Federal Aviation Administration, DOT

Section 171.311

(B) Azimuth angle encoding. Each
guidance angle transmitted must consist 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 function, 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 radiation 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 angles are above the horizontal and zero
angle is along the horizontal.
(iv) Clearance guidance. The timing of
the clearance pulses must be in accordance with Figure 8. For azimuth elements with proportional coverage of
less than Section40 degrees (Section20 degrees for
back azimuth), clearance guidance information must be provided by transmitting 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 positive angles and the fly-left clearance
pulses must represent negative angles.
The duration of each clearance pulse
must be 50 microseconds with a tolerance of Section5 microseconds. The transmitter switching time between the
clearance pulses and the scanning

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