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. In practice, the resulting complexity of the system was so great that it often did not work, and flak troops came to distrust it. By 1943, the use of IFF in Germany was highly confused due to the proliferating number of radar units, Allied jamming, and the fear among the pilots that the Allies were using IFF signals to track their aircraft. Attempts were made to replace Erstling on several occasions, but the chaotic nature of the late-war signals efforts meant the favoured FuG 226 Neuling never reached operational status.
The Royal Air Force did, eventually, use the Erstling signals to track German aircraft. After a Junkers Ju 88 night fighter landed in Scotland in 1944, they were able to reverse engineer the Erstling and introduced the Perfectos system to trigger it. The signals were superimposed on existing radar displays, allowing the Perfectos operator to measure both the direction and range to the Erstling-equipped aircraft. When night fighter losses suddenly spiked, German pilots were told to turn their Erstling units off, leading to friendly fire incidents.
Operation
The basic system consisted of a transmitter/receiver, a "keying unit" to provide a secure day code, an integrated power supply, and a control panel. The power supply was fed by the aircraft's 24 VDC main power, driving a DC motor and alternator that produced 18 VAC 134 Hz output. This was used to power the electronics as well as a motor that drove the keying unit. The vacuum tube heater filaments were driven by the 24 VDC.
The receiver unit was an eight-tube superheterodyne design that was widely sensitive to the 2.4 m band. A 3000 RPM motor drove a tuning capacitor through the Freya range from 123 to 128 MHz, sweeping up and then down the band over a period of 10 ms. The Freya normally used a pulse repetition frequency (PRF) of 500 Hz, so during the 10 ms period, the radar would broadcast five pulses, and given that it was sweeping up and down, the receiver would be tuned to the right frequency during perhaps two of those pulses. The result was a series of pulses of the intermediate frequency turning on and off at about 200 Hz.
This output is used to modulate the transmitter unit. This has two effects. One is that the Erstling transmitter broadcasts a similar pattern of pulses on a selected frequency between 150 and 160 MHz, normally 156 MHz. These are received by a separate receiver at the radar stations, which output the 200 Hz low-frequency audible tone when received. The transmission output also damps down the receiver sensitivity for a short period so other pulses will not be received. This later system means the system will only output a signal for powerful sources, damping down weaker pulses from more distant sources. A small part of the transmitted signal is siphoned off on the way to the antenna and used to light a neon lamp on the instrument panel in the aircraft, indicating that the system was responding to an interrogation, not just receiving it.