<s>
Passive	B-Algorithm
radar	I-Algorithm
(	O
also	O
referred	O
to	O
as	O
parasitic	O
radar	B-Application
,	O
passive	B-Algorithm
coherent	I-Algorithm
location	I-Algorithm
,	O
passive	O
surveillance	O
,	O
and	O
passive	O
covert	O
radar	B-Application
)	O
is	O
a	O
class	O
of	O
radar	B-Application
systems	I-Application
that	O
detect	O
and	O
track	O
objects	O
by	O
processing	O
reflections	O
from	O
non-cooperative	O
sources	O
of	O
illumination	O
in	O
the	O
environment	O
,	O
such	O
as	O
commercial	O
broadcast	O
and	O
communications	O
signals	O
.	O
</s>
<s>
It	O
is	O
a	O
specific	O
case	O
of	O
bistatic	B-Algorithm
radar	I-Algorithm
–	O
passive	B-Algorithm
bistatic	I-Algorithm
radar	I-Algorithm
(	O
PBR	O
)	O
–	O
which	O
is	O
a	O
broad	O
type	O
also	O
including	O
the	O
exploitation	O
of	O
cooperative	O
and	O
non-cooperative	O
radar	B-Application
transmitters	O
.	O
</s>
<s>
Conventional	O
radar	B-Application
systems	I-Application
comprise	O
a	O
colocated	O
transmitter	O
and	O
receiver	O
,	O
which	O
usually	O
share	O
a	O
common	O
antenna	O
to	O
transmit	O
and	O
receive	O
.	O
</s>
<s>
In	O
a	O
passive	B-Algorithm
radar	I-Algorithm
system	O
,	O
there	O
is	O
no	O
dedicated	O
transmitter	O
.	O
</s>
<s>
This	O
allows	O
the	O
bistatic	B-Algorithm
range	I-Algorithm
of	O
the	O
object	O
to	O
be	O
determined	O
.	O
</s>
<s>
In	O
addition	O
to	O
the	O
bistatic	B-Algorithm
range	I-Algorithm
,	O
a	O
passive	B-Algorithm
radar	I-Algorithm
will	O
typically	O
also	O
measure	O
the	O
bistatic	B-Algorithm
Doppler	I-Algorithm
shift	I-Algorithm
of	O
the	O
echo	O
and	O
also	O
its	O
direction	O
of	O
arrival	O
.	O
</s>
<s>
In	O
some	O
cases	O
,	O
multiple	O
transmitters	O
and/or	O
receivers	O
can	O
be	O
employed	O
to	O
make	O
several	O
independent	O
measurements	O
of	O
bistatic	B-Algorithm
range	I-Algorithm
,	O
Doppler	O
and	O
bearing	O
and	O
hence	O
significantly	O
improve	O
the	O
final	O
track	O
accuracy	O
.	O
</s>
<s>
The	O
term	O
"	O
passive	B-Algorithm
radar	I-Algorithm
"	O
is	O
sometimes	O
used	O
incorrectly	O
to	O
describe	O
those	O
passive	O
sensors	O
that	O
detect	O
and	O
track	O
aircraft	O
by	O
their	O
RF	O
emissions	O
(	O
such	O
as	O
radar	B-Application
,	O
communications	O
,	O
or	O
transponder	B-Architecture
emissions	O
)	O
.	O
</s>
<s>
The	O
concept	O
of	O
passive	B-Algorithm
radar	I-Algorithm
detection	O
using	O
reflected	O
ambient	O
radio	O
signals	O
emanating	O
from	O
a	O
distant	O
transmitter	O
is	O
not	O
new	O
.	O
</s>
<s>
The	O
first	O
radar	B-Application
experiments	O
in	O
the	O
United	O
Kingdom	O
in	O
1935	O
by	O
Robert	O
Watson-Watt	O
demonstrated	O
the	O
principle	O
of	O
radar	B-Application
by	O
detecting	O
a	O
Handley	O
Page	O
Heyford	O
bomber	O
at	O
a	O
distance	O
of	O
12	O
km	O
using	O
the	O
BBC	O
shortwave	O
transmitter	O
at	O
Daventry	O
.	O
</s>
<s>
Early	O
radars	B-Application
were	O
all	O
bistatic	O
because	O
the	O
technology	O
to	O
enable	O
an	O
antenna	O
to	O
be	O
switched	O
from	O
transmit	O
to	O
receive	O
mode	O
