Section 171.319

14 CFR Ch. I (1-1-19 Edition)

must not deviate from the true elevation angle at that point by more
than Section0.04 degree for elevation angles
from 2.5Section to 3.5Section. Above 3.5Section these errors
may linearly increase to Section0.1 degree at
7.5Section. Multipath and drift effects are excluded from this requirement.
(3) Antenna alignment. The antenna
must be equipped with suitable optical,
electrical, or mechanical means or any
combination of the three, to align the
lowest operationally required glidepath
to the true glidepath angle with a maximum error of 0.01 degree. Additionally, the elevation antenna bias adjustment must be electronically steerable
at least to the monitor limits in steps
not greater than 0.005 degrees.
(4) Antenna far field patterns in the
plane of scan. On the lowest operationally required glidepath, the antenna
mainlobe pattern must conform to Figure 10, and the beamwidth must be
such that in the installed environment,
no significant ground reflections of the
mainlobe exist. In any case, the beamwidth must not exceed 2 degrees. The
antenna mainlobe may be allowed to
broaden from the value at boresight by
a factor of 1/cosq, where q is the angle
of boresight. Anywhere within coverage, the Section3 dB width of the antenna
mainlobe, while scanning normally,
must not be less than 25 microseconds
(0.5 degrees) or greater than 250 microseconds (5 degrees). The sidelobe levels
must be as follows:
(i) Dynamic sidelobe levels. With the
antenna scanning normally, the dynamic sidelobe level that is detected by
a receiver at any point within the proportional coverage sector must be
down at least 10 dB from the peak of
the mainlobe. Outside the proportional
coverage sector, the radiation from the
scanning beam antenna must be of
such a nature that receiver warnings
will not be removed or a suitable OCI
signal must be provided.
(ii) Effective sidelobe levels. With the
antenna
scanning
normally,
the
sidelobe levels in the plane of scan
must be such that, when reflected from
the ground, the resultant PFE along
any glidepath does not exceed 0.083 degrees.
(5) Antenna far field pattern in the horizontal plane. The horizontal pattern of
the antenna must gradually de-empha-

size the signal away from antenna
boresight. Typically, the horizontal
pattern should be reduced by at least 3
dB at 20 degrees off boresight and by at
least 6 dB at 40 degrees off boresight.
Depending on the actual multipath
conditions, the horizontal radiation
patterns may require more or less deemphasis.
(6) Data antenna. The data antenna
must have horizontal and vertical patterns as required for its function.
(f) False guidance. False courses
which can be acquired and tracked by
an aircraft shall not exist anywhere either inside or outside of the MLS coverage sector. False courses which exist
outside of the minimum coverage sector may be suppressed by the use of
OCI.
NOTE: False courses may be due to (but not
limited to) MLS airborne receiver acquisition of the following types of false guidance:
reflections of the scanning beam and scanning beam antenna sidelobes and grating
lobes.

Section 171.319 Approach elevation monitor
system requirements.
(a) The monitor system must act to
ensure that any of the following conditions do not persist for longer than the
periods specified when:
(1) There is a change in the ground
component contribution to the mean
glidepath error component such that
the path following error on any glidepath exceeds the limits specified in
Section 171.317(d) for a period of more than
one second.
NOTE: The above requirement and the requirement to limit the ground equipment
mean error to Section0.067 degree can be satisfied
by the following procedure. The integral
monitor alarm limit should be set to Section0.067
degree. This will limit the electrical component of mean glidepath error to Section0.067 degree.
The field monitor alarm limit should be set
such that with the mean glidepath error at
the alarm limit the total allowed PFE is not
exceeded on any commissioned glidepath
from the limit of coverage to an altitude of
100 feet.

(2) There is a reduction in the radiated power to a level not less than that
specified in Section 171.317(a)(4) for a period of
more than one second.
(3) There is an error in the preamble
DPSK transmission which occurs more
than once in any one second period.

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