Wideband Antenna Deconvolution for Bistatic Millimeter Wave Radar Reflectivity Measurements

Abstract

Bistatic radar measurements offer unique spatial diversity and enhanced target characterization capabilities, rendering them increasingly vital for contemporary sensing application research. The reliability of such measurements is contingent upon precise system and antenna calibration. The prevailing technique is the substitution method, which involves the use of known reference objects. We propose an over-the-air calibration algorithm for spherical bistatic measurement systems. Our method is both significantly simpler and twice as fast as existing algorithms. The application of our technique to reflectivity measurements of a metal sphere from 76 to 81 GHz demonstrates a dynamic range enhancement of up to 40 dB when compared with uncalibrated data. A comparison with simulation data demonstrates a high degree of agreement between measurement and simulation.

Figures (5)

• Figure 1: Simplified spherical bistatic measurement geometry with in-plane bistatic angle $\beta$. and antenna aperture planes move tangentially on two concentric spheres (here: circles) with the radii $R_\text{tx}$ and $R_\text{rx}$. Diagram combines both calibration $S_\text{cal}$ (blue) and radar measurement $S_\text{radar}$ (red). The target is not present for the calibration measurement. Note that the spherical geometry ensures that both antennas face the center for all bistatic angles $\beta$.
• Figure 2: The Bistatic Radar (BIRA) measurement facility at the Thuringian Center of Innovation in Mobility arxiv24andrich_birahein23_roe_vista. The target, a metallic sphere ⓐ with diameter 30 cm, is positioned in the center on a styrodur pillar ⓑ. The receiver ⓒ remained stationary and the transmitter ⓓ moved to realize a horizontal azimuth cut with both antennas fixed at co-elevation $\theta = 90^\circ$.
• Figure 3: Power delay profile (PDP) of antenna calibration measurement without a target. Line-of-sight between Tx and Rx normalized to 0 m and 0 dB. Raw measurement (black) exhibits shallow roll-off and a discrete parasitic reflection ⓐ off the positioners. In contrast, the deconvolved PDP (blue to green) has sharp edges and up to 40 dB better dynamic range within the sweet spot ⓒ. Note that the dynamic range remains stable over time. A double reflection ⓑ between the Tx and Rx positioners is also visible.
• Figure 4: Bistatic radar reflectivity of a metal sphere with diameter 30 cm measured from 76 to 81 GHz in the horizontal plane with vertical polarization. Top plot with normalization for comparability only. Note the parasitic peak ⓐ, which corresponds to \ref{['fig:calibration_time']} ⓐ. Bottom plot with antenna calibration and deconvolution. Note the significant improvement due to the deconvolution. The imperfect background subtraction leaves residual antenna crosstalk~ⓑ and styrodur pillar response~ⓒ. The strong path represents the tangential reflection off the sphere or the diffraction around the sphere in case of forward scattering, respectively. The measured bistatic range coincides with a geometric path model (circles) of the described propagation effects.
• Figure 5: Comparison of measured (same as \ref{['fig:sphere_impresp']}) and simulated radar reflectivity of sphere. Bottom plot shows absolute reflectivity difference (measurement minus simulation) for values above --55 dBsm. Note that the reflection off the sphere (marked by circles) shows excellent agreement within 3 dB except for forward scattering ($\beta \approx 150^\circ \dots\, 210^\circ$). Antenna crosstalk, i.e., line of sight ⓑ, and the styrodur sphere mount ⓒ are clearly visible.