<s>
The	O
air	B-Application
traffic	I-Application
control	I-Application
radar	I-Application
beacon	I-Application
system	I-Application
(	O
ATCRBS	B-Application
)	O
is	O
a	O
system	O
used	O
in	O
air	B-Application
traffic	I-Application
control	I-Application
(	O
ATC	O
)	O
to	O
enhance	O
surveillance	O
radar	B-Application
monitoring	O
and	O
separation	O
of	O
air	B-Application
traffic	I-Application
.	O
</s>
<s>
It	O
consists	O
of	O
a	O
rotating	O
ground	O
antenna	O
and	O
transponders	B-Architecture
in	O
aircraft	O
.	O
</s>
<s>
When	O
the	O
beam	O
strikes	O
an	O
aircraft	O
,	O
the	O
transponder	B-Architecture
transmits	O
a	O
return	O
signal	O
back	O
giving	O
information	O
such	O
as	O
altitude	O
and	O
the	O
Squawk	O
Code	O
,	O
a	O
four	O
digit	O
code	O
assigned	O
to	O
each	O
aircraft	O
that	O
enters	O
a	O
region	O
.	O
</s>
<s>
ATCRBS	B-Application
assists	O
air	B-Application
traffic	I-Application
control	I-Application
(	O
ATC	O
)	O
surveillance	O
radars	B-Application
by	O
acquiring	O
information	O
about	O
the	O
aircraft	O
being	O
monitored	O
,	O
and	O
providing	O
this	O
information	O
to	O
the	O
radar	B-Application
controllers	O
.	O
</s>
<s>
The	O
controllers	O
can	O
use	O
the	O
information	O
to	O
identify	O
radar	B-Application
returns	O
from	O
aircraft	O
(	O
known	O
as	O
targets	O
)	O
and	O
to	O
distinguish	O
those	O
returns	O
from	O
ground	O
clutter	O
.	O
</s>
<s>
The	O
system	O
consists	O
of	O
transponders	B-Architecture
,	O
installed	O
in	O
aircraft	O
,	O
and	O
secondary	O
surveillance	O
radars	B-Application
(	O
SSRs	O
)	O
,	O
installed	O
at	O
air	B-Application
traffic	I-Application
control	I-Application
facilities	O
.	O
</s>
<s>
The	O
SSR	O
is	O
sometimes	O
co-located	O
with	O
the	O
primary	O
surveillance	O
radar	B-Application
,	O
or	O
PSR	O
.	O
</s>
<s>
These	O
two	O
radar	B-Application
systems	I-Application
work	O
in	O
conjunction	O
to	O
produce	O
a	O
synchronized	O
surveillance	O
picture	O
.	O
</s>
<s>
Transponders	B-Architecture
that	O
receive	O
an	O
interrogation	O
decode	O
it	O
,	O
decide	O
whether	O
to	O
reply	O
,	O
and	O
then	O
respond	O
with	O
the	O
requested	O
information	O
when	O
appropriate	O
.	O
</s>
<s>
Note	O
that	O
in	O
common	O
informal	O
usage	O
,	O
the	O
term	O
"	O
SSR	O
"	O
is	O
sometimes	O
used	O
to	O
refer	O
to	O
the	O
entire	O
ATCRBS	B-Application
system	O
,	O
however	O
this	O
term	O
(	O
as	O
found	O
in	O
technical	O
publications	O
)	O
properly	O
refers	O
only	O
to	O
the	O
ground	O
radar	B-Application
itself	O
.	O
</s>
<s>
An	O
ATC	O
ground	O
station	O
consists	O
of	O
two	O
radar	B-Application
systems	I-Application
and	O
their	O
associated	O
support	O
components	O
.	O