had	O
not	O
been	O
developed	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
British	O
deployed	O
the	O
CHAIN	O
HOME	O
system	O
;	O
the	O
French	O
used	O
a	O
bistatic	O
Continuous	O
Wave	O
(	O
CW	O
)	O
radar	B-Application
in	O
a	O
"	O
fence	O
"	O
(	O
or	O
"	O
barrier	O
"	O
)	O
system	O
;	O
the	O
Soviet	O
Union	O
deployed	O
a	O
bistatic	O
CW	O
system	O
called	O
the	O
RUS-1	O
,	O
and	O
the	O
Japanese	O
developed	O
a	O
bistatic	O
CW	O
radar	B-Application
called	O
"	O
Type	O
A	O
"	O
.	O
</s>
<s>
This	O
system	O
,	O
called	O
the	O
Klein	B-Algorithm
Heidelberg	I-Algorithm
Parasit	I-Algorithm
or	O
Heidelberg-Gerät	B-Algorithm
,	O
was	O
deployed	O
at	O
seven	O
sites	O
(	O
Limmen	O
,	O
Oostvoorne	O
,	O
Ostend	O
,	O
Boulogne	O
,	O
Abbeville	O
,	O
Cap	O
d'Antifer	O
and	O
Cherbourg	O
)	O
and	O
operated	O
as	O
bistatic	O
receivers	O
,	O
using	O
the	O
British	O
Chain	O
Home	O
radars	B-Application
as	O
non-cooperative	O
illuminators	O
,	O
to	O
detect	O
aircraft	O
over	O
the	O
southern	O
part	O
of	O
the	O
North	O
Sea	O
.	O
</s>
<s>
Bistatic	B-Algorithm
radar	I-Algorithm
systems	O
gave	O
way	O
to	O
monostatic	O
systems	O
with	O
the	O
development	O
of	O
the	O
duplexer	O
in	O
1936	O
.	O
</s>
<s>
In	O
the	O
early	O
1950s	O
,	O
bistatic	O
systems	O
were	O
considered	O
again	O
when	O
some	O
interesting	O
properties	O
of	O
the	O
scattered	O
radar	B-Application
energy	O
were	O
discovered	O
,	O
indeed	O
the	O
term	O
"	O
bistatic	O
"	O
was	O
first	O
used	O
by	O
Siegel	O
in	O
1955	O
in	O
his	O
report	O
describing	O
these	O
properties	O
.	O
</s>
<s>
One	O
of	O
the	O
largest	O
and	O
most	O
complex	O
passive	B-Algorithm
radar	I-Algorithm
systems	O
was	O
the	O
UK	O
's	O
RX12874	O
,	O
or	O
"	O
Winkle	O
"	O
.	O
</s>
<s>
Winkle	O
was	O
deployed	O
in	O
the	O
1960s	O
in	O
response	O
to	O
the	O
introduction	O
of	O
the	O
carcinotron	B-Algorithm
,	O
a	O
radar	B-Application
jammer	O
that	O
was	O
so	O
powerful	O
it	O
appeared	O
to	O
render	O
long-distance	O
radars	B-Application
useless	O
.	O
</s>
<s>
Winkle	O
was	O
able	O
to	O
home	O
in	O
on	O
carcinotron	B-Algorithm
broadcasts	O
with	O
the	O
same	O
accuracy	O
as	O
conventional	O
radar	B-Application
,	O
allowing	O
the	O
jammer	O
aircraft	O
to	O
be	O
tracked	O
and	O
attacked	O
at	O
hundreds	O
of	O
miles	O
range	O
.	O
</s>
<s>
Additionally	O
,	O
by	O
indicating	O
the	O
location	O
of	O
the	O
jammer	O
,	O
other	O
radars	B-Application
in	O
the	O
Linesman/Mediator	O
network	O
could	O
reduce	O
the	O
sensitivity	O
of	O
their	O
receivers	O
when	O
pointed	O
in	O
that	O
direction	O
,	O
thereby	O
reducing	O
the	O
amount	O
of	O
jamming	O
received	O
when	O
pointed	O
near	O
the	O
jammer	O
's	O
location	O
.	O
</s>
<s>
The	O
rise	O
of	O
cheap	O
computing	O
power	O
and	O
digital	O
receiver	O
technology	O
in	O
the	O
1980s	O
led	O
to	O
a	O
resurgence	O
of	O
interest	O
in	O
passive	B-Algorithm
radar	I-Algorithm
technology	O
.	O
</s>
<s>
For	O
the	O
first	O
time	O
,	O
these	O
allowed	O
designers	O
to	O
apply	O
