Operations Specialist 2nd Class Carlos Warren records the IFF state of aircraft while standing strike watch aboard the US Navy’s USS Ronald Reagan ‘Nimitz’ class aircraft carrier. The advent of Mode-5 will greatly improve command and control, and situational awareness during air operations.

NATO has set a deadline of 30 June to replace the legacy Mode-4 IFF protocol with Mode-5. We examine what this means for the alliance and the enhancements Mode-5 will bring.

Mode-5 Contract

The unstoppable rise of the Mode-5 Identification Friend or Foe (IFF) protocol continues, illustrated by the news in early March that Leonardo had signed a contract with the Italian ministry of defence worth $90 million to equip an array of Esercito Italiano (Italian Army) and Marina Militare (Italian Navy) ground-based air surveillance, and naval surveillance radars, and fire control/ground-controlled interception radars, with the firm’s New Generation IFF equipment configured to the North Atlantic Treaty Organisation’s (NATO) Mode-5 Baseline-3 protocol.


Mode 5 emerged as the result of a 1995 US Joint Chiefs of Staff requirement for a new IFF waveform to replace the legacy Mode-4 IFF protocol. In 2002 NATO’s membership ratified Standardisation Agreement-4193 (STANAG-4193). This pledged the alliance to roll out Mode-5 across its membership. Mode-5 uses the same civilian frequencies the Mode-S Secondary Surveillance Radar (SSR) system used for air traffic control. A Mode-5 interrogator will transmit its challenge on a frequency of 1.030 gigahertz/GHz and receive the response on 1.090GHz.

To use the Mode-5 military aircraft require a new IFF transponder and/or interrogator, and an accompanying cryptographic computer. These systems are linked to the aircraft’s computer and avionics, and with cockpit controls and displays depicting Mode-5 information. A random reply delay component is included in the Mode-5 software which solves the problem of aircraft flying in close formation sending out their IFF response all at the same time unintentionally jamming their comrade’s reply, and risking several aircraft appearing as one ‘blob’ on an SSR.


Mode-5 uses the Mode-S SSR protocol as its baseline. Athanasios Chouliaras, a former senior officer with the Hellenic Air Force and an airborne ISR, C2BSM platform, EW and datalink evaluator says that Mode-S “allows aircraft-specific selective data communications to support additional air traffic data and airborne collision avoidance systems.”

Mode-S mandates each aircraft to have a unique 24-bit address assigned to an aircraft for its lifetime by the International Civil Aviation Organisation. Mode-S data can also be received by TCAS (Traffic Collision Avoidance System) receivers equipping nearby aircraft, and by the Automatic Dependent Surveillance Broadcast (ADS-B) system: ADS-B is enhancing global air traffic control systems and ascertains an aircraft’s position with the latter transmitting its Global Positioning System coordinates. Mode-S transmits this data across the ADS-B network.


Mode-5 is a cryptographically secure version of Mode-S reserved by NATO for military use. Essentially Mode-5 transmits an encrypted version of the Mode-S information. Crucially, “the Mode 5 waveform uses modern modulation, coding, and cryptographic techniques to overcome performance and security limitations in the current Mk.12 Mode-4 waveform,” says Mr. Chouliaras. Until now Mode-4 has been the standard IFF waveform used by NATO and allied forces. Additional security measures embedded in Mode-5 include “modulation techniques to change the code it transmits every few seconds to frustrate adversaries,” he adds.

Transition to Mode-5

The Mode-5 protocol achieved an initial operating capability with military aircraft in the United States in 2014. NATO is following an aggressive timetable to ensure that aircraft are equipped to use the Mode-5 protocol with a deadline of late June set for the transition.

Fortunately, armed forces in search of Mode-5 suppliers are spoilt for choice: BAE Systems and Leonardo are two of the firms producing Mode-5 compatible IFF interrogators and transponders. Products are also available from Raytheon, Thales, General Dynamics, Hensoldt, Indra and Aselsan.

Neither is Mode-5 confined to NATO. Allied nations will need to have Mode-5 particularly if they wish to join NATO and/or US forces in future air operations. For example, Hanwha and LIG Nex1 are both supplying Mode-5 compatible IFF systems to the Republic of Korea’s armed forces.

Armada Analysis’ figures estimate the unit cost of a Mode-5 compatible IFF interrogator/transponder for a combat aircraft to be between $274,000 and $487,000. Hence the market for equipping new-build aircraft and legacy platforms with such equipment could be lucrative.

Mode-5 will greatly enhance NATO and allied IFF capabilities, although Mr. Chouliaras sounds a note of caution, and emphasises that NATO and other actors who will be using Mode-5 must ensure that the protocol can work effortlessly with other situational awareness tools like the Link-16 and Link-11/22 tactical datalinks used in the air and naval domains respectively for the exchange of track and tactical information.

NATO will employ Mode-4 for some time, as this will inevitably be used by NATO and allied platforms around the world over the coming years as the transition to Mode-5 gathers pace. Mr. Chouliaras stresses that “all legacy IFF modes and their capabilities should be retained and remain operational to facilitate an orderly transition to Mode-5.”

He continues that the advent of Mode-5 could help the further integration of unmanned aerial vehicles into air operations with these aircraft joining their inhabited counterparts in sharing Mode-5, ADS-B and Link-16 data and helping to improve situational awareness between all air platforms during such undertakings.

by Dr. Thomas Withington