Introduction

Woo et. al. published measured and simulated results for a range of simple RCS benchmark targets in 1993, [1]:

• The NASA almond
• Simple ogive
• Double ogive
• Cone-sphere
• Cone-sphere with a gap between cone and sphere.

These targets are defined mathematically so it is easy to ensure that the correct shape is simulated during RCS benchmarking.  These targets were simulated in CADFEKO as to achieve two goals:

• Show that CADFEKO has the means to easily generate simple and complex (NASA almond) mathematical shapes.
• Benchmark FEKO's RCS abilities against accurately measured data.

All 5 targets that were presented in the [1] are simulated and the FEKO results are compared with the measured results by means of graphical overlays, i.e. the FEKO results were scaled to the exact same size and exis dimensions as the measured results before the FEKO results were then pasted over screencaptures of the measured results.  Easy assessments of the accuracy of FEKO's results can thus be made.  In all cases the FEKO result closely matches the measured result.

All figures should be regarded as thumbnails which are linked to higher resolution versions of the same graph.  FEKO only results are also available.

NASA Almond

Constructing the model

The NASA almond is not a simple body-of-revolution (BOR) shape and is essentially flattened in the z-axis, relative to the y-axis, with the x-axis being the longitudinal axis.  The mathematical definition of the curves were implemented in a Python script that outputs 5 line definitions, starting in the xy-plane, rotating in equally spaced angular increments around the x-axis, until the last line is situated in the xz-plane.  The shape is then formed by lofting two adjacent lines to form a quarter of the almond's shape in the first quadrant of the 3D Cartesian axis system.  The 4 faces that are formed in this way are stitched together to account for any mathematical inaccuracies (e.g. rounding errors) and to ensure a close surface.  The resulting shape is then mirrored through the xy-plane and the resulting surfaces again stitched, before mirroring the resulting half almond through the xz-plane to form the complete shape.  In this way a methematically precisely defined shape may be created in CADFEKO to almost arbitrary levels of accuracy.

Results

Simple Ogive

Constructing the model

The metallic ogive was constructed in much the same way as the NASA almond.  A Python script was also used to define the shape of the ogive to create a points list to import into CADFEKO, but differently to the NASA almond, the ogive is a BOR and so only a single curve is imported into CADFEKO, before being spun around the x-axis to form the final ogive shape.

Results

Figure 2: Metallic Ogive RCS Comparisons
(a) Geometry	(b) 1.18 GHz (both polarisations)
(c) 9 GHz (both polarisations)	(d) FEKO results only
	1.18 GHz (both polarisations) 9 GHz (both polarisations)

Double-Ogive

Constructing the model

The double ogive was constructed in the same way as the simple ogive to create a points list to import into CADFEKO, spinning the imported curve around the x-axis to form the final shape.

Results

Figure 3: Metallic Double Ogive RCS Comparisons
(a) Geometry	(b) 1.57 GHz (both polarisations)
(c) 9 GHz (both polarisations)	(d) FEKO results only
	1.57 GHz (both polarisations) 9 GHz (both polarisations)

Cone-Sphere

Constructing the model

The cone-sphere was created directly in CADFEKO as CADFEKO has geometric primitives to easily form half-sphere and cone sections of the shape. 

Results

Cone-Sphere with Gap

Constructing the model

The cone-sphere with gap is created in exactly the same way as the simple cone-sphere, except that the half-sphere and cone are separated by a short cylinder of the appropriate radius. 

Results

Figure 5: Metallic Cone-Sphere with Gap RCS Comparisons
(a) Geometry	(b) 869 MHz (both polarisations)
(c) 9 GHz (both polarisations)	(d) FEKO results only
	869 MHz (both polarisations) 9 GHz (both polarisations)

References