Transponders: They make the whole system work

March 1, 2004

They make the whole system work

By Jim Sparks

Air Traffic Control (ATC) has jokingly been called "an organization funded by the railroads to discourage travel by air." However, considering the total number of aircraft operations daily and the rare incidents of collisions, it is evident that the system works! One device makes all this possible - the transponder.

Transmitting signals
A transponder will send an identifying coded signal in response to a transmitted interrogation from a ground-based radar station. An air traffic controller can then view the identified blip on a screen and know who it is and provide direction to the flight crews maintaining adequate separation with other blips. More recent versions enable aircraft to recognize other aircraft in the area and subsequently provide alerts to the flight crew so they can avoid potential hazards.

What's required
Transponder components on aircraft include a receiver-transmitter, control head, digitizer, and antenna.

On the ground, primary surveillance radar transmits a narrow radio frequency (RF) beam using a rotating antenna. Any target in the beam path will reflect some energy. The elapsed time between the transmission and the energy return can then be calculated and by taking into account antenna position a precise bearing and distance can be displayed on a two-dimensional radar screen. A secondary surveillance radar (SSR) provides the controller with aircraft identification and altitude information. This is accomplished by transmitting groups of pulses. The first provides aircraft identification data and the second gives altitude information.

The basis of TCAS
Transponder Mode "A" is the reference for identification and Mode "C" is the term applied to altitude information. And Mode "S" is now used to enable aircraft to have in-flight communication with other aircraft permitting position and course comparisons. This becomes the basis for a Traffic Collision Avoidance System (TCAS).

Codes distinguish each aircraft. They are typically assigned prior to flight and entered in the transponder control head by a flight crewmember. Certain codes are used when an aircraft is in distress such as 7700 or 7777, and 1200 represents that the aircraft is flying using Visual Flight Rules (VFR). This is recognized by the ground-based equipment and is then highlighted on the air traffic controller's screen making them aware of the situation.

Altitude encoding is a feature associated with transponder Mode C and the response uses a gray code also known as a Gilham code. This is a special binary format using 11 pulses and is located between the main pulses of the transponder output signal. Each pulse represents a specific altitude increment.

Received interrogation signals are passed through a detection circuit called a duplexer within the transponder which will control the switching of the antenna from receive to transmit. The signal is further monitored and conditioned to make it electrically manageable and to validate the pulse pair groups and ensure the aircraft does not respond to an inaccurate signal.

Once the pulses are determined to be valid they are passed on to a decoder. It is here that the determination is made regarding the mode of operation. It is the decision at this point that will decide the format of the transponder response to the ground-based equipment. While the reply is being prepared a suppression signal is also generated to other L band equipment installed on the aircraft. This will include devices such as Distance Measuring Equipment (DME) and is used to block or deactivate the receivers within these systems during the short response transmission.

Transponder systems include an "IDENT" switch. This flight deck located device can be selected by the flight crew at the request of an air traffic controller. Activation will cause the identification of the aircraft on the controller's screen to enlarge allowing the aircraft to stand out on the display. This is accomplished by adding an additional pulse to the transmission and is often referred to as a SPIP.

After creating a coded response including identification and altitude information, the data is sent to be imposed on the radio wave that will carry it back to the ground station. Then the duplexer accomplishes the switching enabling the transmitter to couple with the antenna. This process takes about two microseconds from the time the initial interrogation is received. Once the signal reaches the SSR antenna on the ground the information is decoded, analyzed, and then displayed in front of the controller.

Most systems contain added protection that would prevent excessive interrogations. This means the transponder will reply to a maximum of 2,000 requests per second. The normal interrogation rate is about 400 per second.

The Mode S transponder is an enhancement to the air traffic control process. This feature provides a two-way digital data link for either air to ground or air to air communication. Airborne Separation Assurance (ASA) occurs when the system is interfaced with a Traffic Collision Avoidance System (TCAS). The purpose is to allow a TCAS equipped aircraft to fly within a continuously monitored airspace, allowing the flight crew to acquire and locate other aircraft that may pose a threat. This is accomplished by enabling a TCAS outfitted aircraft to transmit an encoded "All Call" request to any surrounding aircraft using a transponder.

When a nearby aircraft replies to the interrogation it is received and additional antennas can locate the source of the signal on a two-dimensional display. In addition if the target aircraft is using a Mode C or Mode S transponder, vertical data is added further aiding the pilots in recognizing a potentially dangerous condition. Traffic Alerts are displayed 40 seconds prior to a close encounter and a Resolution Advisory (RA) is issued by the TCAS about 25 seconds before the anticipated closest point in the paths of the two aircraft. Some aircraft contain dual transponders. One is operating and the other is used only if the primary unit should fail. Some of these installations will use a Mode S transponder as the primary and a Mode C as the backup. Here the TCAS system will only function with the primary unit in operation.

Regulations and maintenance
Federal Aviation Regulation (FAR) 91.413 addresses transponders and it reads as follows: "No person may

Top: Technician using ramp tester to verify transponder operation. Middle: Transponder test set. Bottom: Transponder L band antennas. Left is No. 1 and right is No. 2.

use an ATC transponder unless, within the last 24 months, the device has been tested and inspected and found to comply with appendix F of FAR Part 43." The regulation goes on to say that following any maintenance where data correspondence error could be introduced, the integrated system must be tested and inspected. In addition the regulation is quite specific regarding who can perform the tests.

Correspondence error could be introduced any time an electrical connection is removed from any component in the transponder or altitude reporting system. In some cases the altitude information comes from an encoder which is fitted to the altimeter. In other situations an air data computer (ADC) may be the provider. In the event an encoding altimeter has to be removed from the instrument panel or if the transponder control head has to be disconnected to gain access to another component the system has been breached and data transmission might be compromised.

Biannual testing includes but is not limited to verification of operating frequencies, power output, response to at least 90 percent of interrogations and verification of the suppression signal. In fact the testing requirements in Part 43 vary with the class of transponder installed. This should always be considered prior to beginning any checks.

Common problems with transponders often involve electrical bonding between the antenna and airframe or faults with the coax cable connecting the antenna to the receiver transmitter.

Many of the systems being produced and installed today use digital technology and include self-test capabilities. When associated with a digital control head and a digital air data computer, the entire system can be self-monitored and theoretically when any connection is broken the self-test should be able to verify system integrity. I asked my local Airworthiness Inspector about using the self-test to satisfy the FAR 91.413 requirement to verify no correspondence error; his comment was, "There is nothing like physically seeing it operate." This is a philosophy that is consistent with the nature of our industry.

What the future holds
Next up, for new aircraft delivered after March 31, 2004 upgraded Mode-S transponders with flight identification will be required for all aircraft regardless of weight for all IFR, and VFR flights within Europe. In service aircraft compliance is slated for March 31, 2005.

Reduced Vertical Separation Minimums (RVSM) is an issue that will impact many of us working with aircraft that routinely operate above Flight Level 290 (29,000 feet). This will go into effect in January 2005 for the continental United States and even though TCAS is not a requirement for RVSM operation, any aircraft incorporating collision avoidance will have to upgrade to TCAS version 7. Prior to this an alert would be issued if an aircraft were perceived to be within 1,250 feet of a TCAS equipped machine. With reduced vertical separation TCAS version 7 will provide the alert at 850 feet.

The transponder is an essential contributor to aviation safety and it is our responsibility as technicians to ensure continued proper operation. This will continue to maintain the well-being of passengers, crew, and the aircraft. After all I don't think most of us would look good carrying oil cans, or lubricating wheels on a train.