</s>
<s>
It	O
is	O
also	O
referred	O
to	O
as	O
skin	O
paint	O
radar	B-Application
because	O
it	O
shows	O
not	O
synthetic	O
or	O
alpha-numeric	O
target	O
symbols	O
,	O
but	O
bright	O
(	O
or	O
colored	O
)	O
blips	O
or	O
areas	O
on	O
the	O
radar	B-Application
screen	O
produced	O
by	O
the	O
RF	O
energy	O
reflections	O
from	O
the	O
target	O
's	O
"	O
skin.	O
"	O
</s>
<s>
The	O
radar	B-Application
detects	O
and	O
displays	O
reflective	O
objects	O
within	O
the	O
radar	B-Application
's	O
operating	O
range	O
.	O
</s>
<s>
Weather	B-Application
radar	I-Application
data	O
is	O
displayed	O
in	O
skin	O
paint	O
mode	O
.	O
</s>
<s>
The	O
primary	O
surveillance	O
radar	B-Application
is	O
subject	O
to	O
the	O
radar	B-Application
equation	O
that	O
says	O
signal	O
strength	O
drops	O
off	O
as	O
the	O
fourth	O
power	O
of	O
distance	O
to	O
the	O
target	O
.	O
</s>
<s>
The	O
second	O
system	O
is	O
the	O
secondary	O
surveillance	O
radar	B-Application
,	O
or	O
SSR	O
,	O
which	O
depends	O
on	O
a	O
cooperating	O
transponder	B-Architecture
installed	O
on	O
the	O
aircraft	O
being	O
tracked	O
.	O
</s>
<s>
The	O
transponder	B-Architecture
emits	O
a	O
signal	O
when	O
it	O
is	O
interrogated	O
by	O
the	O
secondary	O
radar	B-Application
.	O
</s>
<s>
In	O
a	O
transponder	B-Architecture
based	O
system	O
signals	O
drop	O
off	O
as	O
the	O
inverse	O
square	O
of	O
the	O
distance	O
to	O
the	O
target	O
,	O
instead	O
of	O
the	O
fourth	O
power	O
in	O
primary	O
radars	B-Application
.	O
</s>
<s>
The	O
transponder	B-Architecture
can	O
also	O
send	O
encoded	O
information	O
about	O
the	O
aircraft	O
,	O
such	O
as	O
identity	O
and	O
altitude	O
.	O
</s>
<s>
Mode-S	O
interrogators	O
require	O
the	O
sum	O
and	O
difference	O
channels	O
to	O
provide	O
the	O
monopulse	B-Algorithm
capability	O
to	O
measure	O
the	O
off-boresight	O
angle	O
of	O
the	O
transponder	B-Architecture
reply	O
.	O
</s>
<s>
The	O
SSR	O
repetitively	O
transmits	O
interrogations	O
as	O
the	O
rotating	O
radar	B-Application
antenna	O
scans	O
the	O
sky	O
.	O
</s>
<s>
The	O
interrogation	O
specifies	O
what	O
type	O
of	O
information	O
a	O
replying	O
transponder	B-Architecture
should	O
send	O
by	O
using	O
a	O
system	O
of	O
modes	O
.	O
</s>
<s>
There	O
have	O
been	O
a	O
number	O
of	O
modes	O
used	O
historically	O
,	O
but	O
four	O
are	O
in	O
common	O
use	O
today	O
:	O
mode	O
1	O
,	O
mode	O
2	O
,	O
mode	O
3/A	O
,	O
and	O
mode	B-Architecture
C	I-Architecture
.	O
Mode	O
1	O
is	O
used	O
to	O
sort	O
military	O
targets	O
during	O
phases	O
of	O
a	O
mission	O
.	O
</s>
<s>
Mode	O
3/A	O
is	O
used	O
to	O
identify	O
each	O
aircraft	O
in	O
the	O
radar	B-Application
's	O
coverage	O
area	O
.	O
</s>
<s>
