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Earth-bound electronic challenges.

One of the most distressing features of modern battlefields is the possibility for forces on the same side to engage each other. This causes needless death and destruction and sometimes accounts for up to 30 per cent of battlefield losses. There were several incidents during both Gulf conflicts, with one including the destruction of an RAF Tornado by a US Army missile battery.

The Tornado incident may have been due to the failure or combat damage of the aircraft's Identification Friend or Foe (IFF). This was especially tragic, for IFF is supposed to be the electronic talisman that allows a radar operator to detect friends in a screen full of light spots.

The IFF system was originally developed for air warfare, but it quickly migrated to naval warfare. With the evolution of the '24/7 battlefield' it is beginning to enter into the land warfare arena, essentially to avoid what are euphemistically called 'friendly fire' or 'blue-on-blue' incidents.

IFF was developed some 65 years ago as an integral part of the world's first electronically-based air defence system in the United Kingdom. Its basic design has remained unchanged and consists of two elements; an interrogator that asks the question and a transponder that provides the answer.

Modus Operandi

The system is based upon either mission or squadron codes, which may be changed for security purposes, and upon pulse position modulation (PPM) technology. The interrogator, operating at 10.30 MHz, sends pulses that will trigger a response from a friendly platform. The transponder operates at 10.90 MHz and responds with two flaming pulses spaced at 20.3 [micro]s and a combination of up to twelve pulses that can provide up to 4096 combinations. The interrogator converts the pulses into a binary number that matches the pre-arranged code.

The system has developed through three modes based upon an interrogator with two PPM pulses. Mode 1 is for mission identification and the interrogation pulses are at 3[micro]s intervals. Mode 2 is for aircraft identity with interrogation pulses five [micro]s apart while Mode 3 is for air traffic control and the pulses are at eight [micro]s intervals.

All of these are incorporated in the most widely used Western IFF system known as Mk XA. It also includes the civilian equivalent of Mode 3, Mode A, and a height-reporting version known as Mode C. In this the interrogator pulses are al 21 [micro]s intervals and the transponder codes have only 2048 reply combinations. In air traffic control IFF or aircraft identity are designed to provide safety allowing the controller to monitor range through sequence timing, and height.

The Mk XA system remained the norm in Nato's early days but then an issue developed over Strategic Air Command bombers deployed into Europe. It was anticipated they would be returning from their targets at the same time as friendly and hostile aircraft and the existing IFF system would not be sufficiently reliable.

This led to the introduction of the Mk XII system in 1950 with cryptographic enhanced pulses. This introduced Mode 4 with an interrogator providing 36 mode pulses, including a four-pulse preamble, and the transponder having a triple reply group. This system has up to a million possible combinations and sends code words rather than an alphanumeric sequence.

The very complexity of the system, however, meant that its introduction was largely limited to US platforms and most nations retained Mk XA as their basic IFF standard. The British, for example, have tended to add Mk XII to aircraft on an ad hoc basis, especially in areas where they may be operating alongside US forces.

A wide variety of transponder and interrogator products are now available. Dominating the business are AMS, BAE Systems, BAE Systems North America, EADS Defence Electronics, Elta, Honeywell, Northrop Grumman Electronic Systems, Raytheon, Telephonics and Thales Communications. These products are used in fixed and rotary-wing aircraft. in warships and auxiliaries, in radar systems and also with air defence batteries even--including man-portable systems like the Mistral and the RBS 70.

On the Waves

For air-sea use it was clear that, by the late 1970s, a new common system was required and by the mid 1980s a Standard Nato Agreement (Stanag) requirement--Stanag 4162--had emerged for a Nato Identification System (NIS). What was being sought was a more reliable and secure system with cryptographic control but one compatible with civilian air traffic control. It would also have to be compatible with the Link 16 Joint Tactical Information Distribution System (Jtids).

The devil was in the detail and eventually the mirage of an NIS vanished but the foundation of systems compliant with Mk XII capable of reaching the next stage, or Mode 5, emerged. As it would have had to be compatible with new air traffic control requirements it was redesignated Mk XIIA.

Mode 5 is the electronic emitter version of frequency-hopping radio with the pulses 'hopping' the D-band width in pseudo-random fashion. This provides for a more efficient and effective use of a limited (one to two GHz) bandwidth. It is not part of the digital battlefield although it may operate within it.

An added complication is the development of Secondary Surveillance Radar (SSR) for the next generation of air traffic control known as Mode S. As the Vincennes incident of 1987 showed, military and civil aircraft had to start sharing the skies in the post Cold War era.

