The Link-16 radio communications protocol, of which the North Atlantic Treaty Organisation (NATO) commenced development in the 1970s, and which commenced fielding in the mid-1980s, revolutionised air-to-air and surface-to-air/air-to-surface communications.

According to Bruce Eteson, technical director of data link systems at BAE Systems, Link-16 continues to provide its users with “reliable distribution of real-time situational awareness information, targets, and command and control data. This helps them run efficient and effective operations.”

The E-3 family
The E-3 family of airborne early warning aircraft is highly dependent on the Link-16 data link, which is instrumental to it being able to task combat aircraft for specific missions. (USAF)

The workings of Link-16 are fiendishly complex for those without a sound grounding in engineering, computing or communications such as the author, but put simply, Link-16 provides secure voice and data communications across a frequency range of 969 Megahertz/MHz to 1.2 Gigahertz/GHz. This frequency spread is in turn sub-divided into 51 specific frequencies, with the exception of frequencies between 1.008GHz and 1.053GHz and 1.065GHz and 1.113GHz which are reserved by the International Telecommunications Union, the United Nations body which supervises the global allocation of the radio spectrum, for use by radio-based Identification Friend or Foe systems.

The 51 frequencies which Link-16 can use enable frequency hopping to be performed which enhances communications security. The frequency being used by a specific Link-16 networks ‘hops’ across these 51 frequencies in a pseudo-random sequence several times a second. When carrying voice traffic, Link-16 handles such communications at data rates of between 2.4 to 16 kilobits-per-second (kbps).

Modus Operandi

Link-16 uses a ‘nodeless’ architecture, i.e. there is no single platform or base which manages a Link-16 network meaning that the network has no single point of failure. Moreover, the loss of one platform, such as an aircraft, ship or land facility, communicating across the Link-16 network will not cause the entire network to collapse. As well as being secure, in other words, difficult to penetrate and eavesdrop upon, a Link-16 network is highly resistant to electronic jamming.

The data link enables several different types of information to be exchanged between platforms carrying radios capable of handling Link-16 communications, with non-voice data handled at rates of up to 256kbps. Primarily used in the air power domain, Link-16 can support a diverse array of tasks including air defence, anti-surface warfare, Anti-Submarine Warfare (ASW), reconnaissance and intelligence gathering, electronic warfare and air-to-air/air-to-surface targeting. These missions are facilitated by the information which Link-16 can carry. This includes information regarding a Link-16 participants’ location and identification, the status of the platform on the network, surveillance information such as the position of friendly and hostile navigation tracks, command and control information such as the weapons status of a particular platform, information relevant to electronic warfare, plus additional intelligence information and navigation data. Effectively, Link-16 is a situational awareness tool par excellence, and also provides radio channels for voice communications.

The ‘secret sauce’ in a Link-16 network is its use of the Time Division Multiple Access (TDMA) technique. This technique makes several time slots available per second. Each participant on the Link-16 network has their own distinct time slot allocated to them as they join the network. Each second, those Link-16 participants transmit information pertaining to the subjects mentioned above one after the other to the other participants on a network.

A Link-16 network is organised into specific ‘Nets’ stacked one on top of each other. Each of these nets has their time slots (see above) aligned with one another, and each net can be tasked with handling a specific type of situational awareness information, such as electronic warfare, air control or weapons status, for example. Yet these nets are not interconnected, meaning that any platform on the wide Link-16 network can only transmit and receive on a specific Link-16 net at any one time. For instance, it cannot simultaneously transmit across several different nets at once, requiring the platform to transmit its electronic warfare information in its allotted time slot on the electronic warfare net, and then transmit its weapons status information on the weapons information status net during its next allotted slot.

Time slot allocation is crucially important to a Link-16 network. Each 24 hour period of Link-16 operations is divided into 112.5 ‘epochs’ each of which is 12.5 minutes in duration. Each epoch is subdivided into 64 ‘frames’ each of which is twelve seconds in duration. Each frame is divided into three ‘sets’ of 512 timeslots (128 timeslots per second or four seconds in duration). Similarly, a frame can be divided into 1526 timeslots or 128 timeslots per second. In this latter example, a timeslot lasts 7.8 milliseconds.

The information regarding the missions and tasks that Link-16 can support is carried in a series of so-called ‘J-Series’ messages, the J-denoting that the message is a Link-16 message. Each is these has a specific alpha-numeric coding. For example, J14 messages concern electronic warfare control and coordination, J5 messages related to ASW while J9 and J10 messages relate to weapons control and coordination. These J-Series messages are further coded according to the domain they relate to, for example, the .2 reference relates to the air domain, .3 to the maritime domain, and .5 to land. In this instance, a message coded J9.2 indicates that the communications is a Link-16 message related to weapons control and coordination concerning the air domain.

