Form and Function

In the days before widespread radar coverage, pilots flying on instrument flight plans routinely had to report their position to air traffic control, whose job it was to make sure that two airplanes didn't try to occupy the same space at the same time. That's still ATC's primary task, but today we enjoy radar coverage almost everywhere in the United States. And that means the controller on the other end of the radio is as likely to tell you where you are as the other way around. For the vast majority of pilots, radar coverage and its airborne electronic partner, the airborne transponder, have become a simple fact of life.

Air traffic control surveillance radar works on the principle of sending out high-power radio waves. The ground station then listens for the waves to be reflected from anything flying in the coverage area. Endemic to this primary surveillance system are several shortcomings, including relatively inaccurate distance and altitude feedback.

To improve the quality of radar coverage, the Secondary Surveillance Radar System was born, and with it the airborne (meaning that it is installed in the aircraft) transponder. Piggybacked to the standard ground radar system is a second transmitter that sends out a signal at high frequency, specifically 1030 MHz. Upon receiving this signal, the aircraft transponder replies with a signal of its own, at 1090 MHz.

This beacon system greatly im-proves the accuracy of the displayed targets on the controller's radarscope. In addition, more information from the aircraft, including altitude, may be relayed to the ground station. During your initial flight training, one of the first things you probably noticed about the transponder is the four-digit code selectors. By pulsing the transmission back to the ground in a specific fashion, the transponder can respond with as many as 4,096 individual codes. These codes are the numbers that you select on the face of the transponder.

Selecting a response code, or squawk as it is more commonly called, offers several advantages. A universal code, such as 1200 for VFR operations, 7600 for lost communications, and 7700 for emergencies, can be easily read and interpreted by the controller on his scope. What's more, the controller can issue a discrete code to help identity your airplane and thus make it easier to separate you from other aircraft. If you request VFR radar traffic advisories from a controller, and are told to "squawk 1701," for example, that is the discrete code the controller has assigned to you and you alone.

Transponders can have two information-sending modes. Mode A information includes only the squawk code dialed into the face of the transponder. By adding altitude information to that signal, we get Mode C information. For a transponder to transmit Mode C altitude information along with the Mode A squawk code, the aircraft must have an encoder, which is a device that converts the ambient barometric pressure reading to an electronic signal. Mode C is enabled when the mode switch is placed in the Alt position.

There's also something on the face of the transponder unit called an Ident button; this merely adds an additional information blip to the transmission that makes your depiction in the scope glow more brightly for quick identification by the controller. Finally, you will notice a reply light on the face of the transponder. It is connected to the transmitter section and tells you when it is replying to an interrogation. It does not, however, confirm that you are transmitting the correct squawk code or altitude.

Another type of transponder, the Mode S, allows ATC to selectively interrogate transponders and therefore send and receive discrete alpha-numeric messages. By contrast, all Mode A and C units respond whenever they are in range of an interrogating ground facility.

So that's the theory behind how the transponder works. In practice, there are only a few things that you need to consider when you are operating the transponder. Typically we are taught to place the mode switch in Standby before starting the engine and taxiing. Most transponders use a vacuum tube in the transmitter that requires some warmup time. The Standby mode is the warmup mode.

If you select the transponder On or Alt setting while on the ground, it could confuse local radar. If your airport is served by radar on the field or nearby, it is possible that your transponder might respond to the interrogations and create an image on the controller's scope even though the airplane is still on the ground. Obviously, he isn't worried about an aircraft that's taxiing, so it's just a nuisance. Switch the transponder from Standby to On or Alt when taxiing onto the active runway for takeoff.

The aircraft transponder/ATC radar system is prone to occasional problems. One day you may be receiving VFR radar traffic advisories from ATC and the controller will tell you that he is no longer receiving your transponder signal, even though you are using the correct squawk code and have set the unit to the Mode C altitude reporting switch position. The controller may ask you to "recycle your transponder." This means that you should turn the mode switch from Alt or On to Standby and back, which could solve the problem. The problem also could be caused by poor ATC radar coverage or a glitch in the radar, or perhaps a grease-covered transponder antenna on the aircraft belly. (During your preflight, it's a good idea to make sure that the antenna is reasonably clean.)

You may also hear a controller question your altitude. For example, you could be flying at 5,500 feet msl, and the controller will report that your Mode C altitude readout on his radar shows you at 5,100 or perhaps 4,600. Usually, the controller will politely remind you of the local altimeter setting or ask for your altitude-this is a hint that he thinks you are not at your assigned altitude. If your Mode C altitude differs from your true altitude by more than 300 feet, the controller typically will ask you to "stop altitude squawk," which means that you should switch the transponder mode setting from Alt to On. This could be an indication that your transponder or encoder is malfunctioning-or not. A software or mechanical problem in the ATC end of the system also could account for the error.

For example, incorrect codes have been known to appear on the controller's scope even though the right code appears on the transponder's face. That means it's time to select another code. One way to check on where the problem lies is to ask a subsequent controller to confirm your altitude readout on his radar. If your actual altitude differs significantly from what the controller sees, the problem probably is in the airplane's transponder or encoder. Sometimes knobs stick be-tween numbers. If that doesn't seem to be the problem, have the avionics shop take a look.

For reasons of correct transmitter alignment and altitude information, the transponder and the encoder system must be inspected every 24 months. The inspection confirms that the encoder sends the correct altitude information and that the transponder sends the correct squawk code.