RadBase™: A Physics-Based Radar Database Generation Toolkit

Chris Blasband, Jim Jafolla

Surface Optics Corporation

11555 Rancho Bernardo Road

San Diego, CA 92127

Phone: (858) 675-7404

E-mail: cbb@surfaceoptics.com

ABSTRACT

Predicting the Radar Cross Section (RCS) of complex targets is a complicated problem in electromagnetic scattering theory. Radar scattering theory has been studied extensively for many decades. There have been many software packages developed for calculating the RCS of complex targets, however, those that have been fully validated, and are available to the radar scattering community, require the purchase of a very expensive, high-powered UNIX® platform. Surface Optics Corporation (SOC) has developed a state-of-the-art, commercial off-the-shelf (COTS) software package that disposes of the need for buying an expensive computer. RadBase is a user-friendly software product that generates accurate RCS and Amplitude & Phase data for complex targets and cultural features. It is a Java-based application that executes on PC’s running Windows (95®,98®,NT®) and UNIX/Linux based platforms. In this paper, the capabilities of RadBase 2.0 will be described. A brief description of the scattering phenomena, input and output data is presented. A full RadBase validation study is presented as well as RadBase comparisons with the radar scattering code, XPATCH.

INTRODUCTION

SOC has developed a user-friendly software product for generating accurate Radar Cross Section (RCS) and Amplitude and Phase data for complex targets and cultural features as a function of frequency, polarization, incident angle and azimuth angle. RadBase calculates an object's RCS and Amplitude and Phase data using a hybrid geometrical/physical optics approach and includes the following effects:

• Blocking • Multiple Bounce Interaction • Edge Diffraction • Polarization • Dielectric Materials • Bistatic Computations

RadBase is a Java-based application and has a user-friendly Graphical User Interface (GUI), with easy to understand input parameters. This allows it to run on Windows 95, Windows 98, Windows NT and UNIX/Linux operating systems. RadBase currently, supports the following 3-D object model formats:

• STK .mdl Format • Open Flight (.flt extension) • ACAD (e.g. XPATCH facet extension) • Object (.obj extension)

UNIQUE FEATURES

The user does not have to be a radar expert in order to set up and run RadBase. RadBase gives the user:

• Flexibility • RadBase is a true toolkit • Users can easily control the program and vary

parameters • Accuracy on a PC • RadBase generates accurate RCS data for

complex targets • RadBase does not require the purchase of a

high-end SGI • RadBase has been validated against range

measurements • RadBase output compares extremely well with

XPATCH • Speed • RadBase is very efficient

TECHNICAL DESCRIPTION

Most available electromagnetic scattering codes valid in the high frequency regime do not properly include the effects of multiple interactions among components of the target. Several methods of treating the multiple interaction problem have been proposed, and even demonstrated for simple shapes, but have not been implemented in production software in an efficient manner capable of handling arbitrary target configurations. There are several possible approaches to the problem of high frequency multiple scattering. These may be divided into the two general categories of ray methods and field methods. Among the ray methods that have been developed are shooting and bouncing rays (SBR), the generalized ray expansion (ORE) method, and Gaussian beams. All of the ray methods are inherently approximate, and have various shortcomings based on the physics left out of the formulations. For example, SBR requires the construction of an illuminating aperture, even for exterior scattering. It also requires iteration over the number of rays used to define the incident beam. Field solution methods, on the other hand, fall into a hierarchy of methods, which includes inherently exact solutions. Among the inherently exact approaches are integral equation methods, MoM, for example, or

differential equation solutions, such as the finite element (FEM), finite difference (FD), or finite volume (FV) techniques. For high frequencies, or electrically large targets, these rapidly become impractical either because of computer time (MoM) or storage (FEM, FD or FV) requirements. Physical Optics (PO) is a field solution method obtained as an approximation to the inherently exact Stratton-Chu integral form of Maxwell's equations. Physical Optics The Chu-Stratton integrals for the total electric and magnetic fields scattered from an object can be very difficult to solve explicitly. High frequency techniques have been developed for solving these integrals. Physical Optics is an approach that is based upon source currents. PO is valid for cases where the incident wavelength is much smaller than the length of the object that is scattering the energy. In PO theory, the geometry of the object becomes very important in calculating the total scattered electric and magnetic fields. PO uses the integral equation representation for the scattered fields. It also uses the high frequency assumption that the scattered field from one point on an object to any other point is negligible compared to the incident field. Therefore the total field at each point on the surface of the object is approximately equal to the incident field at that point. The scattered field is now reduced to a much simpler equation. The surface current density for PO is

