4-2
4.1.2
Secondary Radar System
ln
the secondary radar system, the pulses mdiated by the secondary radar antenna
are
not
reflected
but
received, decoded and responded
to
by
a transponder installed in the aircrah. This arrangement provides
the grou nd station
with
the active secondary pulses activated
by
the transponder, this being the
reas
on
why
this
is
referred
to
as
active return radiation, and
not
the weak reflected pulses
as
in the primary radar
system. The advantages
of
the secondary radar system
are
:
Low
radia ti
on
energy requ i
red
H igh effective
range
Practically uninfluenced by the weather
Ouicker aircraft
identification
Additional
information
availability (e.g. height
of
aircraft)
4.1.3
1 nterrogation
Pulses
in
the Secondary Radar System
The ground stations
can
usually interrogate the transponder
ATC
2000
in
two
modes, designa
led
rrrude
A
and mode
C.
The former
serves
identification,
the second
to
establish
the
heigh1
of
the
aircraft. A
further
means
of
interrogation
is
mode 8
to
wh ich the transponder
can
be
converted instead
of
mode A. The
conversion procedure
is
described
in
the
Section V , Maintenance and Repair
".The
various interrogation
modes
differ
by
the
spacing between
two
pulses radiated
by
the
ground stations (Pl and
P3)
on the carrier
trequency
1030
MHz.
As
can
be
seen
from
the
following
drawings,
the
pulse spacing in mode A
is
8,v.s,
in
mode B
17p.s
and
in
mode C
21p.s.
Mode D
is
a means
of
interrogation
which
is
currently
not
in
use, its pulse spacing being
25p.s.
Mode
A
Mode
8
1-oo>-----------
'!
7!
0,2).JS
Mode
C
.--------------
21
:!0,2.).lS
Each
interrogation a
Iso
contains a pulse (P2) which
is
aiso sent on the carrier frequency 1030 \tl Hz,
1ot
by the secondary antenna, however,
but
by
an
omnidirectional antenna. This pulse
P2
follows
P1
at a
spacing of
2~-Ls.
With the aid
of
the
P2
ntilse,
thP.
tr;:msrnnrlm determines whether the interrogation
of
the
ground station
is
coming
from
the main lobe
or
from a side lobe
ofthe
secondary antenna radiation. Side
lobe interrogation
is
not
responded
to
by the transponder since
it
would
result in a wrong position
of
the
aircraft being displayed on the radar screen
of
the ATC.
09/80
The document reference is online, please check the correspondence between the online documentation and the printed version.
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