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.
The DME transmitter produces 100-500 watt pulsed signals. The DME is frequency paired
to a DME enabled VOR (military designation is a TACAN), or ILS. The channeling of
the DME is usually done by the NAV frequency selector. The DME transmits and receives
in the 900-1200Mhz band. The equipment manufactured within the past 20 years primarily
uses solid state transmitters. Earlier equipment employed a vacuum tube type transmitter.
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.