Identification, Friend or Foe (IFF)
Last week I told you that the creation of the E-Z pass had been inspired by a World War II technology known as Identification, Friend or Foe (IFF). Both IFF and the first operational radar system – the Chain Home (CH) system – were developed and used in Britain in anticipation of and during World War II.
During the 1930’s, Britain saw the handwriting on the wall about the German threat. The punitive Treaty of Versailles signed at the end of World War I had incensed the Germans. Hitler wanted to punish England and her allies, and was clearly ramping up for war. Separated from the European continent by only the English Channel, the British felt very vulnerable. They began developing an operational radar system to provide early warning of invading German aircraft. Other countries – like the U.S. and Germany – did not feel the same urgency and instead focused on more theoretical radar research. Remember, at this point in time, radar was still very new, and no one yet realized what a game changer it would become.
From the day the CH system was first turned on, its technology was considered obsolete. Huh? Why would anyone do that? People do things like that when they feel like their backs are against the wall. The Brits were desperate. They would have liked to use microwaves (1 GHz to 100+ GHz), but they implemented CH using HF (25 MHz). It was a quick and dirty solution, chosen because they knew how to build HF – but not microwave – transmitters with enough range to warn of an attack from the continent. As built, the CH system was able to detect German airplanes assembling in the French airspace about 150 kilometers (90 miles) away. France would fall to the Germans in 1940.
The CH system dotted the eastern and southern coasts of Britain with antenna towers that transmitted and received pulsed radar. The pulses from different towers were staggered so the round-trip time of a single pulse could be accurately measured without interference from another pulse. This was a necessary condition for calculating how far away an enemy aircraft was. By 1940, airborne radars were being developed for British planes. The pilots used these radars to find and engage enemy aircraft originally detected by ground-based radar. If you want more details about this history, start with Mark Denny’s book Blip, Ping, & Buzz: Making Sense of Radar and Sonar.
IFF was invented and developed by the Brits at the same time as the CH system. It grew out a simple requirement to determine whether a CH-detected airplane was friendly or hostile. In its initial implementation, the IFF equipment on the friendly plane detected the CH radar signal and reflected it back with a characteristic, modified amplitude. The next design enhanced the on-board capability to take in the CH radar signal and return a pulse at the same frequency, now encoded to denote a friendly force. Since the new device received one transmitted signal and responded with a different signal, it came to be known as a “transponder”.
As the war dragged on, more and more radar frequencies began to be used. It became impractical to keep adding new IFF frequencies to the airborne transponders. The British solved this problem by switching to a dedicated IFF frequency band around 150 MHz. As one historian tells us: “IFF interrogating signals, known as ‘challenges’, were transmitted by means of dedicated transmitters known as interrogators whose pulse transmissions were synchronized with the transmissions of the associated radar so that radar echoes and friendly aircraft transponder replies cold be correlated in range.” The United States and Germany were also working on IFF; see https://journals.sagepub.com/doi/pdf/10.1177/002029400303600804.
While IFF helps radar, it’s important to remember that IFF is not radar. Radar involves the echoed return of RF energy and is used to measure range, azimuth, and velocity. IFF is more like a communications system. A transponder sends out signal that is received, identified and answered with another signal. There is no radar return. The IFF message includes things like aircraft identification, altitude, heading, and speed. It works with, but independently from, the radar system.
World War II ended, but the need for radar and IFF continued. The Cold War spurred development of increasingly sophisticated radar and IFF systems for military applications. This work continues today, hidden from public view as classified secrets.
Civil ( non-military) use of radar began in the United States in 1946, when Indianapolis Airport began using radar for air traffic control. By 1960, the US Federal Aviation Agency (now the Federal Aviation Administration or FAA) began requiring the use of IFF transponders on all civilian flights to help air traffic controllers identify each individual aircraft. You can read more of this story at https://www.natca.org/wp-content/uploads/2019/12/NATCA_ATC_History.pdf.
Up until 2020, the US air traffic control system (ATC) relied on ASR-11, a Digital Air Surveillance Radar System which includes both radar and IFF. Beginning in 2020, the US ATC began transitioning to the Automatic Dependent Surveillance Broadcast (ADS-B) system, which provides more precise satellite-based tracking. See https://www.faa.gov/nextgen/programs/adsb/faq/ for more details.
Having covered IFF, we’ll next transition to countermeasures (CM). During World War II, forces began to use electronic CM to protect themselves from enemy activities. Come on back next week, and we’ll talk about cool things like jamming, spoofing, chaff and flares.