Who Goes There?

A Tornado-GR4A of a similar type to that shot down by a US Army MIM-104F battery in March 2003. This loss of the aircraft was partially blamed on the serviceability of the aircraft’s IFF-3500 system. (USAF)

Identifying friends and enemies is a problem as old as warfare itself. The risk of inadvertently attacking one’s own side is an ever-present danger for military operations, particularly those involving aircraft, where the speed of warplanes and missiles makes the timely determination of identity absolutely vital.

Cast your mind back to 2003: It is 0248 local time on 23 March. US-led forces, including the armed forces of the United Kingdom, had two days earlier commenced their campaign to remove the erstwhile dictator Saddam Hussein and his regime from power in Iraq. High above Kuwait, a Royal Air Force (RAF) Panavia Tornado-GR4A ground attack aircraft, numbered ZG710, from the RAF’s 9 Squadron is returning to Ali Al Salem airbase in Kuwait, having completed a mission in support of combat operations, as part of a package of two Tornado-GR4A jets. As ZG710 approaches the airbase, the crew descend to an altitude of 17938 feet/ft (5467.5 metres/m). Their aircraft is then struck by a Surface-to-Air Missile (SAM) fired by a US Army MIM-104F PAC-3 (Patriot Advanced Capability-3) battery. The SAM kills both Tornado-GR4A crewmembers; Flight Lieutenants David Rhys Williams and Kevin Barry Main as the missile hits their aircraft.

According to the official report from the RAF Board of Inquiry convened in cooperation with the US Army, to examine the events which caused the loss of the aircraft, the MIM-104F battery was watching the skies for incoming Iraqi tactical ballistic missiles aimed at Kuwait. When the Tornado-GR4A appeared on the radar screens of the MIM-104F crew, the battery’s Raytheon AN/MPQ-65 C-band (5.25-5.925 Gigahertz/GHz) ground-based air surveillance/fire control radar classified the aircraft as a hostile Anti-Radiation Missile (ARM); displaying the appropriate symbol for an ARM to the crew, the report continued. An interrogation was transmitted to the Tornado-GR4A’s IFF-3500 Identification Friend of Foe (IFF) interrogator/transponder. This, the Board of Inquiry determined, had been: “checked by the ground crew and confirmed to be working correctly,” that the IFF-3500’s “switches were set correctly,” and that this had been checked and noted by the aircrew as the Tornado-GR4 approached Ali Al Salem airbase. Yet, the report adds that the MIM-104F battery crew received “no response” to their IFF interrogation: “Having met all classification criteria, the (MIM-104F) crew launched the (SAM) and the (Tornado-GR4A), mistaken for an Anti-Radiation Missile was engaged in self defence.” The report continued that: “the (MIM-104F) crew had complied with extant self-defence Rules of Engagement (ROE) for dealing with (ARMs).”

An MIM-104F SAM launcher similar to the type that shot down an RAF Tornado-GR4A in March 2003 during the opening stages of Operation IRAQI FREEDOM. (US DoD)

The Board of Inquiry attributed blame for this ‘blue-on-blue’ incident, so-called as friendly forces are depicted with blue pennants on NATO (North Atlantic Treaty Organisation) maps, to a number of factors: These included the classification and ROE criteria used for ARMs by the MIM-104F; MIM-104F SAM firing doctrine and crew training; the autonomous operation criteria for the MIM-104F, and aircraft routing and airspace control. Of particular interest to this article, the report also apportioned blame to the IFF procedures used by the MIM-104F and the serviceability of the IFF-3500 system used onboard the Tornado-GR4A.

IFF Defined

In use since the Second World War, the IFF systems used by aircraft and ground-based air defence installations responsible for protecting airspace, work using a relatively simple principle: A radio signal is transmitted from the ground or from another aircraft which is received by an aircraft’s IFF system. This triggers a second radio signal transmitted as a ‘reply’ to the original ‘interrogation’ which denotes that the aircraft is friendly. The aircraft is then depicted thus on a radar screen.

