IFF Mark II facts for kids
IFF Mark II was a super important system used during World War II to tell if an airplane was friendly or an enemy. The Royal Air Force (RAF) in Britain created it right before the war began. After trying out an earlier version called Mark I in 1939, the Mark II was widely used by late 1940, right after the Battle of Britain. It was used until 1943, when a newer, better system called IFF Mark III took over. The Mark III was used by all the Allied forces (like Britain and the US) even after the war ended.
The Mark I was a basic system. It worked by making the signal from British Chain Home radar systems stronger. This made the airplane's dot, or 'blip,' on the radar display look bigger, showing it was a friendly plane. But the Mark I had problems. Pilots had to constantly adjust a setting called 'gain' while flying to make it work right. Often, it only worked correctly about half the time. Also, it only worked with one radar frequency at a time. This meant pilots had to manually change the setting for different radar stations. In 1939, Chain Home was the main radar, but new radars were coming out with many different frequencies.
The Mark II fixed these issues. It had an automatic gain control that adjusted itself, so pilots didn't need to touch it. This made it much more reliable. To work with many types of radar, it had a clever system of motors and gears. These parts constantly changed the frequency it used, scanning through three wide bands every few seconds. These changes made the device much easier to use and truly helpful for the first time. Before, radar operators couldn't be sure if a blip was an enemy or a friendly plane with a broken IFF. The Mark II was ordered in 1939, but its installation was delayed during the Battle of Britain. It became widely used from the end of 1940.
Even though the Mark II worked with many radars in the early war, by 1942, so many new radars were in use that several different versions of the Mark II were made. Each version worked with specific combinations of radars. Also, new radars using a special device called a cavity magnetron needed very different frequencies. The Mark II couldn't easily adapt to these. This led to the creation of the Mark III. The Mark III worked on just one frequency that could be used with any radar. It also didn't need the complex gear system. Mark III started being used in 1943 and quickly replaced the Mark II.
Contents
How IFF Systems Developed
Early Ideas for Identification
Before Chain Home radar systems were fully set up, a scientist named Robert Watson-Watt thought about how to identify friendly planes on a radar display. He even filed patents for these ideas in 1935 and 1936.
In 1938, scientists at the Bawdsey Manor radar research center started working on one of Watt's ideas. This was a simple "reflector" system. It used special antennas that would vibrate when hit by a radar pulse. This would send an extra signal back to the radar station. A motor would quickly turn the antennas on and off. On the radar screen, this made the plane's 'blip' periodically get longer and shorter. However, this system was very unreliable. It only worked when the plane was in certain spots or flying in specific directions.
Everyone suspected this system wouldn't work well in real battles. When it didn't, the Royal Air Force (RAF) tried a different method. They used a network of tracking stations with HF/DF radio direction finders. Normal aircraft radios were changed to send out a special tone for 14 seconds every minute. This let the tracking stations figure out where the plane was. Several stations worked together for each air defense area. They sent their measurements to a main plotting station. There, they used triangulation (like drawing triangles on a map) to find the plane's exact location.
This system, called "pip-squeak", worked but needed a lot of people. Operators were needed at many stations and at plotting boards in the main headquarters. More people were needed to combine the information from pip-squeak with the radar data. This also meant pilots were often interrupted while talking to ground control. Everyone wanted a system that worked directly with the radar.
The First IFF: Mark I
To make a system as simple as possible, the Bawdsey scientists started working with a special kind of radio receiver called a regenerative receiver. The idea was to make the radar signal much stronger. If the signal became strong enough, the system would actually start sending out its own signal from the plane's antenna. The signals were small, but radar receivers are very sensitive. The signal from the IFF was stronger than just the normal radar reflection from the plane itself.
This extra signal made the airplane's blip on the radar screen suddenly grow much larger. To make sure this big signal wasn't just a large enemy plane, the system was connected to a motor. This motor quickly disconnected and reconnected the receiver, making the blip on the radar screen wiggle. A switch in the cockpit let the pilot control this pattern. One setting sent short pulses, another sent longer pulses, and a third switched between the two with every radar pulse.
The Mark I had two main problems. First, the pilot had to carefully set the feedback control. If it was too low, no signal would be sent back. If it was too high, the system would create its own random signals, called "squitter". This caused a lot of interference for radar operators. It was easy for pilots, especially in single-seat fighter planes, to forget to adjust this setting during a flight. It's thought that the Mark I only sent a usable signal about half the time.
The second problem was that Chain Home radar stations used a few different frequencies, and the Mark I only worked on one frequency at a time. A plane on a typical mission might only be seen by one or two Chain Home stations. To deal with this, the cockpit panel had a card with the frequencies of local Chain Home stations. The pilot had to tune the system as they flew around. Pilots often forgot to do this. If they got lost, they wouldn't know which frequency to use, or the nearest station might not even be on their card.
