IEEE - Aerospace and Electronic Systems - August 2023 - 40

The Moonshine Pulse Repeater Jammer
Mandrel transmitted noise over a bandwidth of 4
MHz, with a preset center frequency that could be tuned
over the frequency band occupied by Freya. From a standoff
position beyond 35 miles, it could reduce the detection
range of Freya against a fighter from about 120 miles to
less than 20 miles.
The presence of noise jamming would have been eviFigure
1.
Freya radar in 1942, showing transmitter and receiver arrays, each
with 12 vertically polarized dipoles [Public domain].
in the development by the U.K. Telecommunications
Research Establishment (TRE) of an airborne barrage
noise jammer, Mandrel, and an airborne pulse deception
jammer, Moonshine (see [1], [3]).
dent to the radar operators, which was not always desirable.
An alternative strategy was to attempt to confuse the longrange
Freya stations into thinking that a large concentration
of aircraft was approaching and so cause their fighter
defenses to be activated and diverted to intercept them,
drawing them away from a real attack and gaining valuable
time. Knowledge of the Freya locations and frequencies
had been obtained from reports from TRE and the Noise
Interference Bureau of the Inter-Service Y Committee (the
Y service was a network of British signals intelligence
receiving stations). The Freya sites were situated along the
coastline at distances varying between 25 and 50 miles, as
shown in Figure 2. At many sites, there was more than one
Freya station, with 5 or 6 stations within 70 miles of the
Cap Gris Nez area. At each site, there would usually be a
W€urzburg which was not being jammed, so the Moonshine
aircraft would not approach closer than 35 miles to a Freya
site so that the deception would not be uncovered.
To determine the appropriate jamming power
required, calibration flights were made to obtain field
strengths at different heights and ranges. It was estimated
that the Moonshine receiver, with a/4 vertical dipole
antenna, would observe a maximum signal of 30 mV at
the minimum desired range of 35 miles, with signals
detectable at well beyond 100 miles.
Figure 2.
Freya sites as identified in 1942 [4].
40
IEEE A&E SYSTEMS MAGAZINE
AUGUST 2023

IEEE - Aerospace and Electronic Systems - August 2023

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