<s>
Very	B-Architecture
low	I-Architecture
frequency	I-Architecture
or	O
VLF	B-Architecture
is	O
the	O
ITU	O
designation	O
for	O
radio	O
frequencies	O
(	O
RF	O
)	O
in	O
the	O
range	O
of	O
3	O
–	O
30kHz	O
,	O
corresponding	O
to	O
wavelengths	O
from	O
100	O
to	O
10km	O
,	O
respectively	O
.	O
</s>
<s>
Due	O
to	O
its	O
limited	O
bandwidth	B-Algorithm
,	O
audio	O
(	O
voice	O
)	O
transmission	O
is	O
highly	O
impractical	O
in	O
this	O
band	O
,	O
and	O
therefore	O
only	O
low	O
data	O
rate	O
coded	O
signals	O
are	O
used	O
.	O
</s>
<s>
The	O
VLF	B-Architecture
band	O
is	O
used	O
for	O
a	O
few	O
radio	O
navigation	O
services	O
,	O
government	O
time	O
radio	O
stations	O
(	O
broadcasting	O
time	O
signals	O
to	O
set	O
radio	O
clocks	O
)	O
and	O
for	O
secure	O
military	O
communication	O
.	O
</s>
<s>
Since	O
VLF	B-Architecture
waves	O
can	O
penetrate	O
at	O
least	O
40meters	O
(	O
131ft	O
)	O
into	O
saltwater	O
,	O
they	O
are	O
used	O
for	O
military	O
communication	O
with	O
submarines	O
.	O
</s>
<s>
Because	O
of	O
their	O
long	O
wavelengths	O
,	O
VLF	B-Architecture
radio	O
waves	O
can	O
diffract	O
around	O
large	O
obstacles	O
and	O
so	O
are	O
not	O
blocked	O
by	O
mountain	O
ranges	O
,	O
and	O
can	O
propagate	O
as	O
ground	O
waves	O
following	O
the	O
curvature	O
of	O
the	O
Earth	O
and	O
so	O
are	O
not	O
limited	O
by	O
the	O
horizon	O
.	O
</s>
<s>
The	O
Earth	O
is	O
surrounded	O
by	O
a	O
conductive	O
layer	O
of	O
electrons	O
and	O
ions	O
in	O
the	O
upper	O
atmosphere	O
at	O
the	O
bottom	O
of	O
the	O
ionosphere	O
called	O
the	O
D	O
layer	O
at	O
60	O
–	O
90km	O
(	O
37	O
–	O
56miles	O
)	O
altitude	O
,	O
which	O
reflects	O
VLF	B-Architecture
radio	O
waves	O
.	O
</s>
<s>
The	O
conductive	O
ionosphere	O
and	O
the	O
conductive	O
Earth	O
form	O
a	O
horizontal	O
"	O
duct	O
"	O
a	O
few	O
VLF	B-Architecture
wavelengths	O
high	O
,	O
which	O
acts	O
as	O
a	O
waveguide	O
confining	O
the	O
waves	O
so	O
they	O
do	O
n't	O
escape	O
into	O
space	O
.	O
</s>
<s>
VLF	B-Architecture
waves	O
have	O
very	O
low	O
path	O
attenuation	O
,	O
2	O
–	O
3dB	O
per	O
1,000	O
km	O
,	O
with	O
little	O
of	O
the	O
"	O
fading	O
"	O
experienced	O
at	O
higher	O
frequencies	O
.	O
</s>
<s>
This	O
is	O
because	O
VLF	B-Architecture
waves	O
are	O
reflected	O
from	O
the	O
bottom	O
of	O
the	O
ionosphere	O
,	O
while	O
higher	O
frequency	O
shortwave	O
signals	O
are	O
returned	O
to	O
Earth	O
from	O
higher	O
layers	O
in	O
the	O
ionosphere	O
,	O
the	O
F1	O
and	O
F2	O
layers	O
,	O
by	O
a	O
refraction	O
process	O
,	O
and	O
spend	O
most	O
of	O
their	O
journey	O
in	O
the	O
ionosphere	O
,	O
so	O
they	O
are	O
much	O
more	O
affected	O
by	O
ionization	O
gradients	O
and	O
turbulence	O
.	O
</s>
<s>
Therefore	O
,	O
VLF	B-Architecture
transmissions	O
are	O
very	O
stable	O
and	O
reliable	O
,	O
and	O
are	O
used	O
for	O
long-distance	O
communication	O
.	O
</s>
<s>
