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.

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.

After a Junkers Ju 88 night fighter landed in Scotland in 1943, 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.

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.

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.

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.

The system was never made to work correctly, and by 1942 as the tempo of night fighter operations began to rapidly increase, an expedient solution was needed.

Rennie Whitehead, who led the design of the British IFF Mark III, read the report and asked if there were circuit diagrams available.

Because the radar now filtered out returns from any of the other bombers, as well as other British aircraft in the area, it became a trivial matter for the operators to guide the specially selected Mosquitos directly at the German pathfinders, quickly shooting down half of them.

In the summer of 1944, the first British Mosquito aircraft were equipped with the "Perfectos", a device that activated the FuG 25a and triggered the response in synchronicity with their own radar signal transmissions.

The crews were told to leave the system turned off until they approached their bases,[1] where local anti-aircraft batteries would have no problems shooting them down while they flew slowly at low altitude.

[2] The Luftwaffe was known for fitting sensitive devices like "Erstling" with small explosive charges to allow their destruction in order to avoid capture.