Mode	B-Architecture
C	I-Architecture
is	O
used	O
to	O
request/report	O
an	O
aircraft	O
's	O
altitude	O
.	O
</s>
<s>
Two	O
other	O
modes	O
,	O
mode	O
4	O
and	O
mode	O
S	O
,	O
are	O
not	O
considered	O
part	O
of	O
the	O
ATCRBS	B-Application
system	O
,	O
but	O
they	O
use	O
the	O
same	O
transmit	O
and	O
receive	O
hardware	O
.	O
</s>
<s>
Mode	O
4	O
is	O
used	O
by	O
military	O
aircraft	O
for	O
the	O
Identification	B-Device
Friend	I-Device
or	I-Device
Foe	I-Device
(	O
IFF	O
)	O
system	O
.	O
</s>
<s>
Mode	O
S	O
transponders	B-Architecture
ignore	O
interrogations	O
not	O
addressed	O
with	O
their	O
unique	O
identity	O
code	O
,	O
reducing	O
channel	O
congestion	O
.	O
</s>
<s>
At	O
a	O
typical	O
SSR	O
radar	B-Application
installation	O
,	O
ATCRBS	B-Application
,	O
IFF	O
,	O
and	O
mode	O
S	O
interrogations	O
will	O
all	O
be	O
transmitted	O
in	O
an	O
interlaced	O
fashion	O
.	O
</s>
<s>
Returns	O
from	O
both	O
radars	B-Application
at	O
the	O
ground	O
station	O
are	O
transmitted	O
to	O
the	O
ATC	O
facility	O
using	O
a	O
microwave	O
link	O
,	O
a	O
coaxial	O
link	O
,	O
or	O
(	O
with	O
newer	O
radars	B-Application
)	O
a	O
digitizer	O
and	O
a	O
modem	B-Application
.	O
</s>
<s>
Once	O
received	O
at	O
the	O
ATC	O
facility	O
,	O
a	O
computer	O
system	O
known	O
as	O
a	O
radar	B-Application
data	O
processor	O
associates	O
the	O
reply	O
information	O
with	O
the	O
proper	O
primary	O
target	O
and	O
displays	O
it	O
next	O
to	O
the	O
target	O
on	O
the	O
radar	B-Application
scope	O
.	O
</s>
<s>
The	O
equipment	O
installed	O
in	O
the	O
aircraft	O
is	O
considerably	O
simpler	O
,	O
consisting	O
of	O
the	O
transponder	B-Architecture
itself	O
,	O
usually	O
mounted	O
in	O
the	O
instrument	O
panel	O
or	O
avionics	O
rack	O
,	O
and	O
a	O
small	O
L	B-Algorithm
band	I-Algorithm
UHF	O
antenna	O
,	O
mounted	O
on	O
the	O
bottom	O
of	O
the	O
fuselage	O
.	O
</s>
<s>
Typical	O
installations	O
also	O
include	O
an	O
altitude	O
encoder	O
,	O
which	O
is	O
a	O
small	O
device	O
connected	O
to	O
both	O
the	O
transponder	B-Architecture
and	O
the	O
aircraft	O
's	O
static	O
system	O
.	O
</s>
<s>
It	O
provides	O
the	O
aircraft	O
's	O
pressure	O
altitude	O
to	O
the	O
transponder	B-Architecture
,	O
so	O
that	O
it	O
may	O
relay	O
the	O
information	O
to	O
the	O
ATC	O
facility	O
.	O
</s>
<s>
The	O
encoder	O
uses	O
11	O
wires	O
to	O
pass	O
altitude	O
information	O
to	O
the	O
transponder	B-Architecture
in	O
the	O
form	O
of	O
a	O
Gillham	B-Device
Code	I-Device
,	O
a	O
modified	O
binary	O
Gray	B-Device
code	I-Device
.	O
</s>
<s>
The	O
transponder	B-Architecture
has	O
a	O
small	O
required	O
set	O
of	O
controls	O
and	O
is	O
simple	O
to	O
operate	O
.	O
</s>