However	O
,	O
atmospheric	O
noise	O
(	O
"	O
sferics	O
"	O
)	O
is	O
high	O
in	O
the	O
band	O
,	O
including	O
such	O
phenomena	O
as	O
"	O
whistlers	O
"	O
,	O
caused	O
by	O
lightning	B-Architecture
.	O
</s>
<s>
VLF	B-Architecture
waves	O
can	O
penetrate	O
seawater	O
to	O
a	O
depth	O
of	O
at	O
least	O
10	O
–	O
40	O
meters	O
(	O
30	O
–	O
130	O
feet	O
)	O
,	O
depending	O
on	O
the	O
frequency	O
employed	O
and	O
the	O
salinity	O
of	O
the	O
water	O
,	O
so	O
they	O
are	O
used	O
to	O
communicate	O
with	O
submarines	O
.	O
</s>
<s>
VLF	B-Architecture
waves	O
at	O
certain	O
frequencies	O
have	O
been	O
found	O
to	O
cause	O
electron	O
precipitation	O
.	O
</s>
<s>
VLF	B-Architecture
waves	O
used	O
to	O
communicate	O
with	O
submarines	O
have	O
created	O
an	O
artificial	O
bubble	O
around	O
the	O
Earth	O
that	O
can	O
protect	O
it	O
from	O
solar	O
flares	O
and	O
coronal	O
mass	O
ejections	O
;	O
this	O
occurred	O
through	O
interaction	O
with	O
high-energy	O
radiation	O
particles	O
.	O
</s>
<s>
A	O
major	O
practical	O
drawback	O
to	O
the	O
VLF	B-Architecture
band	O
is	O
that	O
because	O
of	O
the	O
length	O
of	O
the	O
waves	O
,	O
full	O
size	O
resonant	O
antennas	O
(	O
half	O
wave	O
dipole	O
or	O
quarter	O
wave	O
monopole	O
antennas	O
)	O
cannot	O
be	O
built	O
because	O
of	O
their	O
physical	O
height	O
.	O
</s>
<s>
Vertical	O
antennas	O
must	O
be	O
used	O
because	O
VLF	B-Architecture
waves	O
propagate	O
in	O
vertical	O
polarization	O
,	O
but	O
a	O
quarter-wave	O
vertical	O
antenna	O
at	O
30kHz	O
(	O
10km	O
wavelength	O
)	O
would	O
be	O
high	O
.	O
</s>
<s>
High	O
power	O
VLF	B-Architecture
transmitting	O
stations	O
use	O
capacitively-loaded	O
monopole	O
antennas	O
.	O
</s>
<s>
At	O
VLF	B-Architecture
the	O
design	O
of	O
this	O
coil	O
is	O
challenging	O
;	O
it	O
must	O
have	O
low	O
resistance	O
at	O
the	O
operating	O
RF	O
frequency	O
,	O
high	O
,	O
and	O
must	O
withstand	O
the	O
extremely	O
high	O
voltage	O
on	O
the	O
antenna	O
.	O
</s>
<s>
VLF	B-Architecture
antennas	O
have	O
very	O
narrow	O
bandwidth	B-Algorithm
and	O
to	O
change	O
the	O
transmitting	O
frequency	O
requires	O
a	O
variable	O
inductor	O
(	O
variometer	O
)	O
to	O
tune	O
the	O
antenna	O
.	O
</s>
<s>
The	O
large	O
VLF	B-Architecture
antennas	O
used	O
for	O
high-power	O
transmitters	O
usually	O
have	O
bandwidths	B-Algorithm
of	O
only	O
50	O
–	O
100hertz	O
,	O
and	O
when	O
transmitting	O
frequency-shift	O
keying	O
(	O
FSK	O
)	O
,	O
the	O
usual	O
mode	O
,	O
the	O
resonant	O
frequency	O
of	O
the	O
antenna	O
must	O
sometimes	O
be	O
dynamically	O
shifted	O
with	O
the	O
modulation	B-Application
,	O
between	O
the	O
two	O
FSK	O
frequencies	O
.	O
</s>
<s>
Large	O
VLF	B-Architecture
antennas	O
usually	O
operate	O
in	O
'	O
voltage	O
limited	O
 '	O
mode	O
:	O
the	O
maximum	O
power	O
of	O
the	O
transmitter	O
is	O
limited	O
by	O
the	O
voltage	O
the	O
antenna	O
