(3) Identification of position (IDENT-
signals may be transmitted to interrogating
stations when conditions warrant.
b. Figure 1-2 is a functional block diagram
d. Five independent coding modes are avail-
showing the interrogation and response of the
able to the operator. The first three modes may
RT-859/APX-72. Signals, consisting of pairs
be used independently or in combination. Mode
of pulses spaced to form a code, are transmitted
1 provides 32 possible code combinations, any
to the RT-859/APX-72 which receives the
one of which may be selected in flight. Mode 2
coded signal and transfers it to the decoder.
provides 4,096 possible code combinations but
The decoder checks the incoming signal for
valid code and proper mode (except for mode 4
only one is available since the selection dials
interrogations which are sent directly to the
are not normally available in flight and must
mode 4 board). If valid, the decoded signal is
be preset before flight. Mode 3/A provides
4,096 possible codes any one of which may be
sent to the encoder board which prepares the
selected in flight. Mode C, when connected to
coded reply. The coded reply is sent through the
Pressure Altitude Digitizer CPU-66/A (or
equivalent), wiIl indicate pressure altitude of
aircraft when interrogated. Mode 4, which is
c. The RT-859/APX-72 can be operated in
connected to an external computer, can be
any one of the following categories, each of
selected to display any one of many classified
which may be selected by the operator at Con-
operational codes for security identification.
trol, Transponder Set C-6280(P)/APX.
(1) Low (sensitivity) operation
e. The range of the RT-859/APX-72 is
limited to line-of-sight transmission since its
(2) Normal (sensitivity) operation
Receiver-Transmitter, Radio RT-859/APX-72 functional block diagram.