IFF Mark II

Previous efforts

Even before Chain Home (CH) systems began deployment, Robert Watt had considered the problem of identifying friendly aircraft on a radar display. He filed initial patents on such systems in 1935 and 1936.^[1][2][3]

In 1938, researchers at the Bawdsey Manor radar research establishment began working with the first of Watt's concepts. This was a simple "reflector" system consisting of a set of dipole antennas that were tuned to resonate at the frequency of the CH radars. When a pulse from the radar hit them, they would resonate for a short period and cause an additional signal to be received by the station. The antennas were connected to a motorized switch that periodically shorted the antenna out and cancelled the broadcast, causing the signal to turn on and off. On the CH display, this caused the blip to periodically lengthen and contract. The system proved highly unreliable; it only worked when the aircraft was at certain locations and flying in certain directions.^[1]

It was always suspected this system would be of little use in practice, and when that turned out to be the case the Royal Air Force (RAF) turned to an entirely different system that was also being planned. This consisted of a set of tracking stations using HF/DF radio direction finders. The standard aircraft radios were modified to send out a 1 kHz tone for 14 seconds every minute, allowing the tracking stations ample time to measure the aircraft's bearing. Several such stations were assigned to each sector of the air defence system, and sent their measurements to a plotting station at sector headquarters. There they used triangulation to determine the aircraft's location.^[4]

Known as "pip-squeak",^[4] the system worked but was very labour-intensive, requiring operators at several stations and calculators at sector HQs. More operators were needed to merge the information from the pip-squeak system with that from the radar systems to provide a single view of the airspace. It also meant the pilots were constantly being interrupted when trying to talk their ground controllers. A system that worked directly with the radar was clearly desirable.^[5]

Mark I

Seeking a system that would be as simple as possible, the Bawdsey researchers began work with a regenerative receiver. The idea behind regeneration is to amplify the radio signal and send it into an LC circuit, or "tank", that resonates at a selected frequency. A small part of the tank's output is sent back into the amplifier's input, causing feedback which greatly amplifies the signal. As long as the input signal is relatively constant, like Morse code signals, a single vacuum tube can provide significant amplification.^[6]

One problem with regeneration is that if the feedback is too strong, the signal will grow to the point where it begins to broadcast back out of the antenna and cause interference.^[6] In the case of the IFF system, this is precisely what was desired. When the radar signal was received, and the gain was properly adjusted, the signal grew until it turned the system from a receiver to a broadcaster. The signal levels were still small, but the receivers in the radar systems were extremely sensitive and the signal from the transceiver was larger than what would normally be received from the reflection of the original radar pulse alone.^[7]

This extra signal would cause the aircraft's blip on the radar screen to suddenly grow to be much larger. Since it might be difficult to distinguish the resulting larger signal from IFF from the return of a larger aircraft or formation without IFF, the circuit was connected to a motorized switch that rapidly disconnected and reconnected the receiver, causing the blip to oscillate on the radar display.^[7] A switch on the front panel allowed the pattern to be controlled; one setting sent back 15 microsecond (μs) pulses, the second setting sent 40 μs pulses and the final setting switched between the two with every received pulse.^[8]

There were two major disadvantages of the design. One was that the pilot had to carefully set the feedback control; if it was too low the signal would not be received by the radar station, and if it was too high the circuit would amplify its own electronic noise and give off random signals known as "squitter" across a wide range of frequencies.^[9] This caused significant interference over a large area and was a major problem for radar operators.^[10] It was all too easy to forget to adjust the gain during flight, especially in single-crewed fighters, and it was estimated a usable signal was only returned about 50% of the time.^[7]

The other problem was that the CH stations operated on a small but distinct set of frequencies, and the system only worked at the single tuned frequency. An aircraft on a typical mission profile might only ever be visible to a single CH station, or perhaps two or three over their operational area. To address this, the IIF had a card with the frequencies of local CH stations on it, which the pilot had to tune as they moved about. Unfortunately, pilots often to forgot to do this, and if they were lost or off-course, they would not know which frequency to tune to, or the nearest station might not be on the card at all.^[7]

The Mark I was used only experimentally. Thirty sets were hand-made at AMES and an order for 1,000 was placed with Ferranti in September 1939.^[8]

Mark II

The IFF antenna can be seen on the left of this photo, meeting the fuselage in the RAF roundel. The lengthy antennas, which had to be placed on both sides of the fuselage, slowed the Spitfire by about 2 miles per hour (3.2 km/h).