Mode S can operate in general (all-call) mode with interrogation in three pulse PPM or dedicated mode to specific aircraft. This features a two-pulse PPM preamble followed by 56- or 112-bit Differential Phase Shift Keying (DPSK) that is compatible with data links. The transponder has a four-pulse PPM preamble followed by a 56- or 112-bit Binary PPM (BPPM) code, meaning that each aircraft can have a unique 24-bit address.

EuroControl (the European organisation for the safety of air navigation) requires all aircraft flying as Instrument Flight Rules/General Air Traffic to have Mode S Elementary and Enhanced Surveillance radar capability. This became mandatory on 25 March 2004 with a two-year transitional period to 2006. Without this Mode S capability military aircraft will be banned from civil airways.

These future IFF requirements are sophisticated and expensive, thus defence ministries have the option of trying to jump directly to the new standard or adopting a phased introduction. In the United States a Common IFF Digital Transponder (CXP) has been developed by BAE Systems' Advanced Systems as the AN/APX-117 and APX-118 and its progress has reached the initial production stage.

The CXP will replace four products--the APX-64, -72, -100 and-101--and can support Modes 1-5 and Mode S Level 3. It is compatible with Jtids and the Multifunction Information Distribution System (Mids) and the only difference between the US Air Force's APX-117 and the US Navy's APX-118 is the incorporation of an embedded Mode 4 cryptographic module.

From one Generation to the Next

France, Germany, and originally Italy, agreed to develop what was called the New Generation IFF system leading to the establishment of European Identification Systems (EIS) by Eads and Thales. These companies have won a 160 million[euro] contract to develop a transponder system which will be designated STR2000 by Germany and TSC2000 by France.

The new system, which is to be fitted to a variety of aircraft including transports, attack helicopters and the NH-90 will have a full Mk XI1 capability and Mode S Level 3. The Mode 4 cryptographic computer can be added or operated from an external location and the system is designed for upgrade to Mode 5.

The British have also adopted a phased enhancement in their Successor IFF (Sift) programme designed to meet Staff Requirement (Sea-Land-Air) 924. It seeks to upgrade all three service's Mk XA capability to Mk XII standard, to introduce the Mode S transponder and to provide a 'proven growth path' to Mode 5 standard.

The 500 million[pounds sterling] programme is being implemented in phases called Approaches and will be completed in 2010. Approach A, for which Raytheon Systems Ltd was awarded a 100 million[euro] contract covering some 350 to 400 legacy platforms: including the Royal Navy's Duke (Type 23) class frigates, mine hunters, RAF support aircraft and Army Gazelle helicopters.

Approach B, led by Thales Air Defence, is for surface-to-air missile batteries and Tornado F3 interceptors. The last are receiving Raytheon IFF 4810 transponders and IFF 4500 interrogators. Approach C covers the remaining Tornadoes and Harriers, helicopters, E-3D Sentries and C-130K Hercules transports. Sift does not apply to the latest platforms such as the Eurofighter Typhoon, the Joint Strike Fighter, Nimrod MRA 4 or the Daring (Type 45) class destroyers which have Sift capability and can migrate into Mode 5.

An Now the Unmanned

IFF devices remain vital for all air platforms and while traditionally these have been manned, they will also need to migrate to unmanned platforms. Their importance is growing as armed forces make ever-greater use of unmanned air vehicles and unmanned combat air vehicles for reconnaissance, covert strike missions and strike missions in very hazardous environments.

But the use of an IFF system in unmanned platforms is certainly likely to spread to unmanned surface ships, which are increasingly being considered for a wide variety of roles, and possibly unmanned ground vehicles which are an integral part of the US Army's Future Combat System.

This reflects the growing interest in IFF among ground forces not only for their helicopters but also for combat vehicles such as main battle tanks, infantry combat vehicles and reconnaissance vehicles. Friendly fire incidents have always occurred in battle (the famous Confederate General 'Stonewall' Jackson was one famous victim) but they have never been acceptable, especially after incidents in the First Gulf War highlighted the problem.

Standard?

In an effort to develop a Nato design standard French, German, British and American systems were evaluated in Germany in May 1997, and while they failed to lead to a common system they did result in a common Stanag 4579. The French Battlefield IFF (Biff) and the German Ziel-Erkennung Freund/Feind (Zeff) were later merged when the evaluation failed to achieve its goal, while the American Battlefield Combat Identification System (Bcis) entered low-rate initial production and is being introduced into the 4th Infantry Division (Mechanized). The British Millimetric Target Identification Covert Emitter appears to have been abandoned, or is under low priority development.

