SSR and the Transponder - ATPL 062 — Radio Navigation

The last lesson's radars listened for echoes. Secondary surveillance radar (SSR) listens for answers: the ATC (air traffic control) system is based on the replies provided by the airborne transponders, in response to interrogations from the ATC secondary radar. Keep the roles straight against the DME (distance-measuring equipment), which uses the same vocabulary the other way round: in the DME, the interrogator flies and the transponder stands on the ground; in SSR, the interrogator stands on the ground and the transponder flies. And the ground is not the only questioner: the airborne transponder provides coded-reply signals in response to interrogation signals from the ground secondary radar and from aircraft equipped with TCAS (traffic alert and collision avoidance system).

Why secondary beats primary

The system, then: the ground interrogates, the transponder replies, and TCAS asks the same transponder from the air. What does all that questioning add? The ground SSR uses techniques which provide ATC with information that cannot be acquired by the primary radar: an echo can only say where something is, while a reply can say who it is and how high. The advantages of SSR over a primary radar concern range and collected information, and both come from the transponder's active participation: instead of scraping position from whatever fraction of a pulse reflects back, the SSR's signal only has to reach the aircraft, where the transponder transmits a fresh, full-strength reply of its own. Each leg is one-way, so far less power achieves far more range, and the reply carries data no reflection ever could.

Pulse pairs and the three modes

The question itself is minimal: the interrogator transmits its interrogations in the form of a series of pulse pairs, and the decoding of the time interval between the pulses of a pair determines the operating mode of the transponder. The interrogation modes are Mode A, Mode C and Mode S:

Mode A — asks for the transmission of the aircraft's transponder code;
Mode C — asks for the transmission of the aircraft's pressure altitude;
Mode S — selects an aircraft address and asks for the transmission of flight data for the ground surveillance.

One more separation keeps questions and answers from colliding: the interrogation frequency and the reply frequency are different: the interrogations go up on 1,030 MHz and the replies come down on 1,090 MHz, so neither side ever shouts over the other.

Codes and altitudes

The Mode A answer is the squawk: a Mode A designation is a sequence of four digits, manually selected from 4,096 available codes, and with it an aircraft can be identified by a unique code: the controller assigns it, the crew set it, and the label on the scope follows the aircraft. The Mode C answer is the height: in a Mode C reply, the pressure altitude is reported in 100-ft increments. And when a controller wants one target to raise its hand, a special position identification (SPI) pulse can be transmitted in addition, on request from ATC, but only as a result of a manual selection by the pilot: the IDENT button. Nothing transmits it automatically; the finger on the button is the point.

Mode S: the selective mode

Mode A shouts its question at everyone within the beam; Mode S can whisper to one aircraft. Every aircraft is allocated an ICAO (International Civil Aviation Organization) aircraft address, hard-coded into the Mode S transponder as the Mode S address, and a 24-bit address is used in all Mode S transmissions, so that every interrogation can be directed to a specific aircraft. Accordingly, a Mode S interrogation contains either the aircraft address (a selective call) or an all-call address: the all-call finds transponders the station does not yet know, the selective call works each known aircraft individually. The answers grow up too: Mode S provides enhanced vertical tracking, using a 25-ft altitude increment against Mode C's 100 ft.

Three facts complete the picture. Mode S is compatible with Modes A and C: a Mode S station can still work older transponders, and a Mode S transponder still answers older stations, which is what let the system be introduced aircraft by aircraft. Mode S transponders receive interrogations from TCAS and from SSR ground stations: the TCAS of the Instrumentation course is itself an airborne interrogator, and the transponder answers both without distinction. And SSR can be used for ADS-B (automatic dependent surveillance - broadcast): the same transponder broadcasts its own position and data unasked, the broadcast sibling of the contract-driven ADS of the Instrumentation course, and it rides on SSR technology.

In the cockpit and on the scope

The transponder's selector modes ration what is transmitted, and one lamp shows it happening:

OFF — nothing powered;
Standby — warmed up but not replying;
ON — replies in Mode A: the code, but no altitude;
ALT — replies in Modes A, C and S: code, pressure altitude and the selective services;
TEST — the self-check;
reply lamp — flashes whenever the transponder answers an interrogation, in any transmitting position: the visible pulse of the whole system.

What the controller gains from all this sits on the scope: the ATC display system can present the pressure altitude, the flight level, the flight number or aircraft registration number, and the GS (groundspeed): a labelled, identified, altitude-tagged target where the primary radar alone would show an anonymous blip.

What the exam asks

The mode meanings, the 4,096 codes, the 100-ft and 25-ft increments, the SPI rule and the selector positions carry the marks. The overview, following the learning objectives:

State that ATC is based on transponder replies to SSR interrogations; state that the transponder answers both ground SSR and TCAS

State what SSR provides that primary radar cannot, and its advantages in range and collected information through the aircraft's active participation

State the pulse-pair interrogation, the mode selection by pulse-pair interval, the modes A/C/S and their answers, and the different interrogation and reply frequencies

State the four-digit Mode A code (4,096 available), the unique identifying code, the 100-ft Mode C increments, and the SPI pulse on request, by manual IDENT selection only

Explain the Mode S address family: the hard-coded ICAO 24-bit address, directed (selective) and all-call interrogations, 25-ft vertical tracking, compatibility with A and C, TCAS and ground interrogations, and ADS-B

Explain the selector modes (OFF, Standby, ON, ALT, TEST) and the reply lamp; state what the ATC display can present