Civilian Air Traffic Control (ATC) uses a similar system by which a transponder onboard an aircraft transmits details of an aircraft’s identity and altitude, which is then displayed alongside the aircraft’s radar track. Contemporary IFF systems use several operating ‘Modes’: During Operation IRAQI FREEDOM, military aircraft supporting the US-led coalition employed IFF Modes-1 and -4. NATO has enshrined the requirements for the respective IFF modes into its Standardisation Agreements (STANAGs) intended to harmonise operating procedures across the alliance. Mode-1 uses a cockpit-selectable two-digit unencrypted code which is reserved for use by military aircraft. Mode-4, meanwhile, which is also reserved for use by the military, provides an encrypted reply to an encrypted interrogation. The RAF Board of Inquiry articulated that the MIM-104F battery’s Mode-4 IFF interrogation capability was working correctly during the engagement of the Tornado-GR4A, but that the Mode-1 codes had not been loaded into the AN/MPQ-65’s computer system. To aggravate matters, the MIM-104F battery in question was working in an autonomous capacity: “without voice and data connections to and from (its) battalion headquarters.” This, the report continued, could have: “contributed to the difficulty the Battery had in receiving the Mode-1 IFF codes.” Alongside the inability of the MIM-104F battery to be able to employ Mode-1 IFF interrogation, the report laid blame on the Tornado-GR4’s IFF-3500 IFF system: “There is no firm evidence that (the Tornado-GR4A) responded to any IFF interrogations throughout the entire mission,” adding that the IFF-3500 had both been checked by ground crew, and by the aircrew during the mission. For all intents and purposes, it seemed to these individuals that the IFF system was working correctly. Nevertheless, as the report noted, the MIM-104F battery had transmitted a Mode-4 interrogation which had obviously never been replied to by the Tornado-GR4A, leaving the way open for its engagement by the SAM: “The Board concluded that (the aircraft’s IFF) had a fault, as an IFF Mode-4 response would have prevented the Patriot (ARM) classification and engagement.” The Board added that possible power failures suffered by the IFF may not have been displayed to the aircrew and that: “the most likely explanation for the absence of an IFF response was that there had been a power supply failure.”

A Tornado-GR4A of a similar type to that shot down by a US Army MIM-104F battery in March 2003. This loss of the aircraft was partially blamed on the serviceability of the aircraft’s IFF-3500 system. (USAF)


The loss of Flt. Lts. Main and Williams, and their Tornado-GR4A illustrates not only the split-second decision-making necessary in the conduct of any air campaign to determine whether an aircraft is friendly or hostile, but also the overriding importance of robust and secure IFF systems for military aircraft. While it is probably almost impossible to completely eliminate the danger of future blue-on-blue incidents during air campaigns, ongoing evolutions in the airborne IFF domain could help to significantly reduce the chances of such tragic incidents happening again. Central to these efforts is NATO’s work in realising the Mode-5 IFF standard.

Mode 5 can trace its lineage back to a US Joint Chiefs of Staff requirement issued in 1995 for a new IFF waveform to replace that used by the Mode-4 IFF protocol (see above). This was followed in 2002 with NATO ratifying STANAG-4193 which stipulated that the Mode-5 IFF protocol would be rolled out across the alliance. Mode-5 is essentially a cryptographically secure version of the civilian Mode-S Ultra High Frequency (UHF: 300 megahertz/MHz to three gigahertz/GHz) aviation transponder interrogation protocol. Like military aircraft, civilian aircraft ‘squawk’ a UHF radio transmission in response to an ATC Secondary Surveillance Radar (SSR) interogation. Eurocontrol explained to Armada in a written statement that SSRs work: “along the principle of interrogation and reply (also called question and answer). An interrogator transmits messages that are received and decoded by a transponder onboard an aircraft. In response, the transponder transmits reply messages containing information (such as identity and altitude) requested in the interrogations. The interrogator determines the range of the replying aircraft by the round-trip time of the interrogation and reply, and uses its’ receive antenna to estimate the azimuth from which the reply was transmitted.”