In addition to the operational problems with the Mark I, a more serious problem was the ever-growing number of new radar systems being deployed through this period. Even as the Mk. I was being tested, the RAF, Royal Navy and British Army were all introducing new systems, spanning a wide range of frequencies from the RAF's 200 MHz systems used on night fighters and Chain Home Low through the Army's 75 MHz gun-laying radars and on to the CH at 20 to 30 MHz. Attempting to manually tune among these would be impractical, and completely impossible if the aircraft were visible to more than one radar, which was increasingly the case.^[11]

A solution was already under development by the spring of 1939. This was similar in general terms to the Mark I, but had a number of tuned circuits sensitive to the various radar sets. It used a "complicated system of cams and cogs and Geneva mechanisms"^{[1][lower-alpha 1]} to switch among the different bands by connecting to different oscillators covering a given band, and then used a motorized tuning capacitor to sweep through the frequency range within that band. The system was much more likely to work than the Mark I for the simple reason that the pilot could not forget to tune it, although there was some manual adjustments that were required on a less frequent basis.^[11]

An order for 1000 sets was sent to Ferranti in October 1939, and they had completed the first 100 sets by November. But the rapid expansion of the RAF meant there were not nearly enough to equip an appreciable amount of the fleet by the time of the Battle of Britain in the summer of 1940. Additionally, as the action took place mostly over southern England, IFF would not be very useful as the CH stations were positioned along the coastline and could only see the fighters if they were out over the English Channel. As such there was no pressing need to install the systems, and pip-squeak continued to be used through the Battle.^[7]

The lack of IFF led to a number of problems. Friendly fire was one; the Battle of Barking Creek in September 1939 would not have occurred if IFF had been installed. For the enemy, it meant their own aircraft could not be identified if they were close to known RAF planes. In July 1940 the Germans began to take advantage of this by inserting their bombers into formations of RAF bombers returning from night missions over Europe. To the ground operators these appeared to be more RAF aircraft, and once they crossed the coast there was no way to track them. Even when IFF was available, its general unreliability made it difficult for controllers to trust it.^[7]

As the Battle ended, Mk. II was rapidly installed across the RAF fleet. Its installation on the Supermarine Spitfire required two wire antennas on the tail that slowed the top speed by 2 miles per hour (3.2 km/h) and added 40 pounds (18 kg) of weight. Pip-squeak was still used for areas over land where CH did not cover, as well as an emergency guidance system.^[7] Mark II also found a use on Royal Navy ships, where it was produced as the Type 252 so that ships could identify each other by radar alone.^[12]

A Mark II set was taken to the US as part of the Tizard Mission in November 1940. US researchers were already working on their own IFF system of some complexity. They realized the importance of using a common IFF system, and in early 1941 they decided to install Mark II in their own aircraft.^[12] Production was taken up by Philco with an order for 18,000 sets as the SCR-535 in July 1942. The system was never entirely reliable.^[11]

Mark III

Meanwhile, the profusion of radars that led to the Mk. II continued, and by 1942 there were almost a dozen sub-types with a different set of frequencies. Additionally, the cavity magnetron had matured and an entirely new set of radars operating in the microwave region was about to enter service. These worked on frequencies that the IFF receivers could not operate on.^[13]

In 1940, English engineer Freddie Williams had considered this problem and suggested that all IFF operations move to a single frequency. Instead of responding on the radar's frequency and thus mixing with their signal in the receiver, a completely separate unit would transmit "interrogation" pulses in synchronicity with the radar's pulses, and the received signals would amplified independently and then mixed with the radar's signals on the display. This greatly simplified the airborne equipment because it operated on a single frequency, the only downside being that a second transmitter was needed at the radar stations.^[1]

Production of the IFF Mark III began at Ferranti and was quickly taken up in the US as well by Hazeltine.^[14] It remained the Allies' primary IFF system for the rest of the war, and the 176 MHz common frequency was used for many years after.^[13]

Citations

Bibliography

• Bowden, (Bertram) Vivian (1985). "The story of IFF (identification friend or foe)". Physical Science, Measurement and Instrumentation, Management and Education - Reviews, IEE Proceedings A. 132 (6): 435–437. Retrieved 2014-07-13.
• Brown, Louis (1999). Technical and Military Imperatives: A Radar History of World War 2. CRC Press. ISBN 9781420050660.
• "General IFF principles". United States Fleet. 1945. Retrieved 2012-12-17.
• Howse, Derek (1993). Radar at Sea: The Royal Navy in World War 2. Springer. ISBN 9781349130603.
• Shayler, J.S (2016). "The Royal Navy and IFF". In Kingsley, F.A. The Development of Radar Equipments for the Royal Navy, 1935–45. Springer. pp. 277–289. ISBN 9781349134571.