<s>
The	O
aviation	B-Algorithm
transponder	I-Algorithm
interrogation	I-Algorithm
modes	I-Algorithm
are	O
the	O
standard	O
formats	O
of	O
pulsed	O
sequences	O
from	O
an	O
interrogating	O
Secondary	O
Surveillance	O
Radar	O
(	O
SSR	O
)	O
or	O
similar	O
Automatic	B-Application
Dependent	I-Application
Surveillance-Broadcast	I-Application
(	O
ADS-B	B-Application
)	O
system	O
.	O
</s>
<s>
C	O
Provides	O
the	O
aircraft	O
's	O
pressure	O
altitude	O
and	O
is	O
usually	O
combined	O
with	O
Mode	O
3/A	O
to	O
provide	O
a	O
combination	O
of	O
a	O
4-digit	O
octal	O
code	O
and	O
altitude	O
as	O
Mode	O
3	O
A/C	O
,	O
often	O
referred	O
to	O
as	O
Mode	O
A	O
and	O
C	O
4	O
Provides	O
a	O
3-pulse	O
reply	O
,	O
delay	O
is	O
based	O
on	O
the	O
encrypted	O
challenge	O
5	O
Provides	O
a	O
cryptographically	O
secured	O
version	O
of	O
Mode	O
S	O
and	O
ADS-B	B-Application
GPS	O
position	O
S	O
Provides	O
multiple	O
information	O
formats	O
to	O
a	O
selective	O
interrogation	O
.	O
</s>
<s>
Mode	O
A	O
and	O
Mode	O
C	O
are	O
implemented	O
using	O
air	B-Application
traffic	I-Application
control	I-Application
radar	I-Application
beacon	I-Application
system	I-Application
as	O
the	O
physical	B-Application
layer	I-Application
,	O
whereas	O
Mode	O
S	O
is	O
implemented	O
as	O
a	O
standalone	O
backwards-compatible	O
protocol	O
.	O
</s>
<s>
ADS-B	B-Application
can	O
operate	O
using	O
Mode	O
S-ES	O
or	O
Universal	O
Access	O
Transceiver	O
as	O
its	O
transport	B-Architecture
layer	I-Architecture
:	O
</s>
<s>
The	O
altitude	O
information	O
is	O
passed	O
to	O
the	O
transponder	O
using	O
a	O
modified	O
form	O
of	O
the	O
modified	O
Gray	B-Device
code	I-Device
called	O
a	O
Gillham	B-Device
code	I-Device
.	O
</s>
<s>
This	O
is	O
the	O
type	O
of	O
transponder	O
that	O
is	O
used	O
for	O
TCAS	O
or	O
ACAS	O
II	O
(	O
Airborne	O
Collision	O
Avoidance	O
System	O
)	O
functions	O
,	O
and	O
is	O
required	O
to	O
implement	O
the	O
extended	B-Application
squitter	I-Application
broadcast	O
,	O
one	O
means	O
of	O
participating	O
in	O
ADS-B	B-Application
systems	O
.	O
</s>
<s>
Likewise	O
,	O
a	O
Mode	O
S	O
transponder	O
is	O
required	O
to	O
implement	O
1090ES	O
extended	B-Application
squitter	I-Application
ADS-B	B-Application
Out	O
,	O
but	O
there	O
are	O
other	O
ways	O
to	O
implement	O
ADS-B	B-Application
Out	O
(	O
in	O
the	O
U.S.	O
and	O
China	O
.	O
)	O
</s>
<s>
Upon	O
interrogation	O
,	O
Mode	O
S	O
transponders	O
transmit	O
information	O
about	O
the	O
aircraft	O
to	O
the	O
SSR	O
system	O
,	O
to	O
TCAS	O
receivers	O
on	O
board	O
aircraft	O
and	O
to	O
the	O
ADS-B	B-Application
SSR	O
system	O
.	O
</s>
<s>
In	O
2009	O
the	O
ICAO	O
published	O
an	O
"	O
extended	O
"	O
form	O
of	O
Mode	O
S	O
with	O
more	O
message	O
formats	O
to	O
use	O
with	O
ADS-B	B-Application
;	O
it	O
was	O
further	O
refined	O
in	O
2012	O
.	O
</s>
<s>
Countries	O
implementing	O
ADS-B	B-Application
can	O
require	O
the	O
use	O
of	O
either	O
the	O
extended	B-Application
squitter	I-Application
mode	O
of	O
a	O
suitably-equipped	O
Mode	O
S	O
transponder	O
,	O
or	O
the	O
UAT	O
transponder	O
on	O
978MHz	O
.	O
</s>
