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
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.