<s>
It	O
has	O
a	O
method	O
to	O
enter	O
the	O
four-digit	O
transponder	B-Architecture
code	O
,	O
also	O
known	O
as	O
a	O
beacon	O
code	O
or	O
squawk	O
code	O
,	O
and	O
a	O
control	O
to	O
transmit	O
an	O
ident	O
,	O
which	O
is	O
done	O
at	O
the	O
controller	O
's	O
request	O
(	O
see	O
SPI	O
pulse	O
below	O
)	O
.	O
</s>
<s>
Transponders	B-Architecture
typically	O
have	O
4	O
operating	O
modes	O
:	O
Off	O
,	O
Standby	O
,	O
On	O
(	O
Mode-A	O
)	O
,	O
and	O
Alt	O
(	O
Mode-C	O
)	O
.	O
</s>
<s>
Standby	O
mode	O
allows	O
the	O
unit	O
to	O
remain	O
powered	O
and	O
warmed	O
up	O
but	O
inhibits	O
any	O
replies	O
,	O
since	O
the	O
radar	B-Application
is	O
used	O
for	O
searching	O
the	O
aircraft	O
and	O
exact	O
location	O
of	O
aircraft	O
.	O
</s>
<s>
The	O
steps	O
involved	O
in	O
performing	O
an	O
ATCRBS	B-Application
interrogation	O
are	O
as	O
follows	O
:	O
First	O
,	O
the	O
ATCRBS	B-Application
interrogator	O
periodically	O
interrogates	O
aircraft	O
on	O
a	O
frequency	O
of	O
1030MHz	O
.	O
</s>
<s>
This	O
is	O
done	O
through	O
a	O
rotating	O
or	O
scanning	O
antenna	O
at	O
the	O
radar	B-Application
's	O
assigned	O
Pulse	O
Repetition	O
Frequency	O
(	O
PRF	O
)	O
.	O
</s>
<s>
When	O
the	O
aircraft	O
receives	O
the	O
interrogation	O
,	O
the	O
aircraft	O
transponder	B-Architecture
will	O
send	O
a	O
reply	O
on	O
1090MHz	O
after	O
a	O
3.0	O
μs	O
delay	O
indicating	O
the	O
requested	O
information	O
.	O
</s>
<s>
The	O
jitter	O
problem	O
makes	O
software	O
tracking	O
algorithms	O
problematic	O
,	O
and	O
is	O
the	O
reason	O
why	O
monopulse	B-Algorithm
was	O
implemented	O
.	O
</s>
<s>
Mode	B-Architecture
C	I-Architecture
uses	O
a	O
spacing	O
of	O
21	O
μs	O
,	O
and	O
requests	O
the	O
aircraft	O
's	O
pressure	O
altitude	O
,	O
provided	O
by	O
the	O
altitude	O
encoder	O
.	O
</s>
<s>
The	O
F1	O
and	O
F2	O
pulses	O
are	O
framing	O
pulses	O
,	O
and	O
are	O
always	O
transmitted	O
by	O
the	O
aircraft	O
transponder	B-Architecture
.	O
</s>
<s>
For	O
mode	O
A	O
,	O
each	O
digit	O
in	O
the	O
transponder	B-Architecture
code	O
(	O
A	O
,	O
B	O
,	O
C	O
,	O
or	O
D	O
)	O
may	O
be	O
a	O
number	O
from	O
zero	O
to	O
seven	O
.	O
</s>
<s>
In	O
a	O
mode	B-Architecture
C	I-Architecture
reply	O
,	O
the	O
altitude	O
is	O
encoded	O
by	O
a	O
Gillham	B-Device
interface	I-Device
,	O
Gillham	B-Device
code	I-Device
,	O
which	O
uses	O
Gray	B-Device
code	I-Device
.	O
</s>
<s>
The	O
Gillham	B-Device
interface	I-Device
is	O
capable	O
of	O
representing	O
a	O
wide	O
range	O
of	O
altitudes	O
,	O
in	O
increments	O
.	O
</s>
<s>
If	O
no	O
encoder	O
is	O
attached	O
,	O
the	O
transponder	B-Architecture
may	O
optionally	O
transmit	O
only	O
framing	O
pulses	O
(	O
