An interactive walk through RDR 03. Interrogate aircraft, set a transponder code, and see how garble and reflections can fool the picture.
Primary radar waits for a faint reflection. Secondary radar asks a question. The ground station sends a signal, and a transponder on board the aircraft transmits a reply.
Range and bearing come from the reply in the same way as a primary return. The difference is that the reply is coded, so it carries information as well as position.
Compare the two. Notice that the reply strength doesn’t depend on the size of the aircraft, and that the ground signal only has to travel one way, so less power is needed for the same range.
Switch the transponder off. With SSR alone, an aircraft without a working transponder isn’t displayed at all.
An SSR system has two halves:
The interrogator transmits on one frequency. These interrogations are called modes.
When the transponder hears an interrogation in the mode it’s set to, it replies on a different frequency with coded flight data. These replies are called codes: things like a unique identifier and the aircraft’s level.
Ask with a mode. Answer with a code.
Conventional SSR and Mode S use 1030 MHz for interrogations and 1090 MHz for replies (supplementary reading, Mode S).
The ground station spaces its interrogation pulses at different intervals depending on what it wants to know. Each spacing is a mode.
Modes A and C ask everyone in the beam. Mode S addresses one aircraft, and only that aircraft replies.
Pulse spacing is drawn schematically, not to scale.
ATC allocates the code, and the pilot sets it by hand on the transponder control panel.
An SSR code is four digits, each 0 to 7. That gives 8 × 8 × 8 × 8 = 4,096 codes, from 0000 to 7777. There is no 8 or 9.
Level reporting (Mode C) is automatic. It needs an encoding altimeter, and the only crew action is to select ALT on the transponder.
Special codes indicate an emergency, radio communication failure (RCF) or unlawful interference. Try them.
An SSR‑based ATM system links an aircraft squawking its assigned discrete code to that aircraft’s flight plan. Once they’re correlated, the system can:
Callsigns and flight details are fictional.
When two aircraft are close together, at a similar range and bearing, their reply pulses can arrive on top of each other. The replies are garbled.
The decoder may then show a false code, or both correct codes may be cancelled out. Mode C readouts can be wrong too, until the two radar position indications separate.
De‑garbling equipment helps, but take care using transponder data from aircraft passing close to each other.
Illustrative. The pulse patterns are simplified; overlapping pulses are shown merging.
Interrogations and replies can bounce off buildings or ground features close to the radar head. The radar then hears the same reply from a second direction and displays more than one position for a single aircraft.
This is a split track: two radar position indications for the same aircraft. On older SSR systems this effect was known as FRUIT (False Replies Unsynchronised in Time).
Transponder faults can also show wrong positions or levels. The system is only as good as the information it receives.
SSR depends on a working transponder and correct use of it by the crew. Carriage and use are regulated where transponders are mandatory (ICAO Doc 8168, PANS‑OPS).
Ideally the transponder aerial would radiate equally in every direction. But the aircraft’s own structure acts as a partial reflector, so each aircraft and aerial combination has its own pattern.
Most installations use at least two aerials, giving close to all‑round coverage and less chance of shielding and lost replies.
Bank the aircraft away from the radar with only one aerial fitted.
Like PSR, the interrogator aerial has a narrow beam in azimuth. As it sweeps past, the aircraft replies several times.
Conventional SSR works out the bearing from the whole run of replies. Monopulse SSR can fix the position from a single reply, though generally 4 to 8 replies are received. In many modern systems this is done electronically, giving very accurate bearing.
The result: reduced garbling, greater position accuracy and more stable radar tracks. Compare how tightly the plotted positions sit.
Illustrative. The spread of positions is exaggerated to make the difference visible.
Traditional SSR interrogates every aircraft in range. Mode S uses selective, addressed interrogations, improving detection, identification and level reporting, and allowing more data in each reply.
There are more than 16 million unique ICAO 24‑bit aircraft addresses. With automatic reporting of flight identity, aircraft can be identified unambiguously without relying on the Mode 3/A code.
Benefits include eliminating synchronous replies, resolving over‑interrogation, simpler identification when reflections occur, better tracking, relief from the Mode 3/A code shortage, and better safety nets (STCA, MSAWS, ACAS) with fewer nuisance alerts.
Select an aircraft to see the kind of data it can downlink.
Which parameters appear depends on the aircraft’s equipment and the ATM system. Values shown are fictional.