digital	B-General_Concept
signal	I-General_Concept
processing	I-General_Concept
techniques	O
to	O
exploit	O
a	O
variety	O
of	O
broadcast	O
signals	O
and	O
to	O
use	O
cross-correlation	O
techniques	O
to	O
achieve	O
sufficient	O
signal	O
processing	O
gain	O
to	O
detect	O
targets	O
and	O
estimate	O
their	O
bistatic	B-Algorithm
range	I-Algorithm
and	O
Doppler	O
shift	O
.	O
</s>
<s>
Passive	B-Algorithm
radar	I-Algorithm
systems	O
have	O
been	O
developed	O
that	O
exploit	O
the	O
following	O
sources	O
of	O
illumination	O
:	O
</s>
<s>
GPS	O
satellites	O
(	O
GPS	B-General_Concept
reflectometry	I-General_Concept
)	O
.	O
</s>
<s>
Satellite	O
signals	O
have	O
generally	O
been	O
found	O
more	O
difficult	O
for	O
passive	B-Algorithm
radar	I-Algorithm
use	O
,	O
either	O
because	O
the	O
powers	O
are	O
too	O
low	O
or	O
because	O
the	O
orbits	O
of	O
the	O
satellites	O
are	O
such	O
that	O
illumination	O
is	O
too	O
infrequent	O
.	O
</s>
<s>
The	O
possible	O
exception	O
to	O
this	O
is	O
the	O
exploitation	O
of	O
satellite-based	O
radar	B-Application
and	O
satellite	O
radio	O
systems	O
.	O
</s>
<s>
In	O
a	O
conventional	O
radar	B-Application
system	I-Application
,	O
the	O
time	O
of	O
transmission	O
of	O
the	O
pulse	O
and	O
the	O
transmitted	O
waveform	O
is	O
exactly	O
known	O
.	O
</s>
<s>
A	O
passive	B-Algorithm
radar	I-Algorithm
does	O
not	O
have	O
this	O
information	O
directly	O
and	O
hence	O
must	O
use	O
a	O
dedicated	O
receiver	O
channel	O
(	O
known	O
as	O
the	O
"	O
reference	O
channel	O
"	O
)	O
to	O
monitor	O
each	O
transmitter	O
being	O
exploited	O
,	O
and	O
dynamically	O
sample	B-Algorithm
the	O
transmitted	O
waveform	O
.	O
</s>
<s>
A	O
passive	B-Algorithm
radar	I-Algorithm
typically	O
employs	O
the	O
following	O
processing	O
steps	O
:	O
</s>
<s>
A	O
passive	B-Algorithm
radar	I-Algorithm
system	O
must	O
detect	O
very	O
small	O
target	O
returns	O
in	O
the	O
presence	O
of	O
very	O
strong	O
,	O
continuous	O
interference	O
.	O
</s>
<s>
This	O
contrasts	O
with	O
a	O
conventional	O
radar	B-Application
,	O
which	O
listens	O
for	O
echoes	O
during	O
the	O
periods	O
of	O
silence	O
in	O
between	O
each	O
pulse	O
transmission	O
.	O
</s>
<s>
Passive	B-Algorithm
radar	I-Algorithm
systems	O
use	O
digital	O
receiver	O
systems	O
which	O
output	O
a	O
digitized	O
,	O
sampled	B-Algorithm
signal	O
.	O
</s>
<s>
Most	O
passive	B-Algorithm
radar	I-Algorithm
systems	O
use	O
simple	O
antenna	O
arrays	O
with	O
several	O
antenna	O
elements	O
and	O
element-level	O
digitisation	O
.	O
</s>
<s>
This	O
allows	O
the	O
direction	O
of	O
arrival	O
of	O
echoes	O
to	O
be	O
calculated	O
using	O
standard	O
radar	B-Application
beamforming	B-Algorithm
techniques	O
,	O
such	O
as	O
amplitude	B-Algorithm
monopulse	I-Algorithm
using	O
a	O
series	O
of	O
fixed	O
,	O
overlapping	O
beams	O
or	O
more	O
sophisticated	O
adaptive	B-Algorithm
beamforming	I-Algorithm
.	O
</s>
<s>
Alternatively	O
,	O
some	O
research	O
systems	O
have	O
used	O
only	O
a	O
pair	O
of	O
antenna	O
elements	O
and	O
