<s>
In	O
telecommunications	O
,	O
a	O
transponder	B-Architecture
is	O
a	O
device	O
that	O
,	O
upon	O
receiving	O
a	O
signal	O
,	O
emits	O
a	O
different	O
signal	O
in	O
response	O
.	O
</s>
<s>
In	O
air	O
navigation	O
or	O
radio	B-Application
frequency	I-Application
identification	I-Application
,	O
a	O
flight	O
transponder	B-Architecture
is	O
an	O
automated	O
transceiver	O
in	O
an	O
aircraft	O
that	O
emits	O
a	O
coded	O
identifying	O
signal	O
in	O
response	O
to	O
an	O
interrogating	O
received	O
signal	O
.	O
</s>
<s>
In	O
a	O
communications	O
satellite	O
,	O
a	O
satellite	O
transponder	B-Architecture
receives	O
signals	O
over	O
a	O
range	O
of	O
uplink	O
frequencies	O
,	O
usually	O
from	O
a	O
satellite	O
ground	O
station	O
;	O
the	O
transponder	B-Architecture
amplifies	O
them	O
,	O
and	O
re-transmits	O
them	O
on	O
a	O
different	O
set	O
of	O
downlink	O
frequencies	O
to	O
receivers	O
on	O
Earth	O
,	O
often	O
without	O
changing	O
the	O
content	O
of	O
the	O
received	O
signal	O
or	O
signals	O
.	O
</s>
<s>
A	O
communications	O
satellite	O
’s	O
channels	O
are	O
called	O
transponders	B-Architecture
because	O
each	O
is	O
a	O
separate	O
transceiver	O
or	O
repeater	O
.	O
</s>
<s>
With	O
digital	O
video	O
data	B-General_Concept
compression	I-General_Concept
and	O
multiplexing	B-Architecture
,	O
several	O
video	O
and	O
audio	O
channels	O
may	O
travel	O
through	O
a	O
single	O
transponder	B-Architecture
on	O
a	O
single	O
wideband	O
carrier	O
.	O
</s>
<s>
Original	O
analog	O
video	O
only	O
has	O
one	O
channel	O
per	O
transponder	B-Architecture
,	O
with	O
subcarriers	O
for	O
audio	O
and	O
automatic	O
transmission	O
identification	O
service	O
(	O
ATIS	O
)	O
.	O
</s>
<s>
Non-multiplexed	O
radio	O
stations	O
can	O
also	O
travel	O
in	O
single	O
channel	O
per	O
carrier	O
(	O
SCPC	O
)	O
mode	O
,	O
with	O
multiple	O
carriers	O
(	O
analog	O
or	O
digital	O
)	O
per	O
transponder	B-Architecture
.	O
</s>
<s>
This	O
allows	O
each	O
station	O
to	O
transmit	O
directly	O
to	O
the	O
satellite	O
,	O
rather	O
than	O
paying	O
for	O
a	O
whole	O
transponder	B-Architecture
,	O
or	O
using	O
landlines	B-Application
to	O
send	O
it	O
to	O
an	O
earth	O
station	O
for	O
multiplexing	B-Architecture
with	O
other	O
stations	O
.	O
</s>
<s>
In	O
optical	B-Application
fiber	I-Application
communications	I-Application
,	O
a	O
transponder	B-Architecture
is	O
the	O
element	O
that	O
sends	O
and	O
receives	O
the	O
optical	O
signal	O
from	O
a	O
fiber	B-Architecture
.	O
</s>
<s>
A	O
transponder	B-Architecture
is	O
typically	O
characterized	O
by	O
its	O
data	O
rate	O
and	O
the	O
maximum	O
distance	O
the	O
signal	O
can	O
travel	O
.	O
</s>
<s>
The	O
term	O
"	O
transponder	B-Architecture
"	O
can	O
apply	O
to	O
different	O
items	O
with	O
important	O
functional	O
differences	O
,	O
mentioned	O
across	O
academic	O
and	O
commercial	O
literature	O
:	O
</s>
<s>
according	O
to	O
one	O
description	O
,	O
a	O
transponder	B-Architecture
and	O
transceiver	O
are	O
both	O
functionally	O
similar	O
devices	O
that	O
convert	O
a	O
full-duplex	O
electrical	O
signal	O
into	O
a	O
full-duplex	O
optical	O
signal	O
.	O
</s>
<s>
The	O
difference	O
between	O
the	O
two	O
is	O
that	O
transceivers	O
interface	O
electrically	O
with	O
the	O
host	O
system	O
using	O
a	O
serial	O
interface	O
,	O
whereas	O
transponders	B-Architecture
use	O
a	O
parallel	O
interface	O
to	O
do	O
so	O
.	O
</s>
<s>
In	O
this	O
view	O
,	O
transponders	B-Architecture
provide	O