SSRs are distinct from an ATC primary radar which determines the position of an aircraft using conventional radar techniques which involve transmitting a radio pulse which bounces of an aircraft as an echo. The radar then measures the time difference between the transmission and reception of the radar pulse and its echo to determine the position of the aircraft, and the frequency change between the transmitted pulse and its echo to determine the aircraft’s speed. However, the disadvantage of primary radar for ATC is that, while it provides a physical representation of the aircraft’s location and velocity, it does not provide information regarding the aircraft’s identify. Thus primary radar antennae are often fitted with an SSR antenna to populate radar track data with identity and altitude information.

A conventional ATC radar ensemble. This includes the primary radar which can be seen underneath the SSR antenna atop of the ensemble. (Airbus)


The Mode-S protocol was developed as a result of a joint American and British initiative from the early 1980s to answer deficiencies in the existing civilian Mode-3A (identity) and Mode-3C (altitude) protocols mandated for civil aircraft by the International Civil Aviation Organisation (ICAO); the United Nations organisation supervising civil aviation standards and techniques. Mode-3A was judged to be increasingly insufficient regarding the number of aircraft identity codes used to display an aircraft’s call sign on the radar picture, while Mode-3C, which provides altitude measurements in denominations of 100ft (30.4m) was not judged to be precise enough for use in increasingly congested airspace. The resulting Mode-S protocol was first articulated by an ICAO Circular 174-AN/110 published in 1983. All Mode-S transponders have a unique 24-bit address assigned by the ICAO. A Mode-S response can be triggered by legacy Mode-3A/C interrogation and, as well as providing information to an SSR, the response can be received by TCAS (Traffic Collision Avoidance System) receivers onboard nearby aircraft, and the Automatic Dependent Surveillance Broadcast (ADS-B) system currently being introduced worldwide to enhance ATC. ADS-B determines an aircraft’s position using the Global Positioning System (GPS) satellite navigation system, with Mode-S transmitting across the ADS-B network and providing the aircraft’s GPS position.

The advantages offered by Mode-S include more accurate details regarding an aircraft’s position and altitude, thanks to the GPS functionality offered by the protocol, and the number of identification codes which the 24-bit address enables; over 16.7 million, according to a UK Civil Aviation Authority document published in 2013. The first commercial flight using the Mode-S protocol occurred in 2008, with the European Union (EU) requiring all new civilian aircraft to be equipped to handle the Mode-S protocol by 2017, with Mode-S to be retrofitted on all aircraft using EU airspace by 2020. Meanwhile, the US Federal Aviation Authority requires all aircraft using US airspace to employ the Mode-S protocol by the same year.

Put simply, Mode-5 is a cryptographically secure version of Mode-S which is reserved by NATO for military use. Rob Saracino, technical director for identification and processing solutions at BAE Systems explains that: “From a security perspective, Mode 5 adds better encryption and does not transmit any unprotected data.” The company is hard at work providing interrogators and transponders which are Mode-5/S compatible, such as the AN/PX-7 transponder which has the option of including an embedded GPS (Global Positioning System) card to support the ADS-B system (see above). 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 its aircraft are equipped to use the Mode-5 protocol. Fabrizio Boggiani, marketing and sales director for Leonardo’s airborne and space systems division, states that: “The most important driver for Mode-5 will be the NATO mandate requiring all member countries to move to this standard by 2020.” In July 2016, the firm was selected by the UK Ministry of Defence, alongside Airbus’ defence and space division: “as preferred bidder to upgrade the IFF systems on more than 400 operational platforms, spanning the land, air and sea (domains),” to handle the Mode-5 protocol. Beyond the UK MoD, the firm is also supplying its Mode-5 compatible M-428 IFF transponder to the Swedish and Italian air forces for use onboard the respective, forthcoming Saab JAS-39E Gripen fighters. The firm offers a significantly-sized IFF product family under the M-428 banner. Regarding IFF interrogators, Leonardo provides the M-426/S product which is used alongside the firm’s E-Scan X-band (8.5-10.68GHz) fighter aircraft radar which outfits the Eurofighter Typhoon fighter family, and its M-434/5 interrogator which is used by air surveillance aircraft and, although not confirmed by the company, maybe installed onboard the Aeronautica Militaire (Italian Air Force) forthcoming Gulfstream G-550 business jet outfitted with the Israel Aerospace Industries EL/W-2085 S-band and L-band (2.3-2.5/2.7-3.7GHz/1.215-1.4GHz) air surveillance radars. These aircraft are expected to enter service with the Italian Air Force in the coming two years.