the	O
phase-difference	O
of	O
arrival	O
to	O
calculate	O
the	O
direction	O
of	O
arrival	O
of	O
the	O
echoes	O
(	O
known	O
as	O
phase	B-Algorithm
interferometry	I-Algorithm
and	O
similar	O
in	O
concept	O
to	O
Very	O
Long	O
Baseline	O
Interferometry	O
used	O
in	O
astronomy	O
)	O
.	O
</s>
<s>
This	O
may	O
include	O
high-quality	O
analogue	O
bandpass	O
filtering	O
of	O
the	O
signal	O
,	O
channel	O
equalization	O
to	O
improve	O
the	O
quality	O
of	O
the	O
reference	O
signal	O
,	O
removal	O
of	O
unwanted	O
structures	O
in	O
digital	O
signals	O
to	O
improve	O
the	O
radar	B-Algorithm
ambiguity	I-Algorithm
function	I-Algorithm
or	O
even	O
complete	O
reconstruction	O
of	O
the	O
reference	O
signal	O
from	O
the	O
received	O
digital	O
signal	O
.	O
</s>
<s>
The	O
principal	O
limitation	O
in	O
detection	O
range	O
for	O
most	O
passive	B-Algorithm
radar	I-Algorithm
systems	O
is	O
the	O
signal-to-interference	O
ratio	O
,	O
due	O
to	O
the	O
large	O
and	O
constant	O
direct	O
signal	O
received	O
from	O
the	O
transmitter	O
.	O
</s>
<s>
The	O
key	O
processing	O
step	O
in	O
a	O
passive	B-Algorithm
radar	I-Algorithm
is	O
cross-correlation	O
.	O
</s>
<s>
This	O
step	O
acts	O
as	O
the	O
matched	O
filter	O
and	O
also	O
provides	O
the	O
estimates	O
of	O
the	O
bistatic	B-Algorithm
range	I-Algorithm
and	O
bistatic	B-Algorithm
Doppler	I-Algorithm
shift	I-Algorithm
of	O
each	O
target	O
echo	O
.	O
</s>
<s>
Efficient	O
implementations	O
of	O
the	O
cross-correlation	O
processing	O
based	O
on	O
the	O
discrete	B-Algorithm
Fourier	I-Algorithm
transform	I-Algorithm
are	O
usually	O
used	O
,	O
in	O
particular	O
for	O
OFDM	B-Protocol
waveforms	O
.	O
</s>
<s>
A	O
standard	O
cell-averaging	O
constant	B-Algorithm
false	I-Algorithm
alarm	I-Algorithm
rate	I-Algorithm
(	O
CFAR	O
)	O
algorithm	O
is	O
typically	O
used	O
.	O
</s>
<s>
The	O
line-tracking	O
step	O
refers	O
to	O
the	O
tracking	B-Algorithm
of	O
target	O
returns	O
from	O
individual	O
targets	O
,	O
over	O
time	O
,	O
in	O
the	O
range-Doppler	O
space	O
produced	O
by	O
the	O
cross-correlation	O
processing	O
.	O
</s>
<s>
A	O
better	O
approach	O
is	O
to	O
estimate	O
the	O
target	O
state	O
(	O
location	O
,	O
heading	O
and	O
speed	O
)	O
from	O
the	O
full	O
measurement	O
set	O
of	O
bistatic	B-Algorithm
range	I-Algorithm
,	O
bearing	O
and	O
Doppler	O
using	O
a	O
non-linear	O
filter	O
,	O
such	O
as	O
the	O
extended	O
or	O
unscented	O
Kalman	O
filter	O
.	O
</s>
<s>
The	O
return	O
from	O
this	O
target	O
will	O
appear	O
at	O
a	O
different	O
bistatic	B-Algorithm
range	I-Algorithm
and	O
Doppler	O
shift	O
with	O
each	O
transmitter	O
and	O
so	O
it	O
is	O
necessary	O
to	O
determine	O
which	O
target	O
returns	O
from	O
one	O
transmitter	O
correspond	O
with	O
those	O
on	O
the	O
other	O
transmitters	O
.	O
</s>
<s>
Having	O
associated	O
these	O
returns	O
,	O
the	O
point	O
at	O
which	O
the	O
bistatic	B-Algorithm
range	I-Algorithm
ellipses	O
from	O
each	O
transmitter	O
intersect	O
is	O
the	O
location	O
of	O
the	O
target	O
.	O
