Primary and secondary radar
An ordinary radar only sees a reflection. A secondary radar asks a question ("who are you?") and a transponder in the aircraft answers with a code.
In practiceAir defence uses IFF — military Mode 5, encrypted — to avoid firing on friendly aircraft. No answer does not automatically mean hostile: the transponder may be broken or switched off. Identification therefore remains a combination of IFF, flight route, procedures and other sensors.
Everything in this chapter so far is primary radar: the radar receives a passive reflection from the target. A secondary radar works differently. It asks a question (the interrogation), and a transponder on board the aircraft actively transmits a reply. In the military this is called IFF (Identification Friend or Foe); in civil aviation SSR (secondary surveillance radar).
Because the reply is actively transmitted, it only has to travel one way: the signal decreases with R² instead of R⁴ (Inverse square law). A secondary radar therefore needs much less power, sees no clutter, and receives information that a primary radar can never measure: an identity code and the barometric altitude.
| Interrogation (uplink) | Reply (downlink) | |
|---|---|---|
| Frequency | 1030 MHz | 1090 MHz |
| Transmitter | ground station, ship or aircraft (interrogator) | transponder in the aircraft |
Modes, codes and side lobe suppression
An interrogation consists of pulses whose spacing determines the question — the mode. Pulses P1 and P3 form the question; P2 is a control pulse.
| Mode | Spacing P1 – P3 | Question |
|---|---|---|
| Mode 1 | 3 µs | military: mission or aircraft type |
| Mode 2 | 5 µs | military: individual aircraft code |
| Mode 3/A | 8 µs | identity code (military and civil, the "squawk") |
| Mode C | 21 µs | barometric altitude, in steps of 100 ft |
| Mode 4 / Mode 5 | encrypted | military, cryptographically secured identification |
The reply is a pulse train between two framing pulses 20.3 µs apart. Between them lie 12 information positions, which together make 4096 codes (four octal digits, 0000 – 7777). A few codes are fixed worldwide: 7500 (hijack), 7600 (radio failure) and 7700 (emergency).
The control pulse P2 solves a problem you know from Main lobes, side lobes and back lobes: side lobes. A transponder close to the interrogator also receives a strong question through the side lobes and would reply as if it were in the main beam — in the wrong direction. The interrogator therefore transmits P2 through an omnidirectional control antenna. If P1 is clearly stronger than P2, the transponder is in the main beam and replies; if P2 is as strong or stronger, it stays silent. This is called side lobe suppression (SLS).
Fruit, garbling and Mode S
Busy airspace with many interrogators and many transponders produces two forms of interference:
- Fruit (false replies unsynchronised in time): replies triggered by a different interrogator. They arrive at random moments and are filtered out because they are not synchronised with the radar's own interrogation rate.
- Garbling: two aircraft in the same direction less than about 3 km apart in slant range reply at the same time; their pulse trains (20.3 µs long) overlap and become unreadable.
Mode S (selective) solves both. Every aircraft gets a unique 24-bit address (over 16 million possibilities) and is addressed individually; only that aircraft replies. The question is digitally encoded with phase shifts (DPSK), the reply contains 56 or 112 bits of data. In addition, a Mode S transponder transmits unsolicited squitters: short messages with its address, and in the extended squitter also position, speed and callsign. That is the basis of ADS-B, on which sites such as Flightradar24 run.
IFF and EW
For the EW specialist, IFF is at the same time a means of identification, a vulnerability and a source of intelligence.
- No reply is no proof. IFF provides positive identification of friends, but a missing reply can also mean that the transponder is faulty, wrongly set or switched off. In the 2003 Patriot incident in which a British Tornado was shot down, an IFF problem was among the factors. IFF is therefore one element of identification, alongside ESM, behaviour and procedures (Integrated Air Defense System (IADS)).
- Every reply is an emission. A transponder that replies to interrogation reveals its presence and direction. An adversary can interrogate it himself or passively listen in on the replies to friendly interrogators. That is why strict EMCON rules (EMCON: controlling your own emissions) apply to the use of IFF in operations.
- Open modes can be deceived. Mode 3/A, C and ADS-B are unencrypted: false replies and even complete false ADS-B aircraft are technically easy to generate. Mode 4 and above all Mode 5 (NATO standard, with cryptography and in Level 2 also position data) are designed precisely to prevent spoofing and exploitation.
- ADS-B as OSINT. Military aircraft that have ADS-B or Mode S switched on can be followed worldwide — tankers, AWACS and reconnaissance aircraft along the borders of conflict areas are a staple of open-source analysis.
Radartutorial.eu works out all pulse formats, Mode S messages and the operation of the transponder in detail: secondary radar.