can	O
accept	O
without	O
air	O
breakdown	O
,	O
corona	O
,	O
and	O
arcing	O
from	O
the	O
antenna	O
.	O
</s>
<s>
The	O
bandwidth	B-Algorithm
of	O
large	O
capacitively	O
loaded	O
VLF	B-Architecture
antennas	O
is	O
so	O
narrow	O
(	O
50	O
–	O
100Hz	O
)	O
that	O
even	O
the	O
small	O
frequency	O
shifts	O
of	O
FSK	O
and	O
MSK	O
modulation	B-Application
may	O
exceed	O
it	O
,	O
throwing	O
the	O
antenna	O
out	O
of	O
resonance	B-Application
,	O
causing	O
the	O
antenna	O
to	O
reflect	O
some	O
power	O
back	O
down	O
the	O
feedline	O
.	O
</s>
<s>
The	O
traditional	O
solution	O
is	O
to	O
use	O
a	O
"	O
bandwidth	B-Algorithm
resistor	O
"	O
in	O
the	O
antenna	O
which	O
reduces	O
the	O
,	O
increasing	O
the	O
bandwidth	B-Algorithm
;	O
however	O
this	O
also	O
reduces	O
the	O
power	O
output	O
.	O
</s>
<s>
A	O
recent	O
alternative	O
used	O
in	O
some	O
military	O
VLF	B-Architecture
transmitters	O
is	O
a	O
circuit	O
which	O
dynamically	O
shifts	O
the	O
antenna	O
's	O
resonant	O
frequency	O
between	O
the	O
two	O
output	O
frequencies	O
with	O
the	O
modulation	B-Application
.	O
</s>
<s>
At	O
VLF	B-Architecture
frequencies	O
atmospheric	O
radio	O
noise	O
is	O
far	O
above	O
the	O
receiver	O
noise	O
introduced	O
by	O
the	O
receiver	O
circuit	O
and	O
determines	O
the	O
receiver	O
signal-to-noise	O
ratio	O
.	O
</s>
<s>
Ferrite	O
loop	B-Architecture
antennas	I-Architecture
are	O
usually	O
used	O
for	O
reception	O
.	O
</s>
<s>
Because	O
of	O
the	O
small	O
bandwidth	B-Algorithm
of	O
the	O
band	O
,	O
and	O
the	O
extremely	O
narrow	O
bandwidth	B-Algorithm
of	O
the	O
antennas	O
used	O
,	O
it	O
is	O
impractical	O
to	O
transmit	O
audio	O
signals	O
(	O
AM	B-Device
or	O
FM	B-Application
radiotelephony	O
)	O
.	O
</s>
<s>
A	O
typical	O
AM	B-Device
radio	O
signal	O
with	O
a	O
bandwidth	B-Algorithm
of	O
10kHz	O
would	O
occupy	O
one	O
third	O
of	O
the	O
VLF	B-Architecture
band	O
.	O
</s>
<s>
More	O
significantly	O
,	O
it	O
would	O
be	O
difficult	O
to	O
transmit	O
any	O
distance	O
because	O
it	O
would	O
require	O
an	O
antenna	O
with	O
100	O
times	O
the	O
bandwidth	B-Algorithm
of	O
current	O
VLF	B-Architecture
antennas	O
,	O
which	O
due	O
to	O
the	O
Chu-Harrington	O
limit	O
would	O
be	O
enormous	O
in	O
size	O
.	O
</s>
<s>
In	O
military	O
networks	O
frequency-shift	O
keying	O
(	O
FSK	O
)	O
modulation	B-Application
is	O
used	O
to	O
transmit	O
radioteletype	O
data	O
using	O
5bit	O
ITA2	O
or	O
8bit	O
ASCII	B-Protocol
character	I-Protocol
codes	O
.	O
</s>
<s>
A	O
small	O
frequency	O
shift	O
of	O
30	O
–	O
50hertz	O
is	O
used	O
due	O
to	O
the	O
small	O
bandwidth	B-Algorithm
of	O
the	O
antenna	O
.	O
</s>
<s>
In	O
high	O
power	O
VLF	B-Architecture
transmitters	O
,	O
to	O
increase	O
the	O
allowable	O
data	O
rate	O
,	O
a	O
special	O
form	O
of	O
FSK	O
called	O
minimum-shift	B-Device
keying	I-Device
(	O
MSK	O
)	O
is	O
used	O
.	O
</s>
<s>
The	O
of	O
