Target-aspect-dependent RCS: the effect on assumed beam angle

Traffic speed enforcement is becoming more reliant on automated speed-detection devices, particularly the use of photographically linked slant radar devices. We have conducted an analysis of whether target-aspect-dependent radar cross section (RCS) could have an effect on the accuracy of speed measurements. It was found that there is the potential for a 2.5% error (for a radar aligned at 22° to the road, with a -3 dB beamwidth of 5°) in the speed measured by the radar devices. This is because of the interference between the RCS and the radar beam pattern, if only the main lobe is considered. There are cases, however, when the interference between the RCS and the radar beam patterns are such that the strongest return occurs in the first sidelobe. In these cases, the potential speed error is about 7%. This would be unlikely to occur in practice, however, because of the inverse fourth-power relationship with range. We have not included the details of RCS sidelobe returns in the analysis.