Federal Aviation Administration, DOT

Section 171.313

datum to the coverage limit, the PFE,
PFN and CMN limits, expressed in angular terms, shall be allowed to linearly increase as follows:
(i) With distance along the runway
centerline extended, by a factor of 1.2
for the PFE and PFN limits and to
Section0.10 degree for the CMN limits.
(ii) With azimuth angle, by a factor
of 1.5 at the Section40 degree and a factor of
2.0 at the Section60 degree azimuth angles for
the PFE, PFN and CMN limits.
(iii) With elevation angle from + 9 degrees to + 15 degrees, by a factor of 1.5
for the PFE and PFN limits.
(iv) Maximum angular limits. The
PFE limits shall not exceed Section0.25 degree in any coverage region below an
elevation angle of + 9 degrees nor exceed Section0.50 degree in any coverage region above that elevation angle. The
CMN limits shall not exceed Section0.10 degree in any coverage region within Section10
degrees of runway centerline extended
nor exceed Section0.20 degree in any other region within coverage.
NOTE: It is desirable that the CMN not exceed Section0.10 degree throughout the coverage.

(f) Approach azimuth antenna characteristics are as follows:
(1) Drift. Any azimuth angle as encoded by the scanning beam at any
point within the proportional coverage
must not vary more than Section0.07 degree
over the range of service conditions
specified in Section 171.309(d) without the use
of internal environmental controls.
Multipath effects are excluded from
this requirement.
(2) Beam pointing errors. The azimuth
angle as encoded by the scanning beam
at any point within Section0.5 degree of the
zero degree azimuth must not deviate
from the true azimuth angle at that
point by more than Section.05 degree.
Multipath and drift effects are excluded from this requirement.
TABLE 10 - APPROACH AZIMUTH ACCURACIES AT
THE APPROACH REFERENCE DATUM
Angular error (degrees)
Error type

PFE ........
CMN ......

System

Section20 ft. (6.1m) 1 2
Section10.5 ft. (3.2m) 1 2 4

Ground
subsystem

Airborne
subsystem

Section0.118Section 3 ..
Section0.030Section ....

Section0.017Section
Section0.050Section

Notes:
1 Includes errors due to ground and airborne equipment and
propagation effects.

2 The system PFN component must not exceed Section3.5 meters
(11.5 feet).
3 The mean (bias) error component contributed by the
ground equipment should not exceed Section10 feet.
4 The system control motion noise must not exceed 0.1 degree.
5 The airborne subsystem angular errors are provided for information only.

(3) Antenna alignment. The antenna
must be equipped with suitable optical,
electrical or mechanical means or any
combination of the three, to bring the
zero degree azimuth radial into coincidence with the approach reference
datum (for centerline siting) with a
maximum error of 0.02 degree. Additionally, the azimuth antenna bias adjustment must be electronically steerable at least to the monitor limits in
steps not greater than 0.005 degree.
(4) Antenna far field patterns in the
plane of scan. On boresight, the azimuth antenna mainlobe pattern must
conform to Figure 10, and the beamwidth must be such that, in the installed environment, no significant lateral reflections of the mainlobe exist
along the approach course. In any case
the beamwidth must not exceed three
degrees. Anywhere within coverage the
Section3 dB width of the antenna mainlobe,
while scanning normally, must not be
less than 25 microseconds (0.5 degree)
or greater than 250 microseconds (5 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 off boresight. 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 main beam. Outside the coverage
sector, the radiation from the scanning
beam antenna must be of such a nature
that receiver warning will not be removed or suitable OCI signals must be
provided.
(ii) Effective sidelobe levels. With the
antenna
scanning
normally,
the
sidelobe levels in the plane of scan
must be such that, in the installed environment, the CMN contributed by
sidelobe reflections will not exceed the
angular equivalent of 9 feet at approach reference datum over the required range of aircraft approach
speeds.

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