easier-to-handle	O
lower-rate	O
parallel	O
signals	O
,	O
but	O
are	O
bulkier	O
and	O
consume	O
more	O
power	O
than	O
transceivers	O
.	O
</s>
<s>
according	O
to	O
another	O
description	O
,	O
transceivers	O
are	O
limited	O
to	O
providing	O
an	O
electrical-optical	O
function	O
only	O
(	O
not	O
differentiating	O
between	O
serial	O
or	O
parallel	O
electrical	O
interfaces	O
)	O
,	O
whereas	O
transponders	B-Architecture
convert	O
an	O
optical	O
signal	O
at	O
one	O
wavelength	O
to	O
an	O
optical	O
signal	O
at	O
another	O
wavelength	O
(	O
typically	O
ITU	O
standardized	O
for	O
DWDM	O
communication	O
)	O
.	O
</s>
<s>
As	O
such	O
,	O
transponders	B-Architecture
can	O
be	O
considered	O
as	O
two	O
transceivers	O
placed	O
back-to-back	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
difference	O
in	O
transponder	B-Architecture
functionality	O
also	O
might	O
influence	O
the	O
functional	O
description	O
of	O
related	O
optical	O
modules	O
like	O
transceivers	O
and	O
muxponders	B-Device
.	O
</s>
<s>
Another	O
type	O
of	O
transponder	B-Architecture
occurs	O
in	O
identification	B-Device
friend	I-Device
or	I-Device
foe	I-Device
(	O
IFF	O
)	O
systems	O
in	O
military	O
aviation	O
and	O
in	O
air	B-Application
traffic	I-Application
control	I-Application
secondary	O
surveillance	O
radar	B-Application
(	O
beacon	O
radar	B-Application
)	O
systems	O
for	O
general	O
aviation	O
and	O
commercial	O
aviation	O
.	O
</s>
<s>
Primary	B-Application
radar	I-Application
works	O
best	O
with	O
large	O
all-metal	O
aircraft	O
,	O
but	O
not	O
so	O
well	O
on	O
small	O
,	O
composite	O
aircraft	O
.	O
</s>
<s>
Secondary	O
radar	B-Application
overcomes	O
these	O
limitations	O
but	O
it	O
depends	O
on	O
a	O
transponder	B-Architecture
in	O
the	O
aircraft	O
to	O
respond	O
to	O
interrogations	O
from	O
the	O
ground	O
station	O
to	O
make	O
the	O
plane	O
more	O
visible	O
.	O
</s>
<s>
Depending	O
on	O
the	O
type	O
of	O
interrogation	O
,	O
the	O
transponder	B-Architecture
sends	O
back	O
a	O
transponder	B-Architecture
code	O
(	O
or	O
"	O
squawk	O
code	O
"	O
,	O
Mode	O
A	O
)	O
or	O
altitude	O
information	O
(	O
Mode	B-Architecture
C	I-Architecture
)	O
to	O
help	O
air	B-Application
traffic	I-Application
controllers	O
to	O
identify	O
the	O
aircraft	O
and	O
to	O
maintain	O
separation	O
between	O
planes	O
.	O
</s>
<s>
Another	O
mode	O
called	O
Mode	O
S	O
(	O
Mode	O
Select	O
)	O
is	O
designed	O
to	O
help	O
avoiding	O
over-interrogation	O
of	O
the	O
transponder	B-Architecture
(	O
having	O
many	O
radars	B-Application
in	O
busy	O
areas	O
)	O
and	O
to	O
allow	O
automatic	O
collision	O
avoidance	O
.	O
</s>
<s>
Mode	O
S	O
transponders	B-Architecture
are	O
backward	B-General_Concept
compatible	I-General_Concept
with	O
Modes	O
A	O
and	O
C	O
.	O
Mode	O
S	O
is	O
mandatory	O
in	O
controlled	O
airspace	O
in	O
many	O
countries	O
.	O
</s>
<s>
Transponders	B-Architecture
are	O
used	O
on	O
some	O
military	O
aircraft	O
to	O
ensure	O
ground	O
personnel	O
can	O
verify	O
the	O
functionality	O
of	O
a	O
missile	O
’s	O
flight	O
termination	O
system	O
prior	O
to	O
launch	O
.	O
</s>
<s>
Such	O
radar	B-Application
enhancing	O
transponders	B-Architecture
are	O
needed	O
as	O
the	O
enclosed	O
weapon	O
bays	O
on	O
modern	O
aircraft	O
interfere	O
with	O
prelaunch	O
,	O
flight	O
termination	O
system	O
verification	O
performed	O
by	O
range	O
safety	O
personnel	O
during	O
training	O
test	O
launches	O
.	O
</s>
<s>
The	O
transponders	B-Architecture
re-radiate	O
the	O