large	O
VLF	B-Architecture
antennas	O
is	O
typically	O
over	O
200	O
;	O
this	O
means	O
the	O
antenna	O
stores	O
far	O
more	O
energy	O
(	O
200times	O
as	O
much	O
)	O
than	O
is	O
supplied	O
or	O
radiated	O
in	O
any	O
single	O
cycle	O
of	O
the	O
transmitter	O
current	O
.	O
</s>
<s>
VLF	B-Architecture
antennas	O
typically	O
operate	O
"	O
voltage-limited	O
"	O
,	O
with	O
the	O
voltage	O
on	O
the	O
antenna	O
close	O
to	O
the	O
limit	O
that	O
the	O
insulation	O
will	O
stand	O
,	O
so	O
they	O
will	O
not	O
tolerate	O
any	O
abrupt	O
change	O
in	O
the	O
voltage	O
or	O
current	O
from	O
the	O
transmitter	O
without	O
arcing	O
or	O
other	O
insulation	O
problems	O
.	O
</s>
<s>
The	O
three	O
types	O
of	O
modulation	B-Application
that	O
have	O
been	O
used	O
in	O
VLF	B-Architecture
transmitters	O
are	O
:	O
</s>
<s>
The	O
simplest	O
and	O
earliest	O
form	O
of	O
radio	O
data	O
transmission	O
,	O
this	O
was	O
used	O
from	O
the	O
beginning	O
of	O
the	O
20thcentury	O
to	O
the	O
1960s	O
in	O
commercial	O
and	O
military	O
VLF	B-Architecture
stations	O
.	O
</s>
<s>
Frequency-shift	O
keying	O
(	O
FSK	O
)	O
FSK	O
is	O
the	O
second	O
oldest	O
and	O
second	O
simplest	O
form	O
of	O
digital	O
radio	O
data	O
modulation	B-Application
,	O
after	O
CW	O
.	O
</s>
<s>
The	O
transmitter	O
is	O
periodically	O
switched	O
between	O
these	O
frequencies	O
to	O
represent	O
8bit	O
ASCII	B-Protocol
codes	I-Protocol
for	O
the	O
characters	O
of	O
the	O
message	O
.	O
</s>
<s>
A	O
problem	O
at	O
VLF	B-Architecture
is	O
that	O
when	O
the	O
frequency	O
is	O
switched	O
the	O
two	O
sine	O
waves	O
usually	O
have	O
different	O
phases	O
,	O
which	O
creates	O
a	O
sudden	O
phase-shift	O
transient	O
which	O
can	O
cause	O
arcing	O
on	O
the	O
antenna	O
.	O
</s>
<s>
Minimum-shift	B-Device
keying	I-Device
(	O
MSK	O
)	O
A	O
continuous	B-Device
phase	I-Device
version	O
of	O
FSK	O
designed	O
specifically	O
for	O
small	O
bandwidths	B-Algorithm
,	O
this	O
was	O
adopted	O
by	O
naval	O
VLF	B-Architecture
stations	O
in	O
the	O
1970s	O
to	O
increase	O
the	O
data	O
rate	O
and	O
is	O
now	O
the	O
standard	O
mode	O
used	O
in	O
military	O
VLF	B-Architecture
transmitters	O
.	O
</s>
<s>
If	O
the	O
two	O
frequencies	O
representing	O
'	O
1	O
 '	O
and	O
'	O
0	O
 '	O
are	O
50Hz	O
apart	O
,	O
the	O
standard	O
frequency	O
shift	O
used	O
in	O
military	O
VLF	B-Architecture
stations	O
,	O
their	O
phases	O
coincide	O
every	O
20ms	O
.	O
</s>
<s>
Nations	O
built	O
networks	O
of	O
high-power	O
LF	O
and	O
VLF	B-Architecture
radiotelegraphy	O
stations	O
that	O
transmitted	O
text	O
information	O
by	O
Morse	O
code	O
,	O
to	O
communicate	O
with	O
other	O
countries	O
,	O
their	O
colonies	O
,	O
and	O
naval	O
fleets	O
.	O
</s>
<s>
Early	O
attempts	O
were	O
made	O
to	O
use	O
radiotelephone	O
using	O
amplitude	B-Device
modulation	I-Device
and	O
single-sideband	B-Device
modulation	I-Device
