An interrogation responder system composed of a transmitter stationarily mounted in a certain fixed place and a receiver mounted on a mobile object. The receiver on the mobile side starts the operation when a power is supplied to the entire circuit in accordance with a wake-up signal from the transmitter on the fixed side so as to modulate the electromagnetic signal emitted from the transmitter on the fixed side in accordance with a predetermined code and reflect a response. This enables continuing bilateral communication of a command, data and so forth. By starting communication between the transmitter and the receiver by a direct wave of a carrier signal in a microwave band after sensing that the receiver on the mobile side has entered a good communication zone, secure communication is enabled at a sufficient electric field strength.

A multi-functional radar transponder functions as SART in an emergency condition and as monitor, GPS receiver and VHF transceiver for non-emergency conditions. The radar transponder has an antenna including a SART antenna, a GPS antenna and a Marine VHF antenna. The radar transponder further includes a SART circuit with a monitor output device for monitoring a radar signal, and transfer switches for transferring a monitor process in a normal mode to a responding process in an emergency mode, and vice versa. The GPS antenna closes to a position parallel to the SART antenna in the normal mode and opens vertically against the SART antenna in the emergency mode. The Marine VHF antenna locks the GPS antenna together with the SART antenna for preventing a false emergency response in the normal mode and unlocks the SART antenna for an emergency response in the emergency mode.

A control circuit of a rader transponder produces pulses when the radar transponder detects a radio wave from a search radar. The pulses are sent to a sweep signal generator in which sawtooth waves are obtained through a first constant current circuit for linearizing the voltage-to-time characteristic in a falling portion of a sawtooth. The sweep signal generator further has a second constant current circuit for linearizing the voltage-to-time characteristic in a rising portion of a sawtooth.

This is a radar transponder in which an amplifying means using a FET amplifier is inserted between a receiving antenna and a detecting means. Further, the radar transponder has a receiving on/off control circuit which stops the operation of the amplifying means in transmitting a response signal in order to enhance the receiving sensitivity thereof. Also, the radar transponder mounts the amplifying means, and the detecting means via a metal body having resilience on an antenna housing. Each circuit unit of the radar transponder and connection wirings between each circuit unit are formed by flexible printed circuit boards.

The present invention provides a search and rescue radar transponder easy in maintenance and checking. In the search and rescue radar transponder of the invention, the transmission electric power of the transmitting circuit 3 leaked at the changeover switch 7 is inputted to the receiving circuit 2, and whether the search and rescue radar transponder operates normally or not is judged by comparing the leaked electric power level detected by the receiving circuit 2 with a prescribed level at the controlling circuit 4. Hereby, the operation-checking of the search and rescue radar transponder proper other than an antenna can be executed by a built-in apparatus, and its maintenance and checking can be executed easily and rapidly.