Measurement and analysis of ambiguity functions of passive radar transmissions

Passive coherent location (PCL) radar systems make use of broadcast or communications ´illuminators of opportunity´ in a bistatic configuration. In order to understand the performance limitations of this type of radar it is necessary to know the ambiguity properties of these waveforms, and how they vary with the form of modulation and with the bistatic geometry. This paper presents and analyses the ambiguity functions of a set of off-air measurements of signals that might be used for PCL systems. We find that the ambiguity behavior of analog modulation formats, such as FM radio or analog television, depend significantly on the instantaneous program content, and can be very poor, for example during pauses in speech. Digital modulation formats, in contrast, are much more favorable and much more constant with time. This suggests that the choice of signals to be used for PCL may be made on a dynamic basis, according to the modulation and bistatic geometry.