Given that the number of messages which can be handled by a Link-16 network is so vast, not all of the platforms on a Link-16 network will be required to transmit all of the message types that the network can handle. For example, a tanker is unlikely to need to handle weapons control and coordination messages. For this reason, Link-16 network participants are organised into Network Participation Groups (NPGs). These NPGs are given specific timeslots (see above) within the NPGs to transmit and receive relevant information. As an illustration, fighters having their own NPG will be given timeslots to transmit and receive weapons coordination and control information. A simple description of how a J-Series message can be devised, sent and received is detailed in the ‘How It Works’ box accompanying this article.


How It Works: J-Series Messaging

A USAF Boeing E-3G AEW (Airborne Early Warning) aircraft detects a potential threat using its Northrop Grumman AN/APY-2 radar. The potential threat is viewed by a crewmember on board the E-3G using their Situation Display Console which gives them a radar picture of the airspace in their locale. Using the SDC, the crewmember prepares the information regarding the threat and formats it into a J-Series message, detailing its intended recipient. This message is then encrypted by the E-3G’s JTIDS radio and transmitted to the Link-16 network. This message is coded in such a way as to appear on a specific, relevant net. Also on this net is an F-16C/D fighter which is tasked to receive the message. The E-3G transmits the message during its timeslot on this particular net, in this instance the air defence net, which is then received by the F-16C/D on the same net. The F-16C/D’s JTIDS radio receives the message, decrypts it, and displays the message content on the fighter’s multifunction display screens.


Yet, to an extent, Link-16 has become a victim of its own success regarding how many users a network can accommodate. Mr. Eteson notes there is now “a high demand for access to Link-16 networks. In large operations, this creates difficulties for the network planners who make the Initialization Data Loads (IDLs). The IDLs allocate timeslots to each platform. More platforms, more information flow, and demand for multiple voice channels load the networks so much that it’s impossible to serve everyone with a single net plan.” He continues, the result of this is that “net control officers have to partition nets into operating areas to give users the capacities they need, but this introduces a bridging problem that has to be solved when Link-16 information in one area has to be available in another to build a complete tactical picture.”

A further important part of Link-16’s situational awareness is the navigation information that it can share. All participants on a Link-16 network provide their geographical position which allows track data to be managed and improves situational awareness for command and control assets such as United States Air Force Boeing E-3G Sentry AEW (Airborne Early Warning) aircraft which may be tasked with coordinating all, or some, of an air operation. This navigation information can include the position of participating aircraft, and relevant ground assets such as surface-to-air missile batteries, air defence destroyers or aircraft carriers.

Timeslots are also required on a Link-16 network for voice communications. As noted above, Link-16 can handle voice communications at a rate of 2.4 to 16kbps. However, when operating at full capacity, voice communications at rates of 16kbps can absorb several timeslots, which may be an issue if a Link-16 network is particularly busy. As with all forms of military communications, trade-offs are at work regarding Link-16 communications. While 16kbps offers very clear communications, it absorbs a significant number of timeslots. At the lower end of the spectrum, voice communications at a rate of 2.4kbps absorb fewer timeslots, but can lack clarity. J-Series (see above) messages, and freeform text messages, which Link-16 can also handle, however lack such demands on available timeslots.

Radios

Link-16 communications are handled by a number of radios. In the United States, this includes Joint Tactical Information Distribution Systems (JTIDS) radios which began to be fielded in the US from the mid-1980s initially on USAF E-3A/B aircraft, along with NATO ground control facilities, due to the large physical size of these JTIDS terminals. The high cost of the terminals also meant that only a handful of US fighters, namely the US Navy Grumman/Northrop Grumman F-14D Tomcat and USAF McDonnell Douglas/Boeing F-15C Eagle, would carry JTIDS terminals, developed as the JTIDS Class-2 terminal for these aircraft. The physical and financial incumbent in the JTIDS Class-1 terminal forced the development of the MIDS (Multifunction Information Distribution System) which was a NATO-wide initiative to roll out small, lightweight Link-16 compatible terminals on military aircraft and relevant platforms (see above) across the alliance, via the development of the MIDS-LVT (Low Volume Terminal).

JTIDS network
An early illustration explaining how the JTIDS network would link each of its participants. The realisation of JTIDS was an important step for Link-16 (US DoD)

The MIDS-LVT and MIDS-JTRS (Joint Tactical Radio System) terminals are now the ‘industry standards’ as far as Link-16 terminals are concerned. The MIDS-LVT family includes a number of terminal standards. For example, MIDS-LVT(1) terminals can accommodate voice communications and TACAN (Tactical Air Navigation) radio-based air navigation services. MIDS-LVT(2) terminals have neither voice communications nor TACAN, and are intended for use chiefly by ground-based assets. MIDS-LVT(4) has voice communications but no TACAN, and vice versa with MIDS-LVT(6), while MIDS-LVT(7) is intended for airborne users but is bereft of voice and TACAN. Finally, the MIDS-LVT(2/11) standard has been specifically designed for US Army air defence users and for USAF C2 centres.