most	O
modern	O
transponders	B-Architecture
do	O
)	O
.	O
</s>
<s>
The	O
SPI	O
pulse	O
is	O
turned	O
on	O
by	O
the	O
"	O
identity	O
control	O
"	O
on	O
the	O
transponder	B-Architecture
in	O
the	O
aircraft	O
cockpit	O
when	O
requested	O
by	O
air	B-Application
traffic	I-Application
control	I-Application
.	O
</s>
<s>
The	O
air	B-Application
traffic	I-Application
controller	O
can	O
request	O
the	O
pilot	O
to	O
ident	O
,	O
and	O
when	O
the	O
identity	O
control	O
is	O
activated	O
,	O
the	O
SPI	O
bit	O
will	O
be	O
added	O
to	O
the	O
reply	O
for	O
about	O
20	O
seconds	O
(	O
two	O
to	O
four	O
rotations	O
of	O
the	O
interrogator	O
antenna	O
)	O
thereby	O
highlighting	O
the	O
track	O
on	O
the	O
controllers	O
display	O
.	O
</s>
<s>
When	O
aircraft	O
are	O
close	O
to	O
the	O
ground	O
station	O
,	O
the	O
side	O
lobe	O
signals	O
are	O
often	O
strong	O
enough	O
to	O
elicit	O
a	O
reply	O
from	O
their	O
transponders	B-Architecture
when	O
the	O
antenna	O
is	O
not	O
pointing	O
at	O
them	O
.	O
</s>
<s>
This	O
can	O
cause	O
ghosting	O
,	O
where	O
an	O
aircraft	O
's	O
target	O
may	O
appear	O
in	O
more	O
than	O
one	O
location	O
on	O
the	O
radar	B-Application
scope	O
.	O
</s>
<s>
In	O
extreme	O
cases	O
,	O
an	O
effect	O
known	O
as	O
ring-around	O
occurs	O
,	O
where	O
the	O
transponder	B-Architecture
replies	O
to	O
excess	O
resulting	O
in	O
an	O
arc	O
or	O
circle	O
of	O
replies	O
centered	O
on	O
the	O
radar	B-Application
site	O
.	O
</s>
<s>
By	O
comparing	O
the	O
relative	O
strengths	O
of	O
P2	O
and	O
P1	O
,	O
airborne	O
transponders	B-Architecture
can	O
determine	O
whether	O
or	O
not	O
the	O
antenna	O
is	O
pointing	O
at	O
the	O
aircraft	O
when	O
the	O
interrogation	O
was	O
received	O
.	O
</s>
<s>
The	O
new	O
and	O
improved	O
SLS	O
employs	O
a	O
third	O
pulse	O
,	O
spaced	O
2μs	O
either	O
before	O
P3	O
(	O
a	O
new	O
P2	O
position	O
)	O
or	O
after	O
P3	O
(	O
which	O
should	O
be	O
called	O
P4	O
and	O
appears	O
in	O
the	O
Mode	O
S	O
radar	B-Application
and	O
TCAS	O
specifications	O
)	O
.	O
</s>
<s>
Any	O
requirement	O
at	O
the	O
transponder	B-Architecture
to	O
detect	O
and	O
act	O
upon	O
a	O
P2	O
pulse	O
2μs	O
after	O
P1	O
has	O
been	O
removed	O
from	O
the	O
new	O
and	O
improved	O
TSO	O
C74c	O
specification	O
.	O
</s>
<s>
Most	O
"	O
modern	O
"	O
transponders	B-Architecture
(	O
manufactured	O
since	O
1973	O
)	O
have	O
an	O
"	O
SLS	O
"	O
circuit	O
which	O
suppresses	O
reply	O
on	O
receipt	O
of	O
any	O
two	O
pulses	O
in	O
any	O
interrogation	O
spaced	O
2.0	O
microseconds	O
apart	O
that	O
are	O
above	O
the	O
MTL	O
Minimum	O
Triggering	O
Level	O
threshold	O
of	O
the	O
receiver	O
amplitude	O
discriminator	O
(	O
P1->P2	O
or	O
