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855 

Federal Aviation Administration, DOT 

§ 171.311 

(B)  Azimuth angle encoding. Each 

guidance angle transmitted must con-
sist of a clockwise TO scan followed by 
a counterclockwise FRO scan as viewed 
from above the antenna. For approach 
azimuth functions, increasing angle 
values must be in the direction of the 
TO scan; for the back azimuth func-
tion, increasing angle values must be 
in the direction of the FRO scan. The 
antenna has a narrow beam in the 
plane of the scan direction and a broad 
beam in the orthogonal plane which 
fills the vertical coverage. 

(C) Elevation angle encoding. The radi-

ation from elevation equipment must 
produce a beam which scans from the 
horizon up to the highest elevation 
angle and then scans back down to the 
horizon. The antenna has a narrow 
beam in the plane of the scan direction 
and a broad beam in the orthogonal 
plane which fills the horizontal cov-

erage. Elevation angles are defined 
from the horizontal plane containing 
the antenna phase center; positive an-
gles are above the horizontal and zero 
angle is along the horizontal. 

(iv) Clearance guidance. The timing of 

the clearance pulses must be in accord-
ance with Figure 8. For azimuth ele-
ments with proportional coverage of 
less than ±40 degrees (±20 degrees for 
back azimuth), clearance guidance in-
formation must be provided by trans-
mitting pulses in a TO and FRO format 
adjacent to the stop/start times of the 
scanning beam signal. The fly-right 
clearance pulses must represent posi-
tive angles and the fly-left clearance 
pulses must represent negative angles. 
The duration of each clearance pulse 
must be 50 microseconds with a toler-
ance of ±5 microseconds. The trans-
mitter switching time between the 
clearance pulses and the scanning 

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