Also known as: IFF, Identification friend or foe, secondary surveillance radar
Identification friend or foe (IFF) is a military transponder-and-interrogator system, born in World War II, that answers a single question: is this radar contact a friend?1 A ground or airborne interrogator transmits a coded challenge on 1030 MHz, and a friendly transponder aboard the target replies on 1090 MHz with a coded answer. Civil secondary surveillance radar (SSR) — the Mode A/C and Mode S systems that track airliners — descends directly from IFF and shares the very same frequency pair.2
How it works
IFF is a cooperative system: it works only because the friendly aircraft carries a transponder that recognises the challenge and answers correctly. The interrogator sends a pulse-coded challenge in one of several modes, and a valid transponder replies with a coded response after a fixed delay. Matching the reply to the interrogation confirms the contact is equipped and responding as a friend. This is fundamentally different from primary radar, which merely bounces energy off a target and reports a blip with no identity attached.
The wartime lineage runs straight into today’s civil airspace. The military “modes” of early IFF were opened for air-traffic use as SSR Modes A and C — Mode A returns a four-digit squawk code, Mode C adds pressure altitude — and later Mode S added selective, addressed interrogation and the data link that ADS-B rides on. All of them still use the 1030/1090 MHz pair inherited from IFF, which is why a single antenna and receiver band covers both military and civil secondary surveillance.
Modern military IFF has moved well beyond those open modes. Mode 5 uses encryption and spread-spectrum techniques so that challenges and replies cannot be spoofed or exploited by an adversary, replacing the older Mode 4. The civil and military systems coexist on the same frequencies but the secure military modes are cryptographically protected.
What it can and cannot do
The name is precise and slightly misleading. IFF identifies friends — it cannot mark foes. A correct reply positively confirms a friend; the absence of a reply means only that the contact is not a responding friend. It could be an enemy, or it could be a friendly aircraft with a broken, switched-off, or wrongly keyed transponder. For this reason IFF is one input to identification, never a standalone weapons-release authority. The same cooperative-only limitation applies to civil SSR: an aircraft with no working transponder is invisible to secondary radar even though it is plainly a friend, which is part of why TCAS collision avoidance and primary radar exist alongside it.
Relevance to SDR
For the SDR hobbyist, IFF’s civil descendants are the accessible part: the 1090 MHz replies
that carry Mode A/C squawks, Mode S addresses, and ADS-B position reports are unencrypted
and decodable with a cheap receiver and software like dump1090. The military secure modes
are not. Understanding IFF explains why the aviation surveillance band is arranged the way
it is — a challenge-and-reply architecture from the 1940s, still built on 1030 MHz up and
1090 MHz down. Aircraft surveillance sits outside GopherTrunk’s land-mobile trunking focus,
so it does not decode 1090 MHz itself; the reference stands as context for the wider RF
landscape.
Sources
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Identification friend or foe — Wikipedia, for IFF’s WWII origin, the 1030/1090 MHz challenge-reply pair, modes, and Mode 5. ↩
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Secondary surveillance radar — Wikipedia, for how civil SSR Modes A/C and S descend from IFF and share its frequencies. ↩