Passive detection

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David Lednicer

Aug 21, 1998, 3:00:00 AM8/21/98
Some time ago, someone in this group asked how passive detection
systems work. Recently, I came across the description of one such

Shortly after radar was developed, researchers saw the need for a way
to allow radar observers to know whether a target was friend or foe.
Thus was borne IFF (Identification, Friend or Foe). This was a unit on
an aircraft, that when interrogated by a radar unit, replied saying who
the airplane was. I think more modern units also might tell where the
airplane is. Commercial transponders usually work with encoding
altimeters to provide such info.

The Soviet tactical doctrine was that fighters were to be under ground
control. This entailed the ground controller knowing where the fighter
was. To do this, the fighter's IFF unit needed to be kept on at all

In the mid-1960s, someone working for the USAF found a way to
interrogate Soviet IFF units. The resulting QRC-248 was first used
onboard EC-121s operating in the Vietnam conflict. This capability was
top secret and initially, the EC-121 crews were only allowed to monitor
when the Soviet IFF sets were interrogated by ground radar, a so-called
"passive" mode. Eventually, they were allowed to actually interrogate
the IFF sets themselves, in an "active" mode. This capability was first
used in 1967 and resulted in a dramatic improvement in US air-air
results. The EC-121s could issue MiG warnings to specific aircraft,
with position information.

Eventually, a APX-80 Combat Tree system was installed in F-4s operating
in Vietnam. This allowed the fighters themselves to interrogate their
foe's IFF units. The F-4 crews could then see on a cockpit display
where the opposing MiGs were.

The North Vietnamese caught on and started turning their IFF units
off. However, now the ground controllers had a hard time controlling
them, diminishing their effectiveness.

This information is all from the Marshall Michel book "Clashes; Air
Combat Over North Vietnam 1965-1972". My guess is that the system now
in use on E-2Cs (and E-3s?) is a variant on this idea.

David Lednicer | "Applied Computational Fluid Dynamics"
Analytical Methods, Inc. | email:
2133 152nd Ave NE | tel: (206) 643-9090
Redmond, WA 98052 USA | fax: (206) 746-1299

John Hairell

Aug 26, 1998, 3:00:00 AM8/26/98

Triggering of (i.e. interrogation) and homing in on hostile IFF transmissions
was first done in WWII.

John Hairell (

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