Satgen558 Passive Tracking  by GM4IHJ  (BID SGEN558)   1999-12-04

European newspapers have recently revelled in the ( to them ) , new
discovery , that aircraft and satellites reflect signals sent into
space by ordinary commercial TV and radio stations. Indeed they claim
that the Chinese will now use this " new" discovery , in order to
track hitherto undetectable stealth aircraft. But is this really new,
when in fact professional and amateur radio operators have been doing
it for at least 65 years on a variety of targets.

One early test took place on 26 February 1935. When a massive biplane
bomber of the Royal Air Force , not all that dissimilar from the
monsters used in WW1, flew 20 mile legs up and down the centre of
the main beam of a BBC shortwave ( 6 MHz) transmitter, and to the
delight of a small group gathered in a field some way from the
transmitter, a tiny blip moved along the A trace of their state of the
art, gas focussed soft cathode ray tube. 4 years later Britain had a
network of radar stations, which soon proved their effectiveness. 

Tracking of satellites had to wait until 1957, but from then onwards
several governments used curtain arrays of antennas to flood the sky
across several degrees of longitude, with signals whose reflections
recorded the passing satellites. Radio amateurs were even more astute.
Using a technique pioneered by Dr J Kraus W8JK. W3PK and W2RS got
positive results by monitoring radio reflections from the ionised
trails low flying satellites left in their wake. While in a different
sphere , the advent of monster powered, over the horizon HF radars ,
allowed knowledgable radio amateurs to use these brutes as secondary
radars whose back scatter echoes from land gave an excellent guide to
real time propagation. A relic of world war two activity, this pulse
stealing or ping stealing as it was called in the world of submarine
sonar, allowed you to use another mans radar to give you a picture
without exposing yourself by making your own transmissions.

A related form of passive radar long familiar to radio amateurs is
"Aircraft Flutter". Whereby every aeroplane which passes your station
can contribute fluttering , fast fading signals on many frequencies at
the same time, as it passes by . Indeed if you are looking at a long
range television signal for that other sort of passive reception -
meteor scatter, you can at times get aircraft flutter components all
over an FFT record from both the carrier and the massed array of
sideband mini carriers from VHF TV stations. A situation which can
ruin many a good experiment if you live under an active flightpath
such as the Europe to, Canada and Western USA, which has a procession
of aircraft going south every morning and north every afternoon on the
great circle route Europe- Los Angeles, Vancouver etc.

But will any of these techniques really work on stealth aircraft
specifically designed not to reflect radio signals. Clearly if you
listen to all the radio traffic available at your station it is likely
that one frequency or another may provide some sort of detection,
particularly if you improve your chances by using digital processing,
but even so this, does not readily inform you of the targets azimuth.
elevation , range and track unless you have very complex antennas and
signal processing.