FuG 25a Erstling

FuG 25a Erstling

An Erstling coding unit
Country of origin	Germany
Introduced	1941
Type	Identification Friend or Foe (IFF)

FuG 25a Erstling (German: "Firstborn", "Debut") was an identification friend or foe (IFF) transponder installed in Luftwaffe aircraft starting in 1941 in order to allow German radar stations to identify them as friendly. Later, the FuG 25a became a key component of the EGON night fighter guidance procedure.

Built by the GEMA company, who also designed the Freya radar, the basic concept had been introduced in November 1938, but little work was carried out on it initially. The Luftwaffe's Chief of Signals placed an order for 2,000 to 3,000 units in late 1939, although the prototype was not available until 1941. The system only responded to the Freya, a serious limitation as the Würzburg radar became more widely used. This was addressed starting in July 1942 by equipping Würzburg units with a separate Kuh unit that broadcast pulses on Freya's 2.5 m band.

In theory, the Erstling was much 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 favored 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 then received on older 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 tubes heater filaments were driven by the 24 VDC.

The receiver unit was an eight-tube superheterodyne design that 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 transmitter broadcasts a similar pattern of pulses, which produces a low-frequency audible tone when received, and the other is that the transmitter output 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. The transmitter is set to a selected frequency between 150 and 160 MHz, normally 156 MHz. A small part of the transmitted signal was siphoned off on the way to the antenna and used to light a neon lamp on the front panel, indicating that the system was responding to an interrogation (not just receiving it).

Between the receiver and transmitter is the keying unit. This consisted of two sets of motorized cam switches with ten cams on each shaft. The cams were engaged by two long keys that were inserted into the keying unit, selecting among a set of 1,000 possible patterns. The keys were inserted on the ground and could not be changed in flight, but a switch on the front panel selected which of the two patterns to use. The shafts completed a revolution in about ​1 ¹⁄₂ seconds, alternately allowing the 200 Hz signal through to the transmitter or blocking it. The end result was to reproduce a 10-bit morse code signal when interrogated.

At the Freya site, received signals were sent to a separate unit that filtered low frequency signals into a set of headphones. The radar operator could then listen to the code while the interrogation button was held down. The codes were changed every day, which provided considerable security.

Kuh unit

The FuG 25a had been designed to work only with the Freya radar, while other IFF units were being designed to provide a signal for the Würzburg units. These used a different system where the receiver was set to trigger the transmitter when the PRF of the interrogator changed from the Würzburg's normal 3,750 Hz to 5,000 Hz. 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.

This came in the form of the Q-Gerät or Kuh (German for "cow") transmitter and Gemse (German for "chamois") receiver. The Kuh was a simple system that broadcast low-power signals similar to that of a Freya when triggered by the IFF switch on the Würzburg it was connected to, while the Gemse was an equally simple receiver turned to 156 MHz and then to the same filter as the Freya receiver. Thus when the operator pressed the intorrogate button they heard the same signal as they would on the Freya.

Countermeasures

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, which was detected to reveal the presence of the enemy aircraft. This severely curtailed the use of "Erstling", as German night fighter crews had to switch it off to avoid detection.^[1] A number of German night fighters were shot down by German flak forces when they switched off their units.^[2]

Technical specifications

• Receiver: 125 MHz (Freya) and 550-580 MHz (Würzburg)
• Sensitivity: 2 mV
• Transmitter: 156 MHz
• Power: 0.2 Watt
• Activation: Radar pulses at 5000 Hz
• Encryption: 2x10 bit
• Range: 40 km (FuG 25z) and 270 km (FuG 25a)

Safeguards

The Luftwaffe was known for fitting sensitive devices like "Erstling" with small explosive charges to allow their destruction in order to avoid capture. A short fuse allowed the crew to reach a minimum safe distance.

See also

• Wilde Sau

Literature

• Christian Möller, "Die Einsätze der Nachtschlachtgruppen 1, 2 und 20 an der Westfront von September 1944 bis Mai 1945", ISBN 978-3-938208-67-0
• Fritz Trenkle, "Die deutschen Funkführungsverfahren bis 1945", Dr. Alfred Hüthig Verlag Heidelberg, 1987, ISBN 3-7785-1647-7
• Werner Gierlach, "Flugmeldedienst", issue 8, Freya-Fibel, pages 43-44), Cologne
• TME 11-219 Directory of German Radar Equipment

External links

• Description and pictures of FuG 25a (partly in German)
• Fighter control stations
• Training manuals of the "Jagdschloss" aerial signals training facility (PDF) (German)
• More "Jagdschloss" pictures

References