Meanwhile, the US armed forces are moving towards the MIDS-JTRS terminal which will eventually replace their existing MIDS-LVT terminals. The MIDS-JTRS which has been developed by Data Link Solutions (DLS), a joint BAE Systems/Rockwell Collins initiative, will carry TACAN and optional VHF (Very High Frequency/30-300MHz) and UHF communications, plus the ability to handle the Wideband Networking Waveform (WNW). The WNW is being developed and implemented by the US armed forces and designed to facilitate communications between aircraft and ground vehicles. The MIDS-JTRS has been designed to have the same physical characteristics as MIDS-LVT(1) terminals in service on US military aircraft to enable the latter’s simple plug-and-play replacement with the former.

Industry Providers

As noted above, BAE Systems is involved with Rockwell Collins via DLS in the provision of MIDS-LVT terminals. One of the useful features built into the MIDS-JTRS terminal is its ability to reduce the net congestion which Mr. Eteson discusses above using approaches such as “Concurrent Multinet and its cousin, Concurrent Contention Receive”. He adds that “This give the MIDS-JTRS terminal the ability to receive Link-16 messages from four different sources in the same timeslot. This is a major innovation. For example, it gives a platform the ability to receive situational awareness feeds from two surveillance areas without bridging or using more Link-16 terminals on the platform.”

MIDS-LVT
BAE Systems is an important industry player in the Link-16 domain and provides a number of MIDS terminal products, such as its MIDS-LVT(1) product pictured here. (BAE Systems)

Like BAE Systems, ViaSat of San Diego, California, is heavily involved in Link-16 provision. The firm provides MIDS terminals to the US armed forces, and Andy Kessler, the business area director for the firm’s next-generation tactical data link systems, says that the firm has circa 3500 MIDS-LVT and 500 MIDS-JTRS terminals in service. One area of particular interest focuses on the company’s efforts to enclose the functionality offered by Link-16 into ever-smaller packages. The general trend regarding the miniaturisation of electronics in both the civilian and military domains has helped immeasurably in this regard. This has enabled the firm to realise the KOR-24A Small Tactical Terminal (also provided by Harris). This terminal can equip tactical vehicles, small boats and unmanned aerial vehicles, and small inhabited aircraft for users who may need to be hosted on a Link-16 network. In the US domain, the KOR-24A has been selected by the US Army and will equip that force’s Boeing AH-64E Guardian attack helicopters which are now entering service with the force. Mr. Kessler continues that the product has also been selected by an unnamed international customer. Similarly, for ground users, principally Forward Air Controllers, Link-16 access is provided through ViaSat’s AN/PRC-161 BAT-D (Battlefield Awareness and Targeting System-Dismounted). Providing Link-16 connectivity in a handheld radio the product benefits from the same miniaturisation which has assisted the development of the KOR-24A. Mr. Kessler told Armada that development of the AN/PRC-161 is continuing, and it is possible that it could be ready for acquisition from early 2017. Last, but by no means least, ViaSat also provides MIDS-JTRS radios in tandem with other firms discussed elsewhere in this article such as DLS.

ViaSat’s AN/PRC-161 BAT-D handheld radio
ViaSat’s AN/PRC-161 BAT-D handheld radio is placing Link-16 functionality into the hands of individual soldiers such as Forward Air Controllers. (ViaSat)

The work of ViaSat in migrating Link-16 to comparatively smaller tactical platforms is particularly important. US-led and NATO operations now require Link-16 as a sine qua non for air operations, and joint operations. Richard Gasoin, marketing director for Thales’ radio communications product line, told Armada that Link-16 is “now mandatory for NATO and coalition operations. Non-Link-16 platforms are not able to participate in global operations.” He adds that it has become so important because, “it brings the tactical situation to any assets and allows the acceleration of decision-making processes helping to avoid fratricide.”

Products such as Thales’ TopLink can equip a comparatively small air platform, such as a helicopter, with a standalone Link-16 terminal, avoiding the need to perform large-scale, and potentially costly, modifications to such an aircraft to allow it to participate in a Link-16 network. Similarly, users on the ground in a headquarters can employ the firm’s TopLink Mint which allows them to visualise all Link-16 tracks in a given area. In use with Armée de l’Air (French Air Force) units in Mali, West Africa, supporting operations against Islamist guerrillas in the country, TopLink Mint links back to the French Air Force’s National Air Operations Centre which is tasked with running the air component of these operations in Mali from Lyon-Mont Verdun airbase in south central France. The Link-16 picture generated by the TopLink Mint is carried back to the airbase using the French armed force’s Syracuse X-band (7.9-8.4GHz for uplink/7.25-7.75GHz for downlink) satellite constellation.

40 and Fabulous

Despite its age, the data link still has a lot of life left and Mr. Gasoin believes that “Link-16 will last for a long time.” Interestingly, Link-16 owes much of its genesis to Gordon Welchman, an Anglo-American mathematician who worked at the United Kingdom’s Government Code and Cypher School at Bletchley Park, outside London, with the British Second World War code breaker, and father of modern computing, Alan Turing. Both gentlemen helped to break Nazi Germany’s ENIGMA codes used for military communications and no doubt both would be impressed with both the longevity and capability of Link-16. 

There is a growing need to include an ever-increasing number of participants on a Link-16 network. This can include military helicopters, such as naval support rotorcraft. (US Navy)

by Thomas Withington

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