Part II: Canonical Targets in R&D RCS Measurement Range Operations

The PEC Squat Cylinder

In our previous article, we presented the PEC sphere as the first in a family of canonical targets whose scattering properties enhance practitioner understanding of scattering behavior, provide a mechanism to measure range health, and encourage the use of quality measurement tradecraft. This article is the second in a series of five where we describe the scattering properties and use of the PEC squat cylinder (PSC).

Use of the PSC came to prominence in RCS measurements in the early 1990’s as the community sought to establish a uniform understanding of calibration errors.^[1],[2],[3] The PSC has distinct advantages over the sphere as a calibration target. The sphere’s strong bistatic mechanism illuminating the support structure is replaced in the cylinder by a much weaker diffraction term as shown in Figure 1, especially as compared to the specular term at the front surface of the cylinder. While there exists a creeping wave at H-polarization, additional diffractions at V-polarization, and mutual coupling between the cylinder and its foam mount (or between the cylinder and pylon), all are significantly lower in magnitude than the specular term. These combined advantages ensure a well-behaved cylinder frequency response over a large bandwidth.

[1] H. M. Chizever, R. J. Soerens, and B. M. Kent. "On Reducing Primary Calibration Errors in Radar Cross Section Measurements." Antenna Measurement Techniques Association (AMTA). AMTA 1995, pg 383.

[2] L. A. Muth, R.C. Wittman, and B. M. Kent. “Interlaboratory Comparisons in Radar Cross Section Measurement Assurance.” Antenna Measurement Techniques Association (AMTA). AMTA 1997, pg 297.

[3] B.E. Fischer, B. M. Kent, B.M. Welsh, W.D. Wood, and T.M. Fitzgerald. “Moment Method Inter-code Comparisons and Angular Sensitivity Studies for NIST Calibration (Squat) Cylinders.” Antenna Measurement Techniques Association (AMTA). AMTA 1998, pg 414.

In the mid-1990s, AFRL instituted a novel calibration validation procedure where an operator would calibrate one PSC against another of similar but unequal dimensions over a band of interest. The ratio of this calibrated PSC to its full-wave frequency response became known as the “dual-cal” ratio.” This metric, usually measured at the onset of a measurement series, would produce a frequency response for each polarization with a variance as low as +/-0.25dB across the band of interest as reported by some. This procedure was instituted by numerous chambers across government, academia, and industry in a collaborative measurement exchange program. To aid in this collaborative endeavor, a family of six aluminum PSC targets were constructed by AFRL with the dimensions shown in Table 1.

In the mid-1990s, AFRL instituted a novel calibration validation procedure where an operator would calibrate one PSC against another of similar but unequal dimensions over a band of interest. The ratio of this calibrated PSC to its full-wave frequency response became known as the “dual-cal” ratio.” This metric, usually measured at the onset of a measurement series, would produce a frequency response for each polarization with a variance as low as +/-0.25dB across the band of interest as reported by some. This procedure was instituted by numerous chambers across government, academia, and industry in a collaborative measurement exchange program. To aid in this collaborative endeavor, a family of six aluminum PSC targets were constructed by AFRL with the dimensions shown in Table 1.

The frequency response of each cylinder was measured by multiple ranges around the country in a “round-robin” exchange, sharing results for both indoor and outdoor ranges, with a goal of developing a unified calibration procedure across the RCS community. The “dual-cal” cylinder pairs were usually measured as “375-450”, “750-900”, and “1500-1800”. The RCS of each pair for H-polarization is shown in Figure 2.

The low frequency usable limit for each cylinder pair is determined by the resonance region nulls revealed by the ratio of one PSC to its closest pair. For example, the ratio between the “750” and “900” PSC is shown in Figure 3. The full frequency response ratio is shown on the left with resonance and optical regions shown, with the expanded resonance region ratio is displayed on the right. The “750-900” pair ratio has a first resonance null at about 2 GHz. Extending the usable region below 2 GHz would greatly increase uncertainty due to the slope associated with the other nulls, an unacceptable trait for calibration targets. The “1500-1800” PSC pair has a usable lower limit of 1 GHz while the ”375-400” pair has a usable lower limit of 4 GHz.

The disadvantage of the PSC as compared to the sphere is mounting accuracy on foam column supports. Where the sphere has no “level” position, the PSC must be aligned with the direction of the incident field such that the Poynting vector is orthogonal to the PSC face or parallel to the top and bottom surfaces. This requires an operator to place an inclinometer on the top of the cylinder parallel and perpendicular to the incident field, using low density foam wedges to ensure the level of cylinder. This can be a laborious process, especially when calibrating at higher frequencies, where minor elevation errors can result in significant calibration errors. With practice, most operators gain proficiency in cylinder leveling in a short period of time.

Finally, as in the case of a sphere, the PSC can be offset on a foam column or top hat-style rotator (Figure 4) to examine magnitude and phase variations throughout the target region as the cylinder rotates. This mapping of the scattered electric field into a circle in the complex plane (Figure 5) provides valuable insight regarding the incident field and error sources such as stationary clutter and RFI, without the need for complex and costly field probe machinery.

Conclusion

The PEC squat cylinder provides a stable scattered field frequency response over a wide bandwidth, without the complications from bistatic scattering commonly associated with calibration spheres. While cylinder mounting is more complex than spheres, it becomes second nature for most operators in little time. The cylinder also provides an opportunity to gain insight into clutter, RFI and other errors associated with RCS measurements. Whether used in a calibration context or as a tool for understanding the characteristics of the incident field, the PEC squat cylinder is an indispensable tool for RCS range operators.

[4] L. A. Muth, C. M. Wang and T. Conn, "Robust separation of background and target signals in radar cross section measurements," in IEEE Transactions on Instrumentation and Measurement, vol. 54, no. 6, pp. 2462-2468, Dec. 2005, doi: 10.1109/TIM.2005.858126.
[5] Ibid.