signals	O
allowing	O
for	O
much	O
longer	O
communication	O
distances	O
.	O
</s>
<s>
Although	O
AIS	O
transmitters/receivers	O
are	O
generally	O
called	O
transponders	B-Architecture
they	O
generally	O
transmit	O
autonomously	O
,	O
although	O
coast	O
stations	O
can	O
interrogate	O
class	O
B	O
transponders	B-Architecture
on	O
smaller	O
vessels	O
for	O
additional	O
information	O
.	O
</s>
<s>
In	O
addition	O
,	O
navigational	O
aids	O
often	O
have	O
transponders	B-Architecture
called	O
RACON	O
(	O
radar	B-Application
beacons	O
)	O
designed	O
to	O
make	O
them	O
stand	O
out	O
on	O
a	O
ship	O
's	O
radar	B-Application
screen	O
.	O
</s>
<s>
Many	O
modern	O
automobiles	O
have	O
keys	O
with	O
transponders	B-Architecture
hidden	O
inside	O
the	O
plastic	O
head	O
of	O
the	O
key	O
.	O
</s>
<s>
The	O
user	O
of	O
the	O
car	O
may	O
not	O
even	O
be	O
aware	O
that	O
the	O
transponder	B-Architecture
is	O
there	O
,	O
because	O
there	O
are	O
no	O
buttons	O
to	O
press	O
.	O
</s>
<s>
When	O
a	O
key	O
is	O
inserted	O
into	O
the	O
ignition	O
lock	O
cylinder	O
and	O
turned	O
,	O
the	O
car	O
's	O
computer	O
sends	O
a	O
signal	O
to	O
the	O
transponder	B-Architecture
.	O
</s>
<s>
Unless	O
the	O
transponder	B-Architecture
replies	O
with	O
a	O
valid	O
code	O
,	O
the	O
computer	O
will	O
not	O
allow	O
the	O
engine	O
to	O
be	O
started	O
.	O
</s>
<s>
Transponder	B-Architecture
keys	O
have	O
no	O
battery	O
;	O
they	O
are	O
energized	O
by	O
the	O
signal	O
itself	O
.	O
</s>
<s>
Electronic	O
toll	O
collection	O
systems	O
such	O
as	O
E-ZPass	O
in	O
the	O
eastern	O
United	O
States	O
use	O
RFID	B-Application
transponders	B-Architecture
to	O
identify	O
vehicles	O
.	O
</s>
<s>
Transponders	B-Architecture
are	O
used	O
in	O
motorsport	O
for	O
lap	O
timing	O
purposes	O
.	O
</s>
<s>
Each	O
car	O
has	O
an	O
active	O
transponder	B-Architecture
with	O
a	O
unique	O
ID	O
code	O
.	O
</s>
<s>
Passive	O
and	O
active	O
RFID	B-Application
systems	O
are	O
used	O
in	O
off-road	O
events	O
such	O
as	O
Enduro	B-Application
and	O
Hare	O
and	O
Hounds	O
racing	O
,	O
the	O
riders	O
have	O
a	O
transponder	B-Architecture
on	O
their	O
person	O
,	O
normally	O
on	O
their	O
arm	O
.	O
</s>
<s>
NASCAR	O
uses	O
transponders	B-Architecture
and	O
cable	O
loops	O
placed	O
at	O
numerous	O
points	O
around	O
the	O
track	O
to	O
determine	O
the	O
lineup	O
during	O
a	O
caution	O
period	O
.	O
</s>
<s>
Sonar	B-Application
transponders	B-Architecture
operate	O
under	O
water	O
and	O
are	O
used	O
to	O
measure	O
distance	O
and	O
form	O
the	O
basis	O
of	O
underwater	O
location	O
marking	O
,	O
position	O
tracking	O
and	O
navigation	O
.	O
</s>
<s>
Transponders	B-Architecture
may	O
also	O
be	O
used	O
by	O
residents	O
to	O
enter	O
their	O
gated	O
communities	O
.	O
</s>
<s>
However	O
,	O
having	O
more	O
than	O
one	O
transponder	B-Architecture
causes	O
problems	O
.	O
</s>
<s>
If	O
a	O
resident	O
's	O
car	O
with	O
simple	O
transponder	B-Architecture
is	O
parked	O
in	O
the	O
vicinity	O
,	O
any	O
vehicle	O
can	O
come	O
up	O
to	O
the	O
automated	O
gate	O
,	O
triggering	O
the	O
gate	B-Device
interrogation	I-Device
signal	I-Device
,	O
which	O
may	O
get	O
an	O
acceptable	O
response	O
from	O
the	O
resident	O
's	O
car	O
.	O
</s>
<s>
Such	O
units	O
properly	O
installed	O
might	O
involve	O
beamforming	B-Algorithm
,	O
unique	O
transponders	B-Architecture
for	O
each	O
vehicle	O
,	O
or	O
simply	O
obliging	O
vehicles	O
to	O
be	O
stored	O
away	O
from	O
the	O
gate	O
.	O
</s>