within	O
the	O
band	O
starting	O
from	O
20kHz	O
,	O
but	O
the	O
result	O
was	O
unsatisfactory	O
because	O
the	O
available	O
bandwidth	B-Algorithm
was	O
insufficient	O
to	O
contain	O
the	O
sidebands	O
.	O
</s>
<s>
In	O
the	O
1920s	O
the	O
discovery	O
of	O
the	O
skywave	O
(	O
skip	O
)	O
radio	O
propagation	O
method	O
allowed	O
lower	O
power	O
transmitters	O
operating	O
at	O
high	B-Algorithm
frequency	I-Algorithm
to	O
communicate	O
at	O
similar	O
distances	O
by	O
reflecting	O
their	O
radio	O
waves	O
off	O
a	O
layer	O
of	O
ionized	O
atoms	O
in	O
the	O
ionosphere	O
,	O
and	O
long-distance	O
radio	O
communication	O
stations	O
switched	O
to	O
the	O
shortwave	O
frequencies	O
.	O
</s>
<s>
The	O
Grimeton	O
VLF	B-Architecture
transmitter	O
at	O
Grimeton	O
near	O
Varberg	O
in	O
Sweden	O
,	O
one	O
of	O
the	O
few	O
remaining	O
transmitters	O
from	O
that	O
era	O
that	O
has	O
been	O
preserved	O
as	O
a	O
historical	O
monument	O
,	O
can	O
be	O
visited	O
by	O
the	O
public	O
at	O
certain	O
times	O
,	O
such	O
as	O
on	O
Alexanderson	O
Day	O
.	O
</s>
<s>
Due	O
to	O
its	O
long	O
propagation	O
distances	O
and	O
stable	O
phase	O
characteristics	O
,	O
during	O
the	O
20thcentury	O
the	O
VLF	B-Architecture
band	O
was	O
used	O
for	O
long	O
range	O
hyperbolic	O
radio	O
navigation	O
systems	O
which	O
allowed	O
ships	O
and	O
aircraft	O
to	O
determine	O
their	O
geographical	O
position	O
by	O
comparing	O
the	O
phase	O
of	O
radio	O
waves	O
received	O
from	O
fixed	O
VLF	B-Architecture
navigation	O
beacon	O
transmitters	O
.	O
</s>
<s>
VLF	B-Architecture
was	O
also	O
used	O
for	O
standard	O
time	O
and	O
frequency	O
broadcasts	O
.	O
</s>
<s>
Naturally	O
occurring	O
signals	O
in	O
the	O
VLF	B-Architecture
band	O
are	O
used	O
by	O
geophysicists	O
for	O
long	O
range	O
lightning	B-Architecture
location	O
and	O
for	O
research	O
into	O
atmospheric	O
phenomena	O
such	O
as	O
the	O
aurora	O
.	O
</s>
<s>
Geophysicists	O
use	O
VLF-electromagnetic	O
receivers	O
to	O
measure	O
conductivity	O
in	O
the	O
near	O
surface	O
of	O
the	O
Earth	O
.	O
</s>
<s>
VLF	B-Architecture
signals	O
can	O
be	O
measured	O
as	O
a	O
geophysical	O
electromagnetic	O
survey	O
that	O
relies	O
on	O
transmitted	O
currents	O
inducing	O
secondary	O
responses	O
in	O
conductive	O
geologic	O
units	O
.	O
</s>
<s>
A	O
VLF	B-Architecture
anomaly	O
represents	O
a	O
change	O
in	O
the	O
attitude	O
of	O
the	O
electromagnetic	O
vector	O
overlying	O
conductive	O
materials	O
in	O
the	O
subsurface	O
.	O
</s>
<s>
Powerful	O
VLF	B-Architecture
transmitters	O
are	O
used	O
by	O
the	O
military	O
to	O
communicate	O
with	O
their	O
forces	O
worldwide	O
.	O
</s>
<s>
The	O
advantage	O
of	O
VLF	B-Architecture
frequencies	O
is	O
their	O
long	O
range	O
,	O
high	O
reliability	O
,	O
and	O
the	O
prediction	O
that	O
in	O
a	O
nuclear	B-Application
war	I-Application
VLF	B-Architecture