defined by: PO is based on the tangent plane approximation, so that as originally formulated it would seem to be limited to purely convex bodies. Although some attempts have been made to include multiple interactions, these have not been completely successful. Approach The approach that SOC has adopted is that of hybrid geometrical/physical optics. The methods of physical optics have proven to be extremely useful for predicting exterior, or non-multiple scattering. The accuracy of this method has been shown to be much greater than what might be expected from the approximations on which the method is based. Physical Optics, in combination with the physical theory of diffraction, forms the basis for most industrial RCS prediction codes. Geometrical

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Optics (GO), while it might be considered extremely crude, forms the basis for ray-tracing techniques, and is expected to combine the advantages of simplicity and speed with validity for multiple interactions among electrically large components. The computational approach used in the work described here is a combination of specular beam tracing and bistatic physical optics. It consists of the following steps:

1. Planar polygonal facets represent the scatter

geometry 2. Plane wave illuminated regions of unshadowed and

partially shadowed facets define reflected and transmitted beams

3. First bounce PO contributions to the total scattered field associated with each beam are computed

4. Each reflected and transmitted beam is propagated geometrically to determine illuminated portions of facets that may contribute to scattered fields from second bounces

5. PO currents and fields are computed for visible portions of second bounce facets

6. Second bounce fields are added coherently to first bounce fields

7. Process is repeated for N bounces Figure 1 illustrates schematically the hybrid GO/PO approach.

Figure 1. Schematic example of hybrid GO/PO multiple scattering approach

Additional features of the hybrid GO/PO multiple scattering approach described here may be summarized as follows:

• Closed form algebraic expressions are used to compute monostatic and bistatic PO scattered fields

• Full scattering matrix, co-polarization and cross-polarization, is used at each GO bounce, producing full polarization dependent cross section

• Conducting surface, IBC, or multi-layer coating modeled with Fresnel reflection coefficients

Edge Diffraction Physical Optics does not correctly predict radar scattering properties for objects that contain sharp edges, tips, corners, etc. The PO equations depend upon the surface currents defined at each facet of the object. PO predicts surface currents at edges that differ significantly from measurements.

Many techniques have been developed for predicting the contribution of edge diffraction to the RCS of an object. SOC has incorporated the Sommerfeld technique for computing the RCS and Amplitude & Phase contribution of edges. The Sommerfeld technique is too detailed to go into in this document and the reader is encouraged to examine Reference 2.

Additional Features One of the important features of the RadBase approach is that the GO interactions, which constitute a large percentage of the calculations, are frequency independent. It is only the PO portion, the contribution to the total scattered field from each element visible to the source that depends on frequency. Each time a PO calculation is encountered a frequency sweep can be carried out. Another feature of the SOC approach is that the RadBase code uses table look up rather than direct calculation of Fresnel reflection coefficients, so that the form of the bistatic PO integral is the same for conducting or nonconducting surfaces. These features lead to the following advantages:

• A set of frequencies can be covered at a cost much less than that of doing each one independently

• No more time is required for coated or otherwise nonconducting surfaces that for perfectly conducting

The equation used for the bistatic scattered field Es(r) at an observation point r is given by:

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N are the components of the incident field. The method preserves all phase and polarization information, making it much more useful than conventional ray tracing. RadBase INPUT

A RadBase session begins with the main menu being presented to the user. Figure 2 presents a sample of the RadBase main menu.

Figure 2. RadBase Main Window

RadBase requires as input: 1. 3-D wireframe model of the object

Figure 3. Wireframe Model of a Missile

2. User defined radar, geometry and option parameters User Defined Parameters Under the "Setup" menu, the user chooses the type of run to be performed:

• Custom • RadarWorks™ (RadarWorks is a trademark of

MultiGen-Paradigm, Inc.) • STK™ (STK is a trademark of Analytical

Graphics, Inc.) Custom Setup

RadBase requires specific radar, geometry and option parameters in order to develop the output RCS and Amplitude & Phase databases. Figure 4 presents the RadBase "Setup Custom" panel.

Figure 4. RadBase Custom Setup Panel

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The user defined input parameters are: • Target ID

The Target ID is an integer value placed in the header of the output files. This is useful for simulation systems that contain many targets, allowing the system to uniquely identify each target and assign the proper RCS data.