</s>
<s>
The	O
above	O
description	O
assumes	O
that	O
the	O
waveform	O
of	O
the	O
transmitter	O
being	O
exploited	O
possesses	O
a	O
usable	O
radar	B-Algorithm
ambiguity	I-Algorithm
function	I-Algorithm
and	O
hence	O
cross-correlation	O
yields	O
a	O
useful	O
result	O
.	O
</s>
<s>
Work	O
has	O
been	O
published	O
that	O
has	O
demonstrated	O
the	O
feasibility	O
of	O
this	O
approach	O
for	O
tracking	B-Algorithm
aircraft	O
using	O
the	O
vision	O
carrier	O
of	O
analogue	O
television	O
signals	O
.	O
</s>
<s>
Passive	B-Algorithm
radar	I-Algorithm
performance	O
is	O
comparable	O
to	O
conventional	O
short	O
and	O
medium-range	O
radar	B-Application
systems	I-Application
.	O
</s>
<s>
The	O
detection	O
range	O
can	O
be	O
determined	O
using	O
the	O
standard	O
radar	B-Application
equation	O
,	O
but	O
ensuring	O
proper	O
account	O
of	O
the	O
processing	O
gain	O
and	O
external	O
noise	O
limitations	O
is	O
taken	O
.	O
</s>
<s>
Furthermore	O
,	O
unlike	O
conventional	O
radar	B-Application
,	O
the	O
detection	O
range	O
is	O
also	O
a	O
function	O
of	O
the	O
deployment	O
geometry	O
,	O
as	O
the	O
distance	O
of	O
the	O
receiver	O
from	O
the	O
transmitter	O
determines	O
the	O
level	O
of	O
external	O
noise	O
against	O
which	O
the	O
targets	O
must	O
be	O
detected	O
.	O
</s>
<s>
However	O
,	O
as	O
a	O
rule	O
of	O
thumb	O
,	O
it	O
is	O
reasonable	O
to	O
expect	O
a	O
passive	B-Algorithm
radar	I-Algorithm
using	O
FM	O
radio	O
stations	O
to	O
achieve	O
detection	O
ranges	O
of	O
up	O
to	O
150	O
km	O
,	O
for	O
high-power	O
analogue	O
TV	O
and	O
US	O
HDTV	B-Device
stations	O
to	O
achieve	O
detection	O
ranges	O
of	O
over	O
300	O
km	O
and	O
for	O
lower	O
power	O
digital	O
signals	O
(	O
such	O
as	O
cell	O
phone	O
and	O
DAB	O
or	O
DVB-T	O
)	O
to	O
achieve	O
detection	O
ranges	O
of	O
a	O
few	O
tens	O
of	O
kilometres	O
.	O
</s>
<s>
Passive	B-Algorithm
radar	I-Algorithm
accuracy	O
is	O
a	O
strong	O
function	O
of	O
the	O
deployment	O
geometry	O
and	O
the	O
number	O
of	O
receivers	O
and	O
transmitters	O
being	O
used	O
.	O
</s>
<s>
Systems	O
using	O
only	O
one	O
transmitter	O
and	O
one	O
receiver	O
will	O
tend	O
to	O
be	O
much	O
less	O
accurate	O
than	O
conventional	O
surveillance	O
radars	B-Application
,	O
whilst	O
multistatic	B-Algorithm
radars	I-Algorithm
are	O
capable	O
of	O
achieving	O
somewhat	O
greater	O
accuracies	O
.	O
</s>
<s>
Most	O
passive	B-Algorithm
radars	I-Algorithm
are	O
two-dimensional	O
,	O
but	O
height	O
measurements	O
are	O
possible	O
when	O
the	O
deployment	O
is	O
such	O
there	O
is	O
significant	O
variation	O
in	O
the	O
altitudes	O
of	O
the	O
transmitters	O
,	O
receiver	O
and	O
target	O
,	O
reducing	O
the	O
effects	O
of	O
geometrical	O
dilution	O
of	O
precision	O
(	O
GDOP	O
)	O
.	O
</s>
<s>
Passive	B-Algorithm
radar	I-Algorithm
systems	O
are	O
currently	O
under	O
development	O
in	O
several	O
commercial	O
organizations	O
.	O
</s>
<s>
Several	O
academic	O
passive	B-Algorithm
radar	I-Algorithm
systems	O
exist	O
as	O
well	O
:	O
</s>
<s>
