Date of Award

5-1990

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Electrical Engineering

Abstract

The accuracy of traffic control radars has been in controversy for several years. The purpose of this thesis has been to experimentally investigate the contribution of one facet arising from the doppler radar returns from automobile wheels. By measuring the electromagnetic backscatter profiles of an assortment of automobile wheels over a normal operational range of viewing angles, the locations of the dominant scattering centers on the wheels could be identified. Through knowledge of the locations of these scattering centers, velocity error assessments could be made.

Light truck and car wheels were the targets examined in this thesis. A target pedestal was built to support and rotate the wheels and rear-end housing. This consisted essentially of a ’68 Mercury differential housing with a 2.75:1 gear ratio was used to spin standard 14" and 15" wheels. A 5 hp dc motor was used to turn the wheels at 1130 rpm for the measurements.

A C-band and two X-band CW doppler radar systems were built. These radars were operated at frequencies of 6.115 GHz and 9.9 GHz respectively, and were used to detect the doppler frequency shifts occurring from the rotating wheels.

Measurements were taken for five different hub cap configurations. Vertical and horizontal polarizations were initially employed to examine the relative spectral characteristics of rotating wheels for look angles varying from 0° to 30°. After examination of preliminary data indicated that there was little difference between the returns taken in the two polarizations, it was determined that only vertical polarization data needed to be amassed. Data analysis indicates that the dominant returns from the wheels studied occur at look angles in the range 10° - 20°. Depending upon the physical characteristics of the hub cap (holes, lug nuts, etc.), the velocity of a vehicle as measured by a traffic control radar would have errors on the order of 25 to 35 % if the dominant return would be occurring from the wheel. Thus, a vehicle actually traveling at 55 mph would appear to be traveling 68 to 74 mph.

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