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856 

14 CFR Ch. I (1–1–18 Edition) 

§ 171.311 

beam transmissions must not exceed 10 
microseconds. The rise time at the 
edge of each clearance pulse must be 
less than 10 microseconds. Within the 
fly-right clearance guidance section, 
the fly-right clearance guidance signal 
shall exceed scanning beam antenna 
sidelobes and other guidance and OCI 
signals by at least 5 dB; within the fly- 
left clearance guidance sector, the fly 
left clearance guidance signal shall ex-
ceed scanning beam antenna sidelobes 
and all other guidance and OCI signals 
by at least 5 dB; within the propor-
tional guidance sector, the clearance 
guidance signals shall be at least 5dB 
below the proportional guidance signal. 
Optionally, clearance guidance may be 
provided by scanning throughout the 
approach guidance sector. For angles 
outside the approach azimuth propor-
tional coverage limits as set in Basic 
Data Word One (Basic Data Word 5 for 
back azimuth), proper decode and dis-
play of clearance guidance must occur 
to the limits of the guidance region. 

Where used, clearance pulses shall be 
transmitted adjacent to the scanning 
beam signals at the edges of propor-
tional coverage as shown in Figure 8. 
The proportional coverage boundary 
shall be established at one beamwidth 
inside the scan start/stop angles, such 
that the transition between scanning 
beam and clearance signals occurs out-
side the proportional coverage sector. 
When clearance pulses are provided in 
conjunction with a narrow beamwidth 
(e.g., one degree) scanning antenna, the 
scanning beam antenna shall radiate 
for 15 microseconds while stationary at 
the scan start/stop angles. 

(3)  Data function format. Basic data 

words provide equipment characteris-
tics and certain siting information. 
Basic data words must be transmitted 
from an antenna located at the ap-
proach azimuth or back azimuth site 
which provides coverage throughout 
the appropriate sector. Data function 
timing must be in accordance with 
Table 7a. 

T

ABLE

6—A

NGLE

S

CAN

T

IMING

C

ONSTANTS

 

Function 

Max 

value of 

t

(usec) 

T

o

(usec) 

V(deg/ 

usec) 

T

m

 

(usec) 

Pause 

time 

(usec) 

T

t

(usec) 

Approach azimuth .....................................................................

13,000 

6,800 

0.02 

7,972 

600 

13,128 

High rate approach azimuth ......................................................

9,000 

4,800 

0.02 

5,972 

600 

9,128 

Approach elevation ...................................................................

3,500 

3,350 

0.02 

2,518 

400 

N/A 

Back azimuth .............................................................................

9,000 

4,800 

¥0.02 5,972 

600 9,128 

T

ABLE

7a—B

ASIC

D

ATA

F

UNCTION

T

IMING

 

Event 

Event time slot 

begins at:

1

 

15.625 

kHz clock 

pulse 

(number) 

Time 

(milli-

sec-

onds) 

Preamble ...............................................

0 0 

Data transmission (bits I

13

–I

30

) ..............

25 1 

.600 

Parity transmission (bits I

31

–I

32

) ............

43 2 

.752 

End function (airborne) ..........................

45 

2 .880 

End guard time: end function (ground)

................

3 .100 

1

The previous event time slot ends at this time. 

T

ABLE

7b—A

UXILIARY

D

ATA

F

UNCTION

T

IMING

— 

(D

IGITAL

Event 

Event time slot 

begins at: 

15.625 

kHz clock 

pulse 

(number) 

Time 

(milli-

sec-

onds) 

Preamble ...............................................

0 0 

Address transmission (bits I

13

–I

20

) ........

25 1 

.600 

Data transmission: (bits I

21

–I

69

) .............

33 2 

.112 

T

ABLE

7b—A

UXILIARY

D

ATA

F

UNCTION

T

IMING

— 

(D

IGITAL

)—Continued 

Event 

Event time slot 

begins at: 

15.625 

kHz clock 

pulse 

(number) 

Time 

(milli-

sec-

onds) 

Parity transmission (bits I

70

–I

76

) ............

82 5 

.248 

End function (airborne) ..........................

89 

5 .696 

End guard time; end function (ground)

................

5 .900 

T

ABLE

7c—A

UXILIARY

D

ATA

F

UNCTION

T

IMING

— 

(A

LPHANUMERIC

Event 

Event time slot 

begins at: 

15.615 

kHz clock 

pulse 

(number) 

Time 

(milli-

sec-

onds) 

Preamble ...............................................

0 0 

Address transmission (bits I

13

–I

20

) ........

25 1 

.600 

Data transmission: (bits I

21

–I

76

..............

33 2 

.112 

End function (airborne) ..........................

89 

5 .696 

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