The TDR-90 Transponder is a single unit equipment that is part of the Collins Proline II series. The TDR-90 Transponder is an integral part of the Air Traffic Control Radar Beacon System. The TDR-90 provides identification of transponder-equipped aircraft on the ground controller's plan positions indicator (PPI). The TDR-90 is interrogated by radar pulses received from a ground station and automatically replies with a series of pulses. Reply pulses are coded with an altitude digitizer, automatic altitude reporting.  Antenna, control unit, mount,and adapter kit are required for operation of the TDR-90 system. These units are not supplied as part of the system. The TDR-90 Transponder replies to all valid ATC radar interrogations with a coded reply signal. The reply signal is used by the ATC controller to locate and identify transponder-equipped aircraft. The TDR-90 transmits on a frequency of 1090 mHz and receives on a frequency of 1030 mHz. A side-lobe suppression system is included in the TDR-90. Side-lobe suppression prevents triggering by the side-lobe radiation from the secondary surveillance radar. Side-lobe triggering could supply the ATC controller with false information, thus obscuring the aircraft location. The received interrogation signal is analyzed to determine its validity and mode of operation. For this signal to be valid, it must be from the main lobe of the secondary surveillance radar and be mode A or C. When an interrogation signal is valid, a reply signal is transmitted. The ALT/OFF switch on the control panel enables the transmission of mode C pulses supplied from the altitude digitizer. The TDR-90 is capable of producing 2 to 16 coded reply pulses. The TDR-90 is housed in a 1/4-ATR short low-profile case. The TDR-90  is 3.50 in. high, 2.420 in. wide, 13.844 in. long and has a nominal weight of 3.5 pounds.