FuG 25a Erstling
Type of aircraft transponder
FuG 25a Erstling (German: "Firstborn", "Debut", sometimes FuGe) was an identification friend or foe (IFF) transponder installed in Luftwaffe aircraft starting in 1941 in order to allow German Freya radar stations to identify them as friendly. The system was also used as a navigation transponder as part of the EGON night bombing system during 1943 and 1944. It was the second German IFF system to be used, replacing the FuG 25 Zwilling.
The basic concept of IFF had been introduced in November 1938 but little work was carried out on it initially. In 1939 the Würzburg radar was chosen to replace an earlier fire control radar from Lorenz. This led to the Zwilling, which responded to the Würzburg signals. Meanwhile, the GEMA company introduced the long-range Freya radar and a more advanced IFF system to work with it. This Erstling unit was clearly superior to the Zwilling, but 10,000 Zwilling units had been produced and they were slow to abandon it. Starting in July 1942, the Würzburg units were equipped with a separate Kuh unit that broadcast pulses on Freya's 2.5 m band, allowing them to work with Erstling.
In theory, Erstling was more secure than its Allied IFF Mark III counterpart, as it responded with a morse code signal that changed day-to-day.
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