Leonardo’s M-428 IFF transponder
Leonardo’s M-428 IFF transponder family equips a number of military aircraft worldwide. This includes the forthcoming JAS-39E Gripen fighters of the Italian and Brazilian air forces.


Compared to legacy IFF protocols, such as Mode-4, Mode-5 offers a number of important improvements. According to Christophe Dress, strategy and marketing director for radio communications activities at Thales, these include improved better resistance to Radio Frequency (RF) jamming, a higher probability of correct identification, and improved communications and transmission security due to encryption. While Mode-5 uses the same 1030MHz to 1090MHz frequency range as Mode-4, Mr. Dress adds that the former uses a spread spectrum RF transmission approach. This means that Mode-5 can transmit across this frequency range, and rapidly change between frequencies. This avoids any specific frequency being constantly occupied by a single IFF system, even when it is not transmitting. Mr. Dress continues that to use the Mode-5 protocol, military aircraft require a new IFF transponder and/or interrogator, and an accompanying cryptographic computer. This ensemble, he states, then has to be linked to the aircraft’s computer and avionics systems, with cockpit controls and displays which can depict Mode-5 information also installed. To reduce costs, Mr. Dress says: “in almost all countries Mode-5 and Mode-S modifications have been implemented simultaneously, reducing the cost and the duration of the modernisation.” This is the case for the Finnish which obtained Thales’ TSA-3520 IFF interrogator installed on its twelve Thales GM-400 S-band (2.3-2.5/2.7-3.7GHz) ground-based air surveillance radars which the country acquired from 2013. Moreover, as Mr. Saracino notes: “Mode 5 also incorporates a random reply delay component which eliminates the legacy issues of aircraft flying in formation having their replies interfere with each other.”

Thales GM-400
The IFF interrogator used to determine the identity of aircraft in the locale of this Thales GM-400 ground-based air surveillance radar can be seen here, mounted above the radar’s main antenna. (Thales)


Essentially, Mode-5 includes two distinct approaches, known as Level-1 and Level-2. Eurocontrol states that: “Level 1 employs the traditional interrogation and reply technique to identify and track friendly platforms (aircraft, ships). Level 2 enables an equipped platform to report its identity and position either autonomously or in response to an interrogation.” A written statement provided to Armada from Indra Sistemas notes that Level-2 transmissions will provide the aircraft’s 24-bit identification number, its national origin, mission code and its position in terms of speed, bearing and altitude. Moreover, it will be used constantly: “Mode-5 is to be used across the full range of NATO operations, during peacetime and conflict, from crisis response to major conflicts, in all environmental conditions.”

The Prussian military strategist Carl von Clausewitz, writing in his seminal work Vom Kriege (On War) published posthumously in 1832 argued that: “three quarters of the factors on which action in war is based are wrapped in a fog of greater or lesser uncertainty.” From 2020 onwards, Mode-5 may well help to lift some of this fog and no doubt help to avoid tragic incidents such as the RAF’s loss of their Tornado-GR4A on that fateful night on 23 March 2003. The sheer quantity of information which Mode-5 can impart regarding an aircraft’s identity, position and mission should help to reduce the chances of such accidents occurring in the future. This will be achieved by giving those tasked with air defence, either in the skies or on the ground, as much information as possible about the friendly aircraft in their locale.

by Thomas Withington