The invention of radar to track aircraft immediately led to the issue of how to distinguish “friendly” aircraft from “hostile” aircraft. If an aircraft was just a blip on a PPI scope, there was no telling from that information if it was an enemy that had to be destroyed before it was too late, or if it was an “friendly” who got lost and was now in great danger from his own side.

The answer was to create a scheme, known as “identification friend or foe (IFF)” that allowed electronic identification of a target. A radar site or a fighter could have a radio system known as an “IFF interrogator” that sent a specific signal to an aircraft. The aircraft would, in turn, have a radio system called an “IFF transponder” that picked up the interrogator signal and gave a proper coded response to identify itself as friendly. Incidentally, IFF was also used on ships.

IFF is a tricky issue, since an enemy can not only use IFF to impersonate a friend, but can also trick friendly aircraft or ships into giving away their presence by interrogating IFF. This is the IFF challenge: protecting one’s own IFF while trying to compromise the enemy’s.

Early IFF systems were actually interrogated directly by radars, but as radars evolved into a wide range of different types, that meant that an IFF transponder had to be able to respond to all the different types of radars. That not only made the IFF transponder complicated, it made it easier for an adversary to compromise the IFF system. The solution was to develop specialized interrogator systems designed to be used as an accessory on a radar, with an IFF antenna “piggybacking” in some way on the radar antenna. This scheme was embodied in the British “Mark III” IFF transponder, which became an Allied standard during the war.

* The idea of having a transponder that replied to specific radar signals was a dead end for IFF but had its uses elsewhere, leading to the parallel development of “radar beacons”. These beacons were just transponders that could be used to mark an airfield, or could be carried by advance parties to mark paratrooper landing zones or amphibious landing beaches.

The same approach could be used to give the distance to a fixed station. The idea is conceptually very simple: an interrogator sends a radio pulse to a transponder, which then replies, and the round-trip time is determined to give the range between interrogator and transponder. The approach is very similar to radar, and in fact such a radar beacon scheme is often called a “secondary radar”. The British developed a precision bombing system based on secondary radars named “Gee-H” that permitted highly accurate “blind bombing”, at least for aircraft with a line of sight to a fixed base station in friendly territory.

* As noted above, radar beacons were often used to mark drop zones and landing beaches. The advantage of a radar beacon was that it did not advertise its presence to the enemy, only “speaking when spoken to.” There were times when that wasn’t really a concern, for example to mark rocks that were to be avoided by a landing force, and a cheaper marker could be used, called a “corner reflector” or more formally a “radar signature enhancement device”. This was just some panels of metal joined together in a kite-like configuration to create nice sharp corners that could reflect radio waves.

The problem with this type of corner reflector was that it was somewhat bulky
and inconvenient. After the war, another type of signature enhancement device
was developed, known as a “Luneberg lens”. Electromagnetic waves may have
different propagation velocities in different types of solids, with the
propagation velocity given by an “index of refraction”. If an electromagnetic
wave passes through materials with different indexes of refraction, the wave
will be partly reflected, with the amount of reflection given by the size of the
difference in the index of refraction. A Luneberg lens is a sphere with a graded
index of refraction, making it very reflective to radar. It is compact, and is
often carried by robot aircraft used as decoys or training targets.
Incidentally, a Luneberg lens is still sometimes called a “corner reflector”,
despite the fact that spheres don’t have much in the way of corners.