• Bounces This option gives the user the ability to control the number of multiple bounces off of the object that the radar beam can undergo before returning to the receiver.

• Model Units There are many modeling packages available to develop three-dimensional models of targets and cultural features. Users can create models in a variety of units, therefore, RadBase requires the user to input the units of the input model.

• Output Format: ASCII or Binary The output database can be either ASCII or binary.

• Edge Diffraction (Yes/No) Edge effects can have a significant effect on the RCS of a complex object. RadBase includes edge diffraction effects coupled with the physical optics computations. RadBase is delivered with a routine that automatically generates object edges. It uses the information in the vertex and facet file to generate these edges. If edge diffraction is chosen, RadEdge generates edge information that is passed directly into the RadBase edge diffraction routine.

• Maximum Interior Wedge Angle (MIWA) (Degrees) The maximum interior wedge angle defines the maximum angle at which two facets form an edge. Angles greater than this angle are not included in the edge computations. This prevents inclusion of edges in which the interior angle is close to 180o. The default value for this parameter is 100o. It is recommended that the user choose values between 100o and 170o. The limits are:

90o < MIWA < 180o

• Frequencies (GHz) (.1-100 GHz) Ø Uniform Frequencies (Begin, End, Step)

The user can input a begin, end, and step frequency. The uniform frequency option is useful for generating RCS and Amplitude and Phase data to be used in a SAR processor, or if a known desired frequency spacing is desired. The maximum number of frequencies is 256.

Ø Discrete Frequencies The user can input up to 256 discrete frequencies in any given RadBase run.

• Elevation Angles (Begin, End, Step) The elevation angles must be input in degrees. The range of angles is from -360o to 360o. (0o to 180o) or

(-180o to 0o) defines the upper hemisphere and (180o to 360o) or (-360o to -180o) defines the lower hemisphere. There is no limit on the number of elevation angles allowed.

• Azimuth Angles (Begin, End, Step)

The azimuth angles must be input in degrees. The range of angles is from -360o to 360o. Rotation is defined as counter-clockwise for an observer sitting above the target. Thus, if the nose of the aircraft points down the positive x-axis, 90o is viewing the left wing as defined by the pilot and 270o is viewing the right wing. There is no limit on the number of azimuth angles allowed.

• Bistatic Angles RadBase has the ability to perform monostatic and bistatic computations. If the user “checks” the “Monostatic” box, then no additional input is required. If, however, the “Monostatic” box is not checked, then RadBase will read two additional angles:

• Transmitter Elevation Angle (Degrees) • Transmitter Azimuth Angle (Degrees)

In this case, the begin, end and step elevation and azimuth angles become the receiver angles.

DIELECTRIC MATERIALS Users now have the ability to assign complex treatments to the object being modeled. Each facet of an object can have its own unique material or radar treatment. Users can build their own materials and radar treatments without ever having to edit a file. All material input, treatment design and treatment assignments are performed with RadBase’s easy to use GUI’s.

Material Designation Users can work from the default set of materials delivered with RadBase or add their own materials to the database. Figure 5 presents the RadBase Material Window.

Figure 5. RadBase Material Window

The Material Window lists all of the materials currently available in the database. The process for adding a new material to the database is simply:

1. Enter a material name

2. Enter a material description

3. For each frequency that you have data, enter the frequency, complex permittivity and complex permeability values

4. Click the Add/Replace button and the new material is loaded into the database

Treatment Designation

A treatment is defined as a user specified number of materials (1-N) layered together. Users can work from the default set of treatments delivered with RadBase or develop their own treatment based upon the materials in the materials database. Figure 6 presents the RadBase Treatment Window.

Figure 6. RadBase Treatment Window

The “Select Material” button presents a list of all of the materials available in the RadBase database. The user simply highlights the material for the first (top) layer and then inputs the thickness and sheet impedance values for that layer. If more materials/layers are desired, simply repeat this process until the desired material stack is built. This becomes a new treatment that can be applied to any facet of the object.

Treatment Assignment to Facets

RadBase provides a simple method for assigning treatments to individual facets or groups of facets. The “Assignment” button in the main window presents a list of the object’s components and all available treatments.

Figure 7 shows the assignment window that is presented.