communications	O
will	O
be	O
less	O
disrupted	O
by	O
nuclear	O
explosions	O
than	O
higher	O
frequencies	O
.	O
</s>
<s>
Since	O
it	O
can	O
penetrate	O
seawater	O
VLF	B-Architecture
is	O
used	O
by	O
the	O
military	O
to	O
communicate	O
with	O
submarines	O
near	O
the	O
surface	O
,	O
while	O
ELF	B-Language
frequencies	O
are	O
used	O
for	O
deeply	O
submerged	O
subs	O
.	O
</s>
<s>
Since	O
2004	O
the	O
US	O
Navy	O
has	O
stopped	O
using	O
ELF	B-Language
transmissions	O
,	O
with	O
the	O
statement	O
that	O
improvements	O
in	O
VLF	B-Architecture
communication	O
has	O
made	O
them	O
unnecessary	O
,	O
so	O
it	O
may	O
have	O
developed	O
technology	O
to	O
allow	O
submarines	O
to	O
receive	O
VLF	B-Architecture
transmissions	O
while	O
at	O
operating	O
depth	O
.	O
</s>
<s>
Modern	O
receivers	O
use	O
sophisticated	O
digital	B-General_Concept
signal	I-General_Concept
processing	I-General_Concept
techniques	O
to	O
remove	O
the	O
effects	O
of	O
atmospheric	O
noise	O
(	O
largely	O
caused	O
by	O
lightning	B-Architecture
strikes	O
around	O
the	O
world	O
)	O
and	O
adjacent	O
channel	O
signals	O
,	O
extending	O
the	O
useful	O
reception	O
range	O
.	O
</s>
<s>
Strategic	O
nuclear	O
bombers	O
of	O
the	O
United	O
States	O
Air	O
Force	O
receive	O
VLF	B-Architecture
signals	O
as	O
part	O
of	O
hardened	O
nuclear	O
resilient	O
operations	O
.	O
</s>
<s>
Two	O
alternative	O
character	O
sets	O
may	O
be	O
used	O
:	O
5bit	O
ITA2	O
or	O
8bit	O
ASCII	B-Protocol
.	O
</s>
<s>
Although	O
it	O
is	O
relatively	O
easy	O
to	O
receive	O
the	O
transmissions	O
and	O
convert	O
them	O
into	O
a	O
string	O
of	O
characters	O
,	O
enemies	O
cannot	O
decode	O
the	O
encrypted	O
messages	O
;	O
military	O
communications	O
usually	O
use	O
unbreakable	O
one-time	B-Algorithm
pad	I-Algorithm
ciphers	B-Application
since	O
the	O
amount	O
of	O
text	O
is	O
so	O
small	O
.	O
</s>
<s>
VLF	B-Architecture
can	O
also	O
penetrate	O
soil	O
and	O
rock	O
for	O
some	O
distance	O
,	O
so	O
these	O
frequencies	O
are	O
also	O
used	O
for	O
through-the-earth	O
mine	O
communications	O
systems	O
.	O
</s>
<s>
Despite	O
the	O
low	O
power	O
,	O
stable	O
propagation	O
with	O
low	O
attenuation	O
in	O
the	O
earth-ionosphere	O
cavity	O
enable	O
very	O
narrow	O
bandwidths	B-Algorithm
to	O
be	O
used	O
to	O
reach	O
distances	O
up	O
to	O
several	O
thousand	O
kilometers	O
.	O
</s>
<s>
The	O
modes	O
used	O
are	O
QRSS	O
,	O
MFSK	B-Device
,	O
and	O
coherent	O
BPSK	B-General_Concept
.	O
</s>
<s>
Receivers	O
employ	O
an	O
electric	O
field	O
probe	O
or	O
magnetic	O
loop	B-Architecture
antenna	I-Architecture
,	O
a	O
sensitive	O
audio	O
preamplifier	O
,	O
isolating	O
transformers	O
,	O
and	O
a	O
PC	O
sound	B-Device
card	I-Device
to	O
digitise	O
the	O
signal	O
.	O
</s>
<s>
Extensive	O
digital	B-General_Concept
signal	I-General_Concept
processing	I-General_Concept
is	O
required	O
to	O
retrieve	O
the	O
