Federal Aviation Administration, DOT

Section 171.311

minute, during which time the ground
subsystem is available for operational
use. When the transmissions of the
ground subsystem are not available,
the identification signal must be suppressed. The audible tone in the aircraft is started by setting the Morse
Code bit to logic  - 1 -  and stopped by a
logic  - 0 -  (see Tables 4a and 4b). The
identification
code
characteristics
must conform to the following: the dot
must be between 0.13 and 0.16 second in
duration, and the dash between 0.39 and
0.48 second. The duration between dots
and/or dashes must be one dot plus or
minus 10%. The duration between characters (letters) must not be less than
three dots. When back azimuth is provided, the code shall be transmitted by
the approach azimuth and back azimuth within plus or minus 0.08 seconds.
(B) Airborne antenna selection. A signal for airborne antenna selection shall
be transmitted as a  - zero -  DPSK signal lasting for a six-bit period (see Tables 4a and 4b).
TABLE 4a - APPROACH AZIMUTH FUNCTION
TIMING
Event time slot
begins at - 
Event

15.625
kHz clock
pulse
(number)

Time
(milliseconds)

Preamble ...............................................
Morse code ............................................
Antenna select .......................................
Rear OCI ...............................................
Left OCI .................................................
Right OCI ...............................................
To test ...................................................
To scan 1 ................................................
Pause ....................................................
Midscan point ........................................
FRO scan 1 ............................................
FRO test ................................................
End Function (Airborne) ........................
End guard time; end function (ground)

0
25
26
32
34
36
38
40
................
................
................
................
................
................

0
1.600
1.664
2.048
2.176
2.304
2.432
2.560
8.760
9.060
9.360
15.560
15.688
15.900

AA1 The actual commencement and completion of the TO
and the FRO scan transmissions are dependent on the
amount of proportional guidance provided. The time slots provided shall accommodate a maximum scan of plus or minus
62.0 degrees. Scan timing shall be compatible with accuracy
requirements.

TABLE 4b - HIGH RATE APPROACH AZIMUTH AND
BACK AZIMUTH FUNCTION TIMING
Event time slot
begins at - 
Event

15.625
kHz clock
pulse
(number)

Time
(milliseconds)

Preamble ...............................................
Morse Code ...........................................
Antenna select .......................................
Rear OCI ...............................................
Left OCI .................................................
Right OCI ...............................................
To test ...................................................
To scan 1 ................................................
Pause ....................................................
Midscan point ........................................
FRO scan 1 ............................................
FRO test pulse ......................................
End function (airborne) ..........................
End guard time; end function (ground)

0
25
26
32
34
36
38
40
................
................
................
................
................
................

0
1.600
1.664
2.048
2.176
2.304
2.432
2.560
6.760
7.060
7.360
11.560
11.688
11.900

1 The actual commencement and completion of the TO and
the FRO scan transmissions are dependent on the amount of
proportional guidance provided. The time slots provided will
accommodate a maximum scan of plus or minus 42.0 degrees. Scan timing shall be compatible with accuracy
requirements.

(C) OCI. Where OCI pulses are used,
they must be: (1) greater than any
guidance signal in the OCI sector; (2) at
least 5 dB less than the level of the
scanning beam within the proportional
guidance sector; and (3) for azimuth
functions with clearance signals, at
least 5 dB less than the level of the left
(right) clearance pulses within the left
(right) clearance sector.
TABLE 5 - APPROACH ELEVATION FUNCTION
TIMING
Event time slot
begins at:
Event

15.625
kHz clock
pluse
(number)

Preamble ...............................................
Processor pause ....................................
OCI ........................................................
To scan 1 ................................................
Pause ....................................................
Midscan point ........................................
FRO scan 1 ............................................
End function (airborne) ..........................
End guard time; end function (ground)

0
25
27
29
................
................
................
................
................

Time
(milliseconds)
0
1.600
1.728
1.856
3.406
3.606
3.806
5.356
5.600

1 The actual commencement and completion of the TO and
FRO scan transmissions are dependent upon the amount of
proportional guidance provided. The time slots provided will
accommodate a maximum scan of Section1.5 degrees to + 29.5
degrees. Scan timing shall be compatible with accuracy
requirements.

The duration of each pulse measured
at the half amplitude point shall be at
least 100 microseconds, and the rise and

855

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