The Thales TSE 6010 Biff and Bcis both operate on the same question and answer principle and within the millimetric upper K-band (also Ka-band) at 36 to 38 GHz. These have inherently better directionality than the D-band systems used in airborne and naval applications. bringing benefits in terms of better target discrimination and correlation. They are also relatively covert because they radiate over a limited distance and only when they are interrogated, with the extra benefit that the distance between the two vehicles can be calculated and the target engaged if it proves hostile.

The Biff is a family that consists of a Combined Interrogator Transponder, an Interrogator and a Strap-on Transponder. The identification sequence is conducted simultaneously with the host vehicle's primary sensor (optical, electro-optical or radar) and is manually or semi-automatically initiated. The identification request is transmitted in the form of cryptographically-controlled interrogation sequence over a narrow azimuth centred on the host vehicle's line-of-sight.

Any transponder within the transmission sector authenticates the received interrogation and replies with its own cryptographic reply sequence. Friendly responses are received, processed, authenticated and displayed to the operator as a friend (specific target identity and laser-correlated range together with the identity and number of non-range correlated targets within the sector being illuminated) or as an unknown if interrogations have been unsuccessful. Thales claims the system can establish identities with 98 per cent confidence.

The Biff can also function in a directional Data Exchange Mode (Dem) or as an omni-directional Digital Data Link (DDL). The Dem option is used to acquire specific data (including platform position, type and time) from a previously identified target at ranges of six kilometres or more. DDL is a broadcast mode which facilitates intra-platoon or platoon-to-platoon data exchanges allowing vehicles to exchange tactical situation data in a network type operation within a range of one kilometre.

The French defence procurement agency (DGA) awarded Thales Communications the development and production contract in late 1999 with a view to evaluating a batch of 100 systems that will be delivered by 2005.

Raytheon's Bcis is similar, with the operator activating the interrogator that questions the target in five individual pulses through the directional antenna. The transponder in a friendly vehicle receives the interrogation signal through an omni-directional antenna, validates it and responds with its own identification signal, which is also transmitted in a string of five pulses. At the same time it informs the crew it has been interrogated.

The other vehicle's interrogator receives the response, validates and displays it in the vehicle sighting subsystem and generates a voice cue in its crew's headphones. Overall system response time is approximately 0.30 to 1.00 second and the response signal is time synchronised using Global Positioning System clock data.

A digital datalink add-on is reported to have been developed. With a software enhancement Bcis can use its GPS input to create geographic co-ordinates which can be added to a unique vehicle identification code for transmission via datalink. As a situational awareness tool trials have shown the system can transmit data at ranges of up to 1.2 km.

In 2000 low-rate initial production was approved for 952 units (646 to 2003 with an option for 306), the deliveries of which began in 2001. Interoperability between the Biff and Bcis systems was successfully demonstrated during the first round of laboratory tests held by Nato in July 2003 in the United States.

The problem is that both systems are likely to be expensive. It is interesting to note that M-Tice was designed to be an effective but cheap system allowing the target to make all the emissions. Friendly vehicles would have an inexpensive, M-band (94 GHz) transmitter, emitting a modulated low-power signal with a LPI (low probability of intercept) waveform, and a coherent, high-gain directional receiver. As with Mode 5 the system was also a 'frequency-hopper' whose sequence could literally have been dialled in.

The system produced a visual symbol that indicated whether or not the target was hostile by the lack of any response. The system could also distinguish between two targets which were close to each other and was boresighted to the main armament. The system was not only compact and reliable but also cheap; the estimated price for a vehicle installation in the late 1990s being the equivalent of $ 5000 compared with $15,000 for Bcis.

Aerial and naval IFF is a mature technology regarded as an integral part of these platforms and their roles in all forms of operations. There is no doubt that IFF in these areas will migrate to unmanned platforms out of sheer necessity, but the ground battlefield remains uncertain.

The Gulf wars demonstrated the urgent need for vehicles to have IFF, and this requirement will grow as battlefield surveillance radars take to the air with systems like Jstars, AGS and Astor. The problem will be to achieve this aim in a cost-effective manner.
COPYRIGHT 2004 Armada International
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Title Annotation:Electronics
Author:Hooton, E.R.
Publication:Armada International
Geographic Code:4EUFR
Date:Oct 1, 2004
Words:2542
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