and the scattered fields (equation 2):
????? ????? ???? (2)
The RCS unit is usually given in square meters, or expressed in dB, relative to one square
meter (dBm2 or dBsm) as in equation 3 (Currie, 1989).
??????? ? 10log????????? (3)
These concepts can be applied in the radar equation (Skolnik, 1990), correlating the received
power in terms of transmitted power, scattering, distance and antennas gain (equation 4).
??? ????????
?4??????
(4)
where:
??: radar received power (W);
??: radar transmitted power (W);
R: distance between radar and target (m);
σ: radar cross section of the target (m2);
G: radar antenna gain (dimensionless);
λ: wavelength (m).
The received power by the antenna from the transmitted radar pulse is directly related with
the physical characteristics of the target through the backscattering coefficient. The value of
this backscattering coefficient is basically dependent on the following factors (Jenn, 1995):
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Basics on Radar Cross Section Reduction Measurements
of Simple and Complex Targets Using Microwave Absorbers
353
the target geometry and surface roughness;
the target material composition (electrical and/or magnetic properties);
the wavelength and polarization of the radar; and
the aspect angle related with the incident wave.
The conceptual definition of RCS includes the fact only one part of the radiated energy
reaches the target. The RCS of a target () can be most easily visualized as the product of
three factors:
= Projected cross section x reflectivity x directivity
The reflectivity is defined as the intercepted radiated (scattered) power by the target. The
directivity is given by the ratio of the backscattered power into the radar’s direction to the
power that would have been backscattered, considering an uniform scattering in all
directions (isotropic).
2.1 Scattering matrix
In order to determine the dependence of the RCS in relation to the polarization of the wave,
we must consider the relationship between the transmitted (t) and the received (r) fields, in
terms of the horizontal and vertical components of linear polarization of the wave, EH and
EV, respectively, where the index H denotes horizontal polarization and V the vertical
polarization. Then, EH and EV can be expressed in terms of the reflectivity of a target
illuminated by both polarizations through the proportionality constant aij, where i denotes
the polarization of the transmitter and j the polarization of the receiver (equation 5) (Crispin,
1968) or in matrix notation (equation 6):
??
??
?? ?????
?? ?????
?? ?????
?? ?????
?
(5)
?
???
??
?
?? ? ????
???
???????
???
?
??
??
(6)
The constants aij shall be considered independent of the distance, but in complex notation
due to its phase relationship between the electric field components. For a monostatic radar
configuration, aHV and aVH are equal. Therefore, the scattering matrix defines the relationship
between amplitude and phase components of the electric fields transmitted and received.
A similar matrix to that one mentioned for linear polarization of the wave can be obtained
for elliptical and circular polarizations, using the constants of proportionality describing the
left or right hand polarization (Crispin, 1968).
2.2 Frequency regions
When a target has very small physical dimensions compared to the wavelength of the
incident wave, it is considered that your analysis is being done in the so called Rayleigh
region (Blake, 1986). In this region, the shape of the object does not influence in determining
their RCS, and for some types of objects the RCS is determined from the volume, instead of
considering the dimensions and physical forms. For targets that are comparable in size to
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the wavelength of incident wave, the RCS varies depending on the frequency and is called
the resonant region or Mie region.
When the dimensions of the target are large compared with the wavelength of incident
wave, the RCS can be determined using the methodology of geometrical optics or by the
method of optical physics, and this region is called the optical region. In the following
sections, the analysis of some RCS targets of simple and complex geometry is shown in the
optical region. Figure 1 is a RCS curve of a sphere as a function of the ratio of the target
radius (a)/ wavelength (λ), normalized in the optical region. The analysis of this figure
shows that there are the distinct regions discussed above. Although it is a perfectly
conducting sphere, this behavior is observed for all types of targets (Currie, 1989;
Bhattacharyya & Sengupta, 1991).
Fig. 1. Normalized RCS of a sphere as a function of ka (where k=2π/λ).
3. Radar cross section reduction measurements
RCS measurements are performed for many reasons; the main one is to verify target
detection limits by a radar system to which it is subjected, verifying conformity of the
practical models, designed by theoretical equations (Kouyoumjian, 1965).
The measurement as a whole is a complex task due to the many factors that may affect these
measurements. Instrumental errors, spurious interferences and reflections are some of
contributions to degrade the quality of the data. These problems are compounded when one
is interested in reducing the RCS of an object, as in this case, the magnitude of these effects
can overshadow the true RCS values.
Many methods have been proposed and used for the measurement of several types of
targets. Depending on the size of the targets and radar frequencies used, RCS measurements
can be performed on outdoor or indoor ranges, in the last condition, inside anechoic
chambers. In measurements, it is important that the radar is illuminated by an
electromagnetic wave which is uniform in phase and amplitude. For practical purposes, the
maximum tolerance for amplitude variation over a target is 0.5 dB, and the phase should not
deviate more than 22.5o. These conditions denote the far-field condition (Balanis, 1982; Jasik
& Johnson, 1984), given by:
? ?2??
?
(7)
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of Simple and Complex Targets Using Microwave Absorbers
355
where:
R is the distance between radar and target;
d is the largest dimension of the target; and
λ is the wavelength of the radar.
This condition ensures good measurements. Errors produced by the instrumentation should
neither exceed 0.5 dB nor vary in time in order to avoid instability in the measurement of the
RCS patterns. Thus, it is decisive the careful selection of the experimental parameters. In this
case, the dynamic range of the system should be at least 40 dB when measuring targets with
small RCS. Dynamic range values in the order of 60 dB or higher are preferable when RCS
reduction studies are conducted or when absorber materials are used. Another important
factor to be taken into account is the target support structure that should not interfere with
the incident wave, however in practice such condition isn’t always possible.
Analysis of the electromagnetic energy scattering by metallic objects is very important in the
understanding of RCS of targets, in the same way that reflections of dielectric and magnetic
surfaces are important when studying RCS reduction. Within this context, RCS analysis of
targets with simple shapes is fundamental to support the understanding of RCS patterns of
targets with complex shapes.
To evaluate the electromagnetic behavior of targets, many methods have been proposed and
experimentally used for many decades with well accepted results (Birtcher & Balanis, 1994)
and the RCS measurement errors depend on the nature of the target under test, the distance
at which this target is being measured and the place of measurements.
Systems are projected in such way that respect the parameters described above. However,
they present technical challenges, making adequate the use of experimental procedures that
involve new techniques that can minimize errors. When an anechoic chamber is used, the
emissions and reflections of spurious radiation are controlled and minimized by using
commercial off the shelf ERAM inside the chamber, becoming the background noise levels
of this radiation almost null. On the other hand, by using the outdoor range, the
measurements are affected by environment variations, therefore needing greater control of
the parameters.
Even in measurements performed in indoor range, the RCS patterns of the targets may
become impaired due to the occurrence of noise in the system or on account of low
backward radiation contribution of the targets with lower RCS.
3.1 Basic instrumentation
The basic instrumentation required for RCS measurements consists of four subsystems that
can be controlled by a central station; these subsystems are: positioners and drivers,
receiver, transmitter and data acquisition system.
Using a simple setup and placing the transmitter (TX) and receiver (RX) antennas in the
scheme showed in Figure 2, it is possible to measure the RCS of targets in different frequency
ranges. The distance between both antennas needs to be tested to eliminate the radiation
coupling between them.
In order to have good precision, the transmitter must have means to provide enough power
to allow a good signal/noise ratio for the measurement system and it is necessary to take