855
Federal Aviation Administration, DOT
§ 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 sup-
pressed. The audible tone in the air-
craft 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 char-
acters (letters) must not be less than
three dots. When back azimuth is pro-
vided, the code shall be transmitted by
the approach azimuth and back azi-
muth within plus or minus 0.08 seconds.
(B)
Airborne antenna selection.
A sig-
nal for airborne antenna selection shall
be transmitted as a ‘‘zero’’ DPSK sig-
nal lasting for a six-bit period (see Ta-
bles 4a and 4b).
T
ABLE
4a—A
PPROACH
A
ZIMUTH
F
UNCTION
TIMING
Event
Event time slot
begins at—
15.625
kHz clock
pulse
(number)
Time
(milli-
sec-
onds)
Preamble ...............................................
0 0
Morse code ............................................
25
1 .600
Antenna select .......................................
26
1 .664
Rear OCI ...............................................
32
2 .048
Left OCI .................................................
34
2 .176
Right OCI ...............................................
36
2 .304
To test ...................................................
38
2 .432
To scan
1
................................................
40 2
.560
Pause ....................................................
................
8 .760
Midscan point ........................................
................
9 .060
FRO scan
1
............................................
................
9 .360
FRO test ................................................
................
15 .560
End Function (Airborne) ........................
................
15 .688
End guard time; end function (ground)
................
15 .900
AA
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 pro-
vided shall accommodate a maximum scan of plus or minus
62.0 degrees. Scan timing shall be compatible with accuracy
requirements.
T
ABLE
4b—H
IGH
R
ATE
A
PPROACH
A
ZIMUTH AND
B
ACK
A
ZIMUTH
F
UNCTION
T
IMING
Event
Event time slot
begins at—
15.625
kHz clock
pulse
(number)
Time
(milli-
sec-
onds)
Preamble ...............................................
0 0
Morse Code ...........................................
25
1 .600
Antenna select .......................................
26
1 .664
Rear OCI ...............................................
32
2 .048
Left OCI .................................................
34
2 .176
Right OCI ...............................................
36
2 .304
To test ...................................................
38
2 .432
To scan
1
................................................
40 2
.560
Pause ....................................................
................
6 .760
Midscan point ........................................
................
7 .060
FRO scan
1
............................................
................
7 .360
FRO test pulse ......................................
................
11 .560
End function (airborne) ..........................
................
11 .688
End guard time; end function (ground)
................
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 de-
grees. 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.
T
ABLE
5—A
PPROACH
E
LEVATION
F
UNCTION
T
IMING
Event
Event time slot
begins at:
15.625
kHz clock
pluse
(number)
Time
(milli-
sec-
onds)
Preamble ...............................................
0 0
Processor pause ....................................
25
1 .600
OCI ........................................................
27 1
.728
To scan
1
................................................
29 1
.856
Pause ....................................................
................
3 .406
Midscan point ........................................
................
3 .606
FRO scan
1
............................................
................
3 .806
End function (airborne) ..........................
................
5 .356
End guard time; end function (ground)
................
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
¥
1.5 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
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