Avionics - Pulse Equipment

Transponder DME

Commonly referred to as PULSE equipment. Transponders and DMEs are almost always classed in the same category due to their common frequency spectrum, and similarities in signal types.

Transponder

These avionics systems are the airborne component of the ATCRBS (Air Traffic Control Radar Beacon System), and SSR (Secondary Surveillance Radar) system. Transponders are also an integral part of TCAS (Terminal Collision Avoidance Systems).

A mode A/C transponder receives interrogations from the ground based radar and replies with either the code selected on the transponder or the encoded altitude. The selected code is an octal code, and is either a standard code for the procedure being flown (VFR=1200 for example) or an ATC assigned code. Some codes are for emergency use; 7700=emergency, 7600=no radio, 7500=hijack. The encoded altitude is derived from either an encoding altimeter, or a dedicated blind encoder. The altitude is encoded as a Gillham Code. The transponder transmitted altitude is always referenced to Standard Day (barometric setting of 29.92 inHg), and is converted by the computer system within the ATC ground based RADAR.

The transponder may also be interrogated by a TCAS equipped aircraft flying in relative proximity to provide the TCAS aircraft with the interrogated aircrafts relative position and altitude.

Mode S transponders are capable data linking, and unique aircraft identification coding.

Airborne transponders produce a 100-500 watt pulsed signal at 1090Mhz, and receive replies on 1030Mhz. The latest generation transponders use a solid state transmitter. Older units use a high voltage cavity, which is a style of vacuum tube.

Common Problems

• Alignment: The alignment of the receiver/decoder and transmitter are critical for serviceable performance of the transponder. We have two dedicated benches for Transponder and DME service. All transponder transmitters are aligned and tested in accordance with their approved maintenance manuals. Cavity drifting: The cavity transmitter tube is a vacuum tube excited by a high voltage power supply. As the cavity ages its calibration and linearity drifts. We test all cavity based transponders using our line stretcher to test the cavities VSWR characteristics. If it is 'out of spec', it needs to be replaced. Code Slipping: Erroneous codes can be caused by either switch problems, or logic problems. ATC will report that the wrong code is being received or that your 1500' approach reads out 41,000'. Logic troubleshooting can be laborious. We have veteran technicians that are up to the task.
• DME: Distance Measuring Equipment computes distance by measuring the time it takes for a transmitted pulse to go from the aircraft to a ground based receiver, and back to the aircraft. The measured distance is the 'slant range', or distance plus altitude hypotenuse. The ground speed is also computed by the DME by averaging the change in distance. Distance is displayed in NM to the station, and either time to/from station or ground speed.

Common Problems

• Low Power: If either the transmitter or receiver is weak the DME will have poor range. With our test equipment and experience we can isolate the problem and effect a serviceable repair.
• Receiver: Because the frequency of the DME signal is so high, the troubleshooting of the receivers is very difficult. We have dedicated test equipment including signal generators, spectrum analyzers, and digital oscilloscopes to assist our experienced technicians.
• Transmitters: As with the DME receivers the signals are in the microwave spectrum. The DME transmitters usually use stripline wave guides, and very fast solid state electronics. Troubleshooting transmitters is difficult and alignment usually requires high frequency sweep generators.