The Mark I was only used for experiments. Thirty units were made by hand, and an order for 1,000 was placed in September 1939.
The Improved Mark II
Besides the problems with the Mark I, a bigger issue was the increasing number of new radar systems being used. Even while the Mark I was being tested, the RAF, Royal Navy, and British Army were introducing new radars. These radars used a wide range of frequencies, from the RAF's 200 MHz systems on night fighters and Chain Home Low to the Army's 75 MHz gun-laying radars and the Chain Home at 20 to 30 MHz. Trying to manually tune between all these would be impossible, especially if a plane was seen by more than one radar, which was happening more and more.
A solution was already being developed in early 1939. It was similar to the Mark I but could work with many different radar sets. It used a "complicated system of cams and cogs and Geneva mechanisms" to switch between different frequency bands. It would connect to circuits that covered a band, then use a motor to sweep through the frequencies within that band. To make sure the signal was the right strength and didn't cause "squitter", an automatic gain control was added. These changes meant pilots didn't need to tune or adjust the gain during flight. This greatly improved the chance that the system would respond correctly to a radar. Only occasional adjustments on the ground were needed to keep it working.
An order for 1,000 Mark II sets was sent in October 1939. The first 100 were ready by November. However, the RAF was growing so fast that not many planes had the system by the time of the Battle of Britain in mid-1940. The battle mostly happened over southern England, where IFF wasn't as useful because Chain Home stations were along the coast. They could only see fighters if they were over the English Channel. There wasn't an urgent need to install the systems, so pip-squeak continued to be used during the battle.
The lack of IFF led to problems, including friendly fire (when friendly forces accidentally attack each other). For example, the Battle of Barking Creek in September 1939 wouldn't have happened if IFF had been installed. It also meant that enemy planes couldn't be identified if they were flying close to known RAF aircraft. In July 1940, the Germans started taking advantage of this. They would fly their bombers within formations of RAF bombers returning from night missions over Europe. To ground operators, these looked like more RAF planes. Once they crossed the coast, there was no way to track them. Even if one of the rare Mark I sets was available, its unreliable signals made it hard for controllers to trust it.
As the Battle of Britain ended, the Mark II was quickly installed in RAF aircraft. Installing it on the Supermarine Spitfire required two wire antennas on the tail. These antennas slowed the plane's top speed by about 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 that Chain Home didn't cover, and as an emergency guidance system. The Mark II was also used on Royal Navy ships, where it was called Type 252. This allowed ships to identify each other using radar.
A Mark II set was taken to the US in November 1940. US scientists were already working on their own IFF system. They realized how important it was to use a common IFF system. In early 1941, they decided to install the Mark II in their own aircraft. Production was started by an American company, with an order for 18,000 sets called SCR-535 in July 1942. However, the system was never completely reliable.
The Next Generation: Mark III
The number of different radars kept growing, and by 1942, there were almost a dozen different versions of the Mark II. Each version covered a specific set of frequencies. Some, like the IIIN, were tuned for radars commonly used by the Navy. Others, like the IIIG, were for ground radars used by the Army and Air Force. No single Mark II unit could respond to all of them. To make things more complicated, the cavity magnetron had improved. A new generation of radars using very high frequencies (microwaves) was about to be used. The IFF receivers couldn't work on these new frequencies.
In 1940, an English engineer named Freddie Williams thought about this problem. He suggested that all IFF systems should use just one frequency. Instead of responding on the radar's frequency, a separate unit would send out "interrogation" pulses at the same time as the radar's pulses. The signals received from the IFF would then be amplified separately and combined with the radar's signals on the display. This made the equipment on the plane much simpler because it only needed to work on one frequency. The only downside was that radar stations needed a second transmitter.
Production of the IFF Mark III began in Britain and was quickly taken up in the US. It became the main IFF system for the Allies for the rest of the war. The 176 MHz common frequency it used was important for many years after the war.
Different Versions of IFF Mark II
- Mark I – This was the first experimental version that worked with Chain Home radars.
- Mark II – This version automatically scanned three different frequency bands. It worked with Chain Home, gun-laying (GL), and Navy radars.
- Mark IIG – The "G" stood for "Ground". This version had bands that covered common ground-based radars like Chain Home, CHL, GL, and AMES Type 7.
- Mark IIN – The "N" stood for "Naval". This version had bands that covered various Royal Navy radars like Type 286.
- ABE (SCR-535 and SCR-535/A) – This was the US version. It covered US Army radars like SCR-268, SCR-270, SCR-271, and SCR-516.
- ABK – This was another US version. It covered US Navy radars as well as common ground radars.
Images for kids
-
The IFF Mark II antenna on this Spitfire can just be made out, stretching across the rear fuselage from the roundel to the tip of the horizontal stabiliser.