P2->P3	O
or	O
P3->P4	O
)	O
.	O
</s>
<s>
The	O
FAA	O
refers	O
to	O
the	O
non-responsiveness	O
of	O
new	O
and	O
improved	O
TSO	O
C74c	O
compliant	O
transponders	B-Architecture
to	O
Mode	O
S	O
compatible	O
radars	B-Application
and	O
TCAS	O
as	O
"	O
The	O
Terra	O
Problem	O
"	O
,	O
and	O
has	O
issued	O
Airworthiness	O
Directives	O
(	O
ADs	O
)	O
against	O
various	O
transponder	B-Architecture
manufacturers	O
,	O
over	O
the	O
years	O
,	O
at	O
various	O
times	O
on	O
no	O
predictable	O
schedule	O
.	O
</s>
<s>
The	O
ghosting	O
and	O
ring	O
around	O
problems	O
have	O
recurred	O
on	O
the	O
more	O
modern	O
radars	B-Application
.	O
</s>
<s>
It	O
is	O
highly	O
likely	O
that	O
one	O
of	O
those	O
software	O
algorithms	O
was	O
the	O
proximate	O
cause	O
of	O
a	O
mid-air	O
collision	O
recently	O
,	O
as	O
one	O
airplane	O
was	O
reported	O
at	O
showing	O
its	O
altitude	O
as	O
the	O
pre-flight	O
paper	O
filed	O
flight	O
plan	O
,	O
and	O
not	O
the	O
altitude	O
assigned	O
by	O
the	O
ATC	O
controller	O
(	O
see	O
the	O
reports	O
and	O
observations	O
contained	O
in	O
the	O
below	O
reference	O
ATC	O
Controlled	O
Airplane	O
Passenger	O
Study	O
of	O
how	O
radar	B-Application
worked	O
)	O
.	O
</s>
<s>
See	O
the	O
reference	O
section	O
below	O
for	O
errors	O
in	O
performance	O
standards	O
for	O
ATCRBS	B-Application
transponders	B-Architecture
in	O
the	O
US	O
.	O
</s>
<s>
See	O
the	O
reference	O
section	O
below	O
for	O
FAA	O
Technician	O
Study	O
of	O
in-situ	O
transponders	B-Architecture
.	O
</s>
<s>
The	O
beacon	O
code	O
and	O
altitude	O
were	O
historically	O
displayed	O
verbatim	O
on	O
the	O
radar	B-Application
scope	O
next	O
to	O
the	O
target	O
,	O
however	O
modernization	O
has	O
extended	O
the	O
radar	B-Application
data	O
processor	O
with	O
a	O
flight	O
data	O
processor	O
,	O
or	O
FDP	O
.	O
</s>
<s>
Although	O
the	O
ATCRBS	B-Application
does	O
not	O
display	O
aircraft	O
heading	O
.	O
</s>
<s>
Mode	O
S	O
,	O
or	O
mode	O
select	O
,	O
despite	O
also	O
being	O
called	O
a	O
mode	O
,	O
is	O
actually	O
a	O
radically	O
improved	O
system	O
intended	O
to	O
replace	O
ATCRBS	B-Application
altogether	O
.	O
</s>
<s>
A	O
few	O
countries	O
have	O
mandated	O
mode	O
S	O
,	O
and	O
many	O
other	O
countries	O
,	O
including	O
the	O
United	O
States	O
,	O
have	O
begun	O
phasing	O
out	O
ATCRBS	B-Application
in	O
favor	O
of	O
this	O
system	O
.	O
</s>
<s>
Mode	O
S	O
is	O
designed	O
to	O
be	O
fully	O
backward	O
compatible	O
with	O
existing	O
ATCRBS	B-Application
technology	O
.	O
</s>
<s>
Mode	O
S	O
,	O
despite	O
being	O
called	O
a	O
replacement	O
transponder	B-Architecture
system	O
for	O
ATCRBS	B-Application
,	O
is	O
actually	O
a	O
data	O
packet	O
