A receive-only radar used for search, tracking, surveillance, identification, guidance, and mapping. The techniques used are similar to those in radiometry; passive radar could be said to be a branch of radiometry. Military applications are quite different, however, from other uses of radiometry in that the source is usually not known to exist until detected by the radar, and the operator has no control over the position of the source but must determine it and sometimes track it. There are often other sources in the vicinity, so that discriminating between competing sources is important in military applications. The operation of passive radars depends upon the detection of microwave or infrared radiation from warm bodies.