RCS Simulations and Measurements of an Autonomous Spherical RCS Calibration Device

Abstract

Ultra-Wideband (UWB) calibration of RCS measurement radar systems, particularly outdoor, ground-to-air dynamic signature measurement radars, are conventionally accomplished using calibration devices (CD) attached to static towers, tethered from balloons, dropped from helicopters or other air vehicles, or with active RF repeater systems. These methods contain errors from background-target interactions, or are operationally compromised, or are expensive. For high accuracy RCS radar calibration, a revolutionary methodology and architecture is mandatory to support demanding new radar metrology requirements. Within this paper, an update of ongoing efforts toward developing an autonomous, airborne drone-based CD is provided. The radar reflectivity of such a device must have a highly reproducible RCS, as manifested by a narrow, well defined sampling distribution. Computational electromagnetic simulations were used to predict the RCS of a 60.9-cm diameter autonomous Spherical Passive/Active Calibration Device (SPARCS) with periodic, hexagonal "honeycomb" electromagnetic screens for inlet/outlet ports of the propulsion system. Two methods were used to predict the RCS, including Large Element Physical Optics (LE-PO) and Physical Optics (PO). Both methods show good agreement with the measured RCS magnitude. PO had a better angular resolution than LE-PO, while LE-PO resulted in a 6-fold decrease in computational expense relative to PO. Narrowband RCS measurements show periodic oscillations indicative of grating lobes, which are not well captured by the PO methods. Sampling distributions from measured and simulated data are approximately normal and have standard deviations on the order of 0.08 dB. These results validate the RCS signature of the autonomous, airborne, spherical CD incorporating inlet/outlet RF screens as a valid calibration target.