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The Gripen data link

March 5, 2019

 

SA Flyer January edition featured an in-depth look at the Saab Gripen. While lauded for its agility, diverse ordnance load, and ability to use short runways, one aspect of the Gripen that makes as big an impact on its combat performance is its world-leading integration and use of digital tactical data links. It’s worth taking a dive into the history behind how Sweden developed the Gripen’s data link.

 

 

 

Sweden has long been a pioneer in the adoption of tactical digital data links, defining its first tactical data links protocols in the 1960s and flying the first operational aircraft-to-aircraft data links a decade later. The requirement arose from its early understanding that it was in a weak strategic situation and could not hope to fight the Soviet Union directly from traditional bases, using easily overheard radio calls, and with traditional tactics.

This led to Bas 60 (Flygbassystem 60, Air Base System 60), a strategic plan which involved the construction of dozens of reserve, temporary, and road bases around Sweden to which its Saab 35 Draken and Saab 32 Jansen would rapidly disperse in the event of an attack, and a country-wide distributed digital command and control (C2) system called STRIL 60 (Stridsledning och Luftbevakning) that could send digital data link commands to the Draken from 40 strategically-placed 10 kW VHF transmitters.

The STRIL 60 data link in the Draken was a mechanical and electrical genius, given it entered operational service in the early 1960s, when even the USAF was still only experimenting with similar systems. Each 103-bit STRIL control data message contained a header containing synchronisation info and its source ID, the stage of the attack process, and the ID of the aircraft to which it was addressed. After this came the height, course, bearing, and distance of the target relative to the intercepting aircraft, along with a command message of one of 20 pre-defined phrases such as “STIG” (track).

When these messages, broadcast at 3000 bits/second (or 26 messages / second), reached the intended aircraft it routed them through specialist circuitry that converted the bits into electrical signals to drive mechanical instruments in the cockpit. The Distance-Altitude-Command (AHK) instrument, for instance, had a rotating drum on which Saab had printed all 20 STRIL commands, with the incoming signal determining which one was visible through the viewing window. It had an altitude indicator with a ribbon showing own height and an arrow being set to target height by the incoming messages.

In an era before digital displays and powerful computers it was a remarkable system that gave the Swedish Air Force the ability to disperse country-wide and yet accurately meet any attack with the help of ground radar stations, all without a single spoken word over the radio. Today, more than 50 years on, most air forces still can’t do that.

Introducing the JA-37 Viggen (the earlier attack-oriented AJ-37 Viggen lacked a data link) took the STRIL 60 system to a new level of capability. It was the first aircraft to fly operationally with a modern-style integrated circuit onboard computer, the CK 37, which handled onboard navigation, radar control, and a host of other functions. This meant that while it still received the same basic 103-bit message format as the Draken, it could also receive the absolute position of the target as opposed to its relative position, freeing up the ground control systems to perform other calculations and allowing for the transmission of multiple target tracks.

The Viggen’s PS-46/A radar was linked to the CK 37 computer, so it became possible to slave it to the STRIL data link which meant that the ground control systems could position the radar’s antenna to the most advantageous position for a lock. The cockpit was fitted with both a Head-Up Display and a CRT-based multi-function display, meaning it could show more complex data and more message types.

This combination was extraordinarily effective. So much so that the Swedish Air Force could use it to perform the only successful intercepts ever conducted against the US Air Force’s SR-71 Blackbirds. The latter would skim the edge of Swedish air space about once a week while flying their “Baltic Express” surveillance missions along the Soviet coast, passing through a narrow 3 km-wide sliver of international air space between the Swedish mainland and Gotland at 70,000 ft and Mach 3.

The ground control stations would scramble Viggens once they detected an incoming SR-71 and position them for a head-on intercept, first climbing to 26 000 ft at Mach 1.35, then a three to five degree upward climb on afterburner to 60,000 ft directly in the path of the oncoming SR-71 and into the best firing position for a simulated launch of their Skyflash air-to-air missiles. Throughout the intercept the Viggen pilots had the entire tactical picture on their multi-function display, merging the information from ground radars and their own radar to provide the accuracy necessary for an intercept with closing speeds of Mach 4-5 and only the tiniest margins of error.

The Swedish Air Force recorded over 50 successful intercepts - defined as those where the simulated Skyflash firing was statistically likely to have scored a direct hit. It’s difficult to imagine their having the same success without the pin-point accuracy, jamming resistance, and radar-slaving provided by the Stril system and it proved to the Swedish Air Force that similar tactics would work against Soviet MiG-25s and other high-altitude, high-performance aircraft.

As powerful as the Stril ground-controlled intercept data link was, it had a glaring shortcoming in its reliance on ground-based control systems. In an era where Soviet offensive systems seemed to advance ever further and faster, Sweden could no longer depend on the integrity of the Stril system being maintained in the event of an attack.