Research	O
on	O
passive	B-Algorithm
radar	I-Algorithm
systems	O
is	O
of	O
growing	O
interest	O
throughout	O
the	O
world	O
,	O
with	O
various	O
open-source	O
publications	O
showing	O
active	O
research	O
and	O
development	O
in	O
the	O
United	O
States	O
(	O
including	O
work	O
at	O
the	O
Air	O
Force	O
Research	O
Labs	O
,	O
Lockheed-Martin	O
Mission	O
Systems	O
,	O
Raytheon	O
,	O
University	O
of	O
Washington	O
,	O
Georgia	O
Tech/Georgia	O
Tech	O
Research	O
Institute	O
and	O
the	O
University	O
of	O
Illinois	O
)	O
,	O
in	O
the	O
in	O
The	O
Netherlands	O
,	O
in	O
the	O
United	O
Kingdom	O
(	O
at	O
Roke	O
Manor	O
Research	O
,	O
QinetiQ	O
,	O
University	O
of	O
Birmingham	O
,	O
University	O
College	O
London	O
and	O
BAE	O
Systems	O
)	O
,	O
France	O
(	O
including	O
the	O
government	O
labs	O
of	O
ONERA	O
)	O
,	O
Germany	O
(	O
including	O
the	O
labs	O
at	O
)	O
,	O
Poland	O
(	O
including	O
Warsaw	O
University	O
of	O
Technology	O
)	O
.	O
</s>
<s>
The	O
US	O
HDTV	B-Device
standard	O
is	O
particularly	O
good	O
for	O
passive	B-Algorithm
radar	I-Algorithm
,	O
having	O
an	O
excellent	O
ambiguity	B-Algorithm
function	I-Algorithm
and	O
very	O
high	O
power	O
transmitters	O
.	O
</s>
<s>
The	O
DVB-T	O
digital	O
TV	O
standard	O
(	O
and	O
related	O
DAB	O
digital	O
audio	O
standard	O
)	O
used	O
throughout	O
most	O
of	O
the	O
rest	O
of	O
the	O
world	O
is	O
more	O
challenging	O
transmitter	O
powers	O
are	O
lower	O
,	O
and	O
many	O
networks	O
are	O
set	O
up	O
in	O
a	O
"	O
single	O
frequency	O
network	O
"	O
mode	O
,	O
in	O
which	O
all	O
transmitters	O
are	O
synchronised	O
in	O
time	O
and	O
frequency	O
.	O
</s>
<s>
Without	O
careful	O
processing	O
,	O
the	O
net	O
result	O
for	O
passive	B-Algorithm
radar	I-Algorithm
is	O
like	O
multiple	O
repeaters	O
jammers	O
.	O
</s>
<s>
Researchers	O
at	O
the	O
University	O
of	O
Illinois	O
at	O
Urbana	O
–	O
Champaign	O
and	O
Georgia	O
Institute	O
of	O
Technology	O
,	O
with	O
the	O
support	O
of	O
DARPA	O
and	O
NATO	O
C3	O
Agency	O
,	O
have	O
shown	O
that	O
it	O
is	O
possible	O
to	O
build	O
a	O
synthetic	O
aperture	O
image	O
of	O
an	O
aircraft	O
target	O
using	O
passive	O
multistatic	B-Algorithm
radar	I-Algorithm
.	O
</s>
<s>
Herman	O
,	O
Moulin	O
,	O
Ehrman	O
and	O
Lanterman	O
have	O
published	O
reports	O
based	O
on	O
simulated	O
data	O
,	O
which	O
suggest	O
that	O
low-frequency	O
passive	B-Algorithm
radars	I-Algorithm
(	O
using	O
FM	O
radio	O
transmissions	O
)	O
could	O
provide	O
target	O
classification	O
in	O
addition	O
to	O
tracking	B-Algorithm
information	O
.	O
</s>
<s>
Researchers	O
at	O
the	O
University	O
of	O
Washington	O
operate	O
a	O
distributed	O
passive	B-Algorithm
radar	I-Algorithm
exploiting	O
FM	O
broadcasts	O
to	O
study	O
ionospheric	O
turbulence	O
at	O
altitudes	O
of	O
100	O
km	O
and	O
ranges	O
out	O
to	O
1200	O
km	O
.	O
</s>
<s>
Silentium	O
Defence	O
has	O
launched	O
the	O
first	O
operational	O
ground	O
passive	B-Algorithm
radar	I-Algorithm
specifically	O
designed	O
to	O
track	O
LEO	O
.	O
</s>