protocol	O
which	O
can	O
be	O
used	O
to	O
augment	O
ATCRBS	B-Application
transponder	B-Architecture
positioning	O
equipment	O
(	O
radar	B-Application
and	O
TCAS	O
)	O
.	O
</s>
<s>
With	O
old	O
ATCRBS	B-Application
technology	O
,	O
all	O
aircraft	O
within	O
the	O
beam	O
pattern	O
of	O
the	O
interrogating	O
station	O
will	O
reply	O
.	O
</s>
<s>
In	O
an	O
airspace	O
with	O
multiple	O
interrogation	O
stations	O
,	O
ATCRBS	B-Application
transponders	B-Architecture
in	O
aircraft	O
can	O
be	O
overwhelmed	O
.	O
</s>
<s>
By	O
interrogating	O
one	O
aircraft	O
at	O
a	O
time	O
,	O
workload	O
on	O
the	O
aircraft	O
transponder	B-Architecture
is	O
greatly	O
reduced	O
.	O
</s>
<s>
The	O
half	O
split	O
method	O
is	O
computed	O
by	O
recording	O
the	O
azimuth	O
of	O
the	O
first	O
and	O
last	O
replies	O
from	O
the	O
aircraft	O
,	O
as	O
the	O
radar	B-Application
beam	O
sweeps	O
past	O
its	O
position	O
.	O
</s>
<s>
With	O
MSSR	O
(	O
monopulse	B-Algorithm
secondary	O
surveillance	O
radar	B-Application
)	O
and	O
Mode	O
S	O
,	O
the	O
radar	B-Application
can	O
use	O
the	O
information	O
of	O
one	O
reply	O
to	O
determine	O
azimuth	O
.	O
</s>
<s>
This	O
is	O
calculated	O
based	O
on	O
the	O
RF	O
phase	O
of	O
the	O
aircraft	O
reply	O
,	O
as	O
determined	O
by	O
the	O
sum	O
and	O
difference	O
antenna	O
elements	O
,	O
and	O
is	O
called	O
monopulse	B-Algorithm
.	O
</s>
<s>
This	O
monopulse	B-Algorithm
method	O
results	O
in	O
superior	O
azimuth	O
resolution	O
,	O
and	O
removes	O
target	O
jitter	O
from	O
the	O
display	O
.	O
</s>
<s>
Diversity	O
Mode	O
S	O
transponders	B-Architecture
may	O
be	O
implemented	O
for	O
the	O
purpose	O
of	O
improving	O
air-to-air	O
surveillance	O
and	O
communications	O
.	O
</s>
<s>
The	O
high	O
coverage	O
of	O
radar	B-Application
service	O
available	O
today	O
means	O
that	O
some	O
radar	B-Application
sites	O
receive	O
transponder	B-Architecture
replies	O
from	O
interrogations	O
that	O
were	O
initiated	O
by	O
other	O
nearby	O
radar	B-Application
sites	O
.	O
</s>
<s>
Finally	O
,	O
technology	O
improvements	O
have	O
made	O
transponders	B-Architecture
increasingly	O
affordable	O
such	O
that	O
today	O
almost	O
all	O
aircraft	O
are	O
equipped	O
with	O
them	O
.	O
</s>
<s>
The	O
system	O
also	O
has	O
provisions	O
for	O
transferring	O
arbitrary	O
data	O
both	O
to	O
and	O
from	O
a	O
transponder	B-Architecture
.	O
</s>
<s>
This	O
aspect	O
of	O
mode	O
S	O
makes	O
it	O
a	O
building	O
block	O
for	O
many	O
other	O
technologies	O
,	O
such	O
as	O
TCAS	O
2	O
,	O
Traffic	O
Information	O
Service	O
(	O
TIS	O
)	O
,	O
and	O
Automatic	B-Application
Dependent	I-Application
Surveillance-Broadcast	I-Application
.	O
</s>