It therefore developed the next evolution of tactical data link: A world-first two-way encrypted aircraft-to-aircraft, ground-to-aircraft, and aircraft-to-ground digital radio “fighter link” that entered operational service on the JA-37 Viggen in 1985. This was a generational leap ahead of the existing system because for the first time it meant that it could share aircraft data with other Viggens and ground systems in real time. Not only did this mean that status information like position, fuel levels, armament remaining and so on could be shared securely with no spoken radio calls, which enhanced situational awareness and reduced pilot workload, but the link shared real time radar information with such a high level of accuracy that the receiving Viggen could use the data to fire its own Skyflash air-to-air missiles without ever turning on its own radar.

Thus a Viggen flight could launch an attack with only one or two aircraft using their radars and being seen by the on-board self-defence systems of enemy aircraft, while the ‘nose cold’ Viggens raced ahead into the most advantageous firing position. The level of tactical situational awareness available to Viggen pilots was like nothing seen before, and it let themthey could experiment with ever more interesting attack profiles that treated the entire flight as a coherent system which had combined capabilities that were greater than the sum of its constituent parts.

It was an early glimpse of the power that networked warfare could bring.

By 1980, concept work had already begun on the next generation Swedish fighter, the Saab JAS 39 Gripen, to replace the Viggen and the few remaining Drakens in service. The new aircraft took advantage of advances in computing and the miniaturisation of electronics to become a swing-role aircraft, defining its operating roles and functions in software, rather than hardware. So they could change roles in-flight as a result of processing the output of all of its onboard sensors into an integrated set of fused views. This took both internal data processing and external data sharing to the next level.

On the Gripen the ‘fighter link’ expanded into the Tactical Information Data Link System (TIDLS) or ‘TAU-link’ (Tactical Air Unit), a TDMA-based high-bandwidth bi-directional UHF data link connecting up to four Gripens in a flight (hence ‘tactical air unit’) and to a Saab Erieye-carrying airborne early warning aircraft. Gripens can share almost every onboard function across TIDLS, including position, altitude, airspeed, and heading; fuel, weapons, and countermeasures status; target position and movement data, whether in the air or on the ground; active engagements; threats; and even the position of the cursor on multi-function displays to allow for pilots to highlight targets or items of interest for other aircraft.

TIDLS has a range of up to 500 km and is both semi-directional and highly resistant to jamming. So Gripen operators have been able to redefine the concept of a flight, from requiring the aircraft to stay near each other, to one where the four aircraft can spread across hundreds of kilometres while still actively sharing radar, status, and other sensor data at high-speed and in real-time. This creates both tactical surprise and ambiguity, because an enemy flight can no longer easily predict from which direction an attack might come, and opens new opportunities for sensor fusion across multiple aircraft.

By being so far apart, yet sharing radar and EW data in real time, a flight of four Gripens can each combine the measurements and perform onboard calculations for faster lock-on, better tracking, and jamming avoidance. Where pilots could treat a Viggen flight as a coherent unit in the tactical sense, Gripen pilots can go a step further and treat their flights as a coherent and combined sensor suite made up of four connected nodes. As onboard sensors and computers become more powerful, so the capability of Gripen flights will continue to increase.

While an intra-flight data link is convenient, there’s always a need to speak to ground control and other platforms. The first Gripen variants also used the Swedish national data link, TARAS (Tactical Radio System), linked to the updated Stril 90 system for broader communication. The Gripen C/D variant ditched TARAS for the NATO Link-16 standard, as Sweden had by then become increasingly involved in NATO operations and needed to communicate with other NATO platforms during missions. This caused a controversial capability gap as it has taken time for other Swedish military systems to transition to Link-16 and there’s no easy mechanism to translate between it and TARAS.

In South Africa’s case, the locally developed Link-ZA is the chosen data link in place of TARAS and Link-16. Link-ZA is a mostly TDMA (time division multiple access) based data link that can run on HF, VHF, or UHF bands at a rate of up to 16,000 bits/sec, accommodating 16 platforms per net and with at least three concurrent nets in operation. Messages are defined according to the Variable Message Format standard, split into the broad categories of Image, Awareness and Text Messages.

Through Link-ZA the South African Air Force has adapted the TIDLS concept to share radar situational awareness data from Gripens to its Hawk Mk120 lead-in fighter trainers, thus making the Hawks cheap interceptors, despite their own lack of onboard radars.

Looking at the near future, tactical data links are undergoing another generational change. Where current approaches like Link-16 and Link-ZA depend on reliable but dated technologies like TDMA to prevent contention, which severely limits network sizes and flexibility, manufacturers are building the next generation of data links on top of internet technologies with IP (internet protocol) addressing, software-defined radio waveforms, ad hoc mesh networks, and flexible adaptation and self-repair.

When these are used with AESA radars and other new-generation sensors we will see a whole new range of capabilities and combinations arise.

 

 

 

 

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