weak	O
signals	O
from	O
beneath	O
interference	O
from	O
power	O
line	O
harmonics	O
and	O
VLF	B-Architecture
radio	O
atmospherics	O
.	O
</s>
<s>
VLF	B-Architecture
signals	O
are	O
often	O
monitored	O
by	O
radio	O
amateurs	O
using	O
simple	O
homemade	O
VLF	B-Architecture
radio	O
receivers	O
based	O
on	O
personal	O
computers	O
(	O
PCs	O
)	O
.	O
</s>
<s>
An	O
aerial	O
in	O
the	O
form	O
of	O
a	O
coil	O
of	O
insulated	O
wire	O
is	O
connected	O
to	O
the	O
input	O
of	O
the	O
soundcard	B-Device
of	O
the	O
PC	O
(	O
via	O
a	O
jack	O
plug	O
)	O
and	O
placed	O
a	O
few	O
meters	O
away	O
from	O
it	O
.	O
</s>
<s>
Fast	O
Fourier	O
transform	O
(	O
FFT	O
)	O
software	O
in	O
combination	O
with	O
a	O
sound	B-Device
card	I-Device
allows	O
reception	O
of	O
all	O
frequencies	O
below	O
the	O
Nyquist	O
frequency	O
simultaneously	O
in	O
the	O
form	O
of	O
spectrogrammes	O
.	O
</s>
<s>
Because	O
CRT	B-Device
monitors	I-Device
are	O
strong	O
sources	O
of	O
noise	O
in	O
the	O
VLF	B-Architecture
range	O
,	O
it	O
is	O
recommended	O
to	O
record	O
the	O
spectrograms	O
with	O
any	O
PC	O
CRT	B-Device
monitors	I-Device
turned	O
off	O
.	O
</s>
<s>
These	O
spectrograms	O
show	O
many	O
signals	O
,	O
which	O
may	O
include	O
VLF	B-Architecture
transmitters	O
and	O
the	O
horizontal	O
electron	O
beam	O
deflection	O
of	O
TV	O
sets	O
.	O
</s>
<s>
These	O
cause	O
the	O
ionization	O
level	O
to	O
increase	O
in	O
the	O
ionosphere	O
producing	O
a	O
rapid	O
change	O
to	O
the	O
amplitude	O
and	O
phase	O
of	O
the	O
received	O
VLF	B-Architecture
signal	O
.	O
</s>
<s>
Callsign	O
Frequency	O
Location	O
of	O
transmitter	O
Remarks	O
—	O
11.905	O
kHz	O
various	O
locations	O
(	O
Russia	O
)	O
Alpha-Navigation	O
—	O
12.649	O
kHz	O
various	O
locations	O
(	O
Russia	O
)	O
Alpha-Navigation	O
—	O
14.881	O
kHz	O
various	O
locations	O
(	O
Russia	O
)	O
Alpha-Navigation	O
HWU	O
15.1	O
kHz	O
Rosnay	O
(	O
France	O
)	O
400	O
kW	O
—	O
15.625	O
kHz	O
—	O
Frequency	O
for	O
horizontal	O
deflection	O
of	O
electron	O
beam	O
in	O
CRT	B-Device
televisions	O
(	O
576i	O
)	O
—	O
15.734	O
kHz	O
—	O
Frequency	O
for	O
horizontal	O
deflection	O
of	O
electron	O
beam	O
in	O
CRT	B-Device
televisions	O
(	O
480i	O
)	O
JXN	O
16.4	O
kHz	O
Gildeskål	O
(	O
Norway	O
)	O
SAQ	O
17.2	O
kHz	O
Grimeton	O
(	O
Sweden	O
)	O
Only	O
active	O
at	O
special	O
occasions	O
—	O
~	O
17.5	O
kHz	O
?	O
</s>
<s>
Twenty	O
second	O
pulses	O
NAA	O
17.8	O
kHz	O
VLF	B-Architecture
station	O
(	O
NAA	O
)	O
at	O
Cutler	O
,	O
Maine	O
(	O
US	O
)	O
RDL	O
UPD	O
UFQE	O
UPP	O
UPD8	O
18.1	O
kHz	O
various	O
locations	O
,	O
including	O
Matotchkinchar	O
(	O
Russia	O
)	O
HWU	O
18.3	O
kHz	O
Le	O
Blanc	O
(	O
France	O
)	O
Frequently	O
inactive	O
for	O
long	O
periods	O
RKS	O
18.9	O
kHz	O
various	O
locations	O
(	O
Russia	O
)	O
Rarely	O
active	O
GQD	O
19.6	O
kHz	O
Anthorn	O
(	O
UK	O
)	O
Many	O
operation	O
modes	O
.	O
</s>
