Doppler spectrum of single automobiles

The Doppler spectrum from an extended object illuminated by a finite beam is analyzed theoretically. The long-time spectrum is the product of an antenna spectrum and an object spectrum, which may lead to significant systematic errors in determination of velocity. The error probability is larger for short-time spectra. Experimental results on automobiles also indicate the existence of a very wide spectrum, which may be multipeaked with incorrect dominating peaks for short periods of time. Physically the errors may be interpreted as a result of moving phase centers.