A radar system in which the object to be detected is fitted with cooperative equipment in the form of a radar receiver/transmitter or a trans-ponder. Radio pulses from the interrogator (transmitter/receiver) site are received in the cooperative equipment and used to trigger a distinctive transmission from the transponder. This latter transmission, rather than a reflected signal, is then received back at the transmitter/receiver or interrogator site. The interrogation is done at 1030 MHz, whereas the response is at 1090 MHz. The system provides an automatic altitude reporting facility.