Figure 7. RadBase Assignment Window

The user simply clicks the component and then clicks the treatment desired for that component. These choices are highlighted in blue in their respective windows. The next step is to click the “Assign” button. The actual component will be highlighted with a new color in the main image window and the treatment assigned to that component will now be listed. Figure 8 presents the new main window.

Figure 8. RadBase Main Window with Treatment Assignment

Upon execution, RadBase internally calculates the reflection and transmission coefficients for all treatments assigned through this process.

RadBase OUTPUT

RadBase outputs one binary or ASCII file. The prefix of the input file is used for output. For example, if the file, "barn.flt" is input, the output file will be named, "barn.rcs". The .rcs file contains RCS data and complex field data for all user-defined frequencies, four linear polarizations and all user defined elevation and azimuth angles. Figure 9 presents an RCS versus elevation angle plot generated by RadBase.

Figure 9. RCS vs. Elevation Angle Plot Generated by RadBase

VALIDATION

RadBase has been validated against range measurements of an aircraft. Figure 10 presents an image of the aircraft measured in the radar range and Figure 11 presents an image of the modeled aircraft.

Figure 10. True Figure 11. Modeled

Figures 12 and 13 present plots of the RadBase predicted RCS versus the measured data. It can be seen that RadBase compares extremely well with the measured data.

Figure 12. Measured vs RadBase Modeled Aircraft 5 GHz; VV; 42o Elevation

Figure 13. Measured vs RadBase Modeled Aircraft 5GHz; HH; 42o Elevation

COMPARISON WITH XPATCH

RadBase has been compared to the radar scattering software package, XPATCH. XPATCH is a set of high-frequency radar signature prediction codes that are based on a method called the Shooting & Bouncing Ray (SBR) technique. XPATCH was chosen for this study because it has been well validated and documented. Figure 14 presents a three dimensional image of a Scud Launcher used as input into RadBase and XPATCH. Figures 15 and 16 present comparisons of RadBase and XPATCH for the Scud Launcher. The figures show that RadBase compares extremely well with XPATCH, and the two produce almost exact results at the major peaks (0o, 76 o, 90 o and 180 o). Figure 17 presents a comparison of RadBase and XPATCH for a faceted missile. Again, the two software systems produce very comparable results. These results are only a small sample from a complete RadBase/XPATCH study performed by Surface Optics.

Figure 14. Scud Launcher used for RadBase/XPATCH Comparison

10 GHz; 22=30 o; VV

Figure 15. RadBase/XPATCH Comparison for a Scud Launcher – 10 GHz; 22=30o; VV

10 GHz; 22=30 o; HH

Figure 16. RadBase/XPATCH Comparison for a Scud Launcher – 10 GHz; 22=30o; HH

10 GHz; MM=0 o; VV

Figure 17. RadBase/XPATCH Comparison for a Faceted Missile – 10 GHz; MM=0o; VV

APPLICATIONS RadBase has applications in many areas of radar research, design and simulation. It is currently being used by radar design engineers, radar experts in the real-time visual simulation/sensor simulation community, and scientists performing Synthetic Aperture Radar (SAR) image interpretation and simulation.

RadBase gives the user the ability to generate radar databases for a variety of applications:

• Radar Simulation

• Target Signature Analysis

• Radar System Analysis

• Radar System Performance

• Radar Design

• SAR Image Interpretation

• Human Factor Studies

• Radar Operator Training

• Input to other COTS Products

Ø STK/Radar™

Ø RadarWorks™

CONCLUSIONS RadBase is a user-friendly software product for generating accurate Radar Cross Section (RCS) and Amplitude and Phase data for complex targets and cultural features as a function of frequency, polarization, incident angle and azimuth angle. It allows labs to produce radar simulation data on every PC available within their facility. Previously, this was only possible using their handful of very expensive UNIX platforms.

RadBase has been validated against range measurements and been compared extensively to validated radar scattering software packages such as XPATCH. RadBase will continue to advance significantly with new releases.

REFERENCES [1] Eugene Knott, John Shaeffer, Michael Tuley, “Radar Cross Section”, Artech House, Inc. 1985

[2] George Ruck, Donald Barrick, et al, “Radar Cross Section Handbook”, Plenum Press 1970

[3] R.T. Brown, “Bistatic Physical Optics Scattering from a Surface Described by an Acoustic or Electromagnetic Impedance Boundary Condition,” Seventh Annual Review of Progress in Applied Computational Electromagnetics, Monterey, California, March 1991