Digital Technologies: Operational Flight and Tactics Trainer (OFTT) link (1964)

Linking the Operational Flight and Tactics Trainer (OFTT)

In the mid-sixties, the Royal Netherlands Air Force (KLu) started to deploy F-104G fighter jets, better known under the name Starfighter. The 322 and 323 Squadrons at the Leeuwarden Air Base received this fighter aircraft for its hostile aircraft interception task. The aircraft’s pulse radar had to be used to intercept low-flying invaders. Unfortunately, that use of the radar produced a lot of ground clutter. For that reason, the pilot had to be led by a ground station to a favourable attack position. At the moment the pilot caught sight of his opponent, he could execute the attack task.

 RNthAF Starfighter is in amazingly original condition and was parked on the flightline as if it was still operational (wishful thinking!). c/n 683-8114. On display at the '100th Anniversary of Dutch military aviation'. Volkel Airbase, Netherlands. 14-6-2013 by Alan Wilson [CC BY-SA 2.0 (], via Wikimedia Commons
RNthAF Starfighter is in amazingly original condition and was parked on the flightline as if it was still operational (wishful thinking!). c/n 683-8114. On display at the ‘100th Anniversary of Dutch military aviation’. Volkel Airbase, Netherlands. 14-6-2013 by Alan Wilson [CC BY-SA 2.0 (], via Wikimedia Commons
In the initial period, the pilot was controlled by the Navigation Station North (NS “N”) in Den Helder (the term Navigation Station was a concealed name for a Ground Controlled Interception operations station). The horizontal scanning radar and a number of altitude radars were set up to monitor the airspace of the northern Netherlands in the Cold War from the command & control station. That station controlled the F-104 interceptors and guided them to intercept the intruder by radio. As one can understand, such interceptions (very fortunately) did not take place very often.

Flight simulator

To practice the pilots’ flying skills and the attack procedures, a flight simulator at Leeuwarden Air Base was used. In the flight simulator the pilots could not only make a simulated flight with the Starfighter as fighter. A simulated opponent, the target, could also fly around. The target was controlled by an instructor/operator. The setup was called “Operational Flight and Tactics Trainer” (OFTT). OFTT was manufactured by Canadian Aviation Electronics (CAE). The “Linking OFTT” project focused on this task: practicing the pilots to manoeuvre into a good attack position.

The flight simulator consisted of a cockpit and a room full of high cabinets with racks full of servo systems and an analogue computer system. The electronics tracked every movement that the fighter and the target made. The servos controlled the cockpit to be able to simulate all movements of the aircraft and to give all cockpit instruments the correct value. This gave the pilot in the cockpit a realistic experience of his flight and the attack. Finally, there was a large room with a model of the Northern Netherlands area including the aircraft shooting range on Vlieland. The surrounding image was recorded with video cameras and projected on a screen in front of the cockpit by a beamer. This setup made the visual perception as realistic as possible for the pilot.

Cockpit of the OFTT Lockheed F-104G Starfighter (source: Beeldbank van het Nederlands Instituut voor Militaire Historie (NIMH)
Cockpit of the OFTT Lockheed F-104G Starfighter (source: Beeldbank van het Nederlands Instituut voor Militaire Historie (NIMH)

Training exercises

As described, the pilot had to practice intercept procedures on a simulated target that was operated by the instructor. To make the exercise even more realistic, the KLu proposed to link the flight simulator with the radar station in Den Helder. The idea was to inject both the position and height of the fighter and the target as simulated radar echoes into the radar system. The NS “N” operators could then watch and track this information on their radar screen. This allowed them to control the simulated interception as if it were a real attack.
The advantages of this design were enormous. The observers in Den Helder were able to command & control many interceptions; a chance they didn’t have often or two (or more) real aircraft planes had to take off to practice this. The pilot could receive realistic operational instructions from real radar observers, a task that the instructor of the simulator could only partially perform. In short: a training course both for observers and pilots under operational conditions. A major extra advantage was that it did not require expensive flight hours.

The TNO Physics Laboratory was commissioned by the KLu to design, develop and establish this link. A feasibility study was carried out in 1964. In 1968, the link became operational. An article (in Dutch) about the OFTT was published in the Militaire Spectator in 1967.

Linking the OFTT

The positions and height of fighter and target were only available in the servos of the analog computer system. The range of each servo between its maximum and minimum values was ten revolutions. A ten-rotations potentiometer was connected to the axis of each of the position and height servos. The value of the resistor could therefore be digitised. Each of the recorded values ​​was then put in a digital message.
These messages were transferred via a modem connection (FSK, 750 Baud) over a leased (analog) telephone line between Leeuwarden and Den Helder. A second telephone line was used to transfer voice information in order to simulate the radio connection between pilot and radar station operator.

It should be noted that integrated circuits do not yet exist. All digital electronics were built with transistors where only one flip-flop or a single port was built on a small printed circuit board. A large cabinet, one meter wide and one meter high, with two racks one behind the other filled with printed circuit boards (PCBs) was the usual electronics construction method at that time. One of such cabinets was placed in Leeuwarden and one in Den Helder.
In 1968, the OFTT link was completed by TNO and put into operational use by the KLu. Every year, two TNO employees had to travel to Leeuwarden and then to Den Helder to recalibrate the system.

2nd generation OFTT-link

In 1972, the task of the five Dutch airspace monitoring stations (navigation stations) was taken over by Nieuw Milligen, which received a new and more modern three-dimensional long-distance Medium Power Radar (Thompson-CSF MPR a.k.a. ARES-3D). The Den Helder airspace radar station was closed as a result.

ARES-3D radar at Nieuw Miiligen
ARES-3D radar at Nieuw Milligen, source: Beeldbank van het Nederlands Instituut voor Militaire Historie

In 1973, TNO received a request by the KLu to move the OFTT link to Nieuw Milligen. Because the position and height data of fighter and target had to be entered in a different way, it was decided to build a completely new link system for Leeuwarden. All electronics could be replaced with the technology of that time. The full system could be placed in just one 19-inch rack adjacent to one of the servo systems. In Nieuw Milligen, only a modem was required and the data could be entered directly into the computers. The data from fighter and target could be presented on the screens in the same way as with real aircraft. This second generation system was delivered in 1974.

This version of the OFTT link attracts the attention of the Belgian Air Force. They then use equivalent facilities: an F-104 flight simulator in Bevekom (Beauvechain) near Leuven and a command/radar observation station in Glons. The Belgian Air Force subsequently commissioned TNO to develop an equivalent OFTT link for them in 1975. The electronics were delivered in 1976.

In 1980, the last Starfighter is decommissioned at the Leeuwarden air base. In the same year, the Belgian Starfighters were replaced as well by F-16s. Because flight simulators for those new aircraft are based on modern computer technology, transferring data is no longer a problem. No special link electronics is needed anymore, which means the end of the OFTT project.




This page is based on text written by Ing. W.(Wim) F.M. van der Heijden