IEEE - Aerospace and Electronic Systems - August 2023 - 39

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resolution, a signal simulating a large number of aircraft in
the formation may comprise a range-extensive modulated
pulse extending over a few tens of microseconds. A single
aircraft fitted with such a jammer might then be able to
simulate the appearance of a raid of multiple aircraft
extending over, say, 5 miles down-range (i.e., over about
50 ms in time) from the real aircraft.
The transmitter power required by a repeater jammer
that is sending back pulses into a radar antenna main
beam may be very low. The basic radar equation shows
that the peak power flux density at a radar due to reflection
from a target at a range R is given by
Fr ¼
PrGr
s
4pR2L1 4pR2
where
Pr : radar peak transmitter power;
Gr : radar antenna mainbeam gain;
s : target RCS;
L1 : transmission losses (mainly radar feeder losses).
The flux density at the radar from a jammer at the
same range and bearing as the target is
Fj ¼
PjGj
4pR2L2
where
Pj : peak jammer power;
Gt : jammer antenna gain in the direction of the radar.
If a jammer located on a target is to replicate the
power received at the radar from the target it can be seen
that
PjGj PrGr
4pR2 ¼ kPrGr:
s
Now the jammer effective radiated power (ERP),
PjGj, should be a factor k less than the radar ERP. If s ¼
100 m2 and R ¼ 35 miles, it can be seen that k86 dB.
At longer ranges, the factor will be even smaller. So for a
peak radar transmitter power of 10 kW and an antenna
gain of 13 dB (see later values for Freya), a jammer that
AUGUST 2023
JAMMING FREYA
It was decided that active radar countermeasures should be
deployed against the Freya radar systems and this resulted
IEEE A&E SYSTEMS MAGAZINE
39
has an antenna gain of3 dB would require a peak transmitter
power of 0.5 mW. Even allowing for significant feeder
losses, a mismatch at the radar and higher radar power or
target RCS, it can be seen that a jammer transmitter power
only of the order of milliwatts is required.
GERMAN AIR DEFENSE SYSTEMS ACROSS THE
ENGLISH CHANNEL
In 1942, RAF Fighter Command was working to establish
air superiority over France. It was known that the
enemy had two coastal chains of radar (known at that
time in the U.K. as RDF) stations providing early warning
of air attack. The long-range Freya systems operated
on different frequencies in the band 119.5-128 MHz and
were believed to be able to detect aircraft at ranges up to
120 miles. There were also W€urzburg systems used
for gun laying, operating on 53 cm wavelength ( 570
MHz) with detection ranges of either 35 miles for the
standard system or 70 miles for the Giant W€urzburg (the
W€urzburg-Riese).
The Freya systems used a PRF of either 1000 Hz or
500 Hz, giving a maximum instrumented range of 93
miles or 186 miles, respectively. It was determined from
intercepted enemy reports that the maximum detection
range against a fighter aircraft was about 120 miles and at
that range an aircraft might be observed by 10 to 12 stations
simultaneously, each potentially operating on a different
frequency. Freya had separate transmitter and
receiver arrays (see Figure 1), each comprising two rows
of 6 vertical dipoles. The horizontal aperture was 6.2 m.
From photographic reconnaissance, it was estimated that
the antenna gain of the transmit and the receive arrays was
about 13 dB. The measured pulse width was about 2 ms,
suggesting a receiver bandwidth of 1/2 MHz. From intercepted
reports of detection ranges, it was also estimated
that Freya had a peak transmitter power between 5 kW
and 50 kW.
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IEEE - Aerospace and Electronic Systems - August 2023

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