Federal Aviation Administration, DOT

Section 171.317

(4) Within the elevation coverage sector defined in paragraphs (a) (1), (2) and
(3) of this section, the power densities
must not be less than those shown in
Table 9, but the equipment design must
also allow for:
(i) Transmitter power degradation
from normal by Section1.5 dB.
(ii) Rain loss of Section2.2 dB at the coverage extremes.
(b) Elevation siting requirements. The
Elevation Antenna System must:
(1) Be located as close to runway centerline as possible (without violating
obstacle clearance criteria).
(2) Be located near runway threshold
such that the asymptote of the minimum glidepath crosses the threshold
of the runway at the Approach Reference Datum height. Normally, the
minimum glidepath should be 3 degrees
and the Approach Reference Datum
height should be 50 feet. However,
there are circumstances where other
glideslopes
and
reference
datum
heights are appropriate. Some of these
instances are discussed in FAA Order
8260.34 (Glide Slope Threshold Crossing
Height Requirements) and Order 8260.3
(IFR Approval of MLS.)
(3) Be located such that the MLS Approach Reference Datum and ILS Reference Datum heights are coincident
within a tolerance of 3 feet when MLS
is installed on a runway already served
by an ILS. This requirement applies
only if the ILS glide slope is sited such
that the height of the reference datum
meets the requirements of FAA Order
8260.34.
(c) Antenna coordinates. The scanning
beams transmitted by the elevation
subsystem must be conical.
(d) Elevation accuracy. (1) The accuracies shown in Table 13 are required at
the approach reference datum. From
the approach reference datum to the
coverage limit, the PFE, PFN and CMN
limits shall be allowed to linearly increase as follows:
(i) With distance along the runway
centerline extended at the minimum
glide path angle, 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, from runway
centerline extended to the coverage extreme, by a factor of 1.2 for the PFE

and PFN limits and by a factor of 2.0
for the CMN limits;
(iii) With increasing elevation angles
from + 3 degrees to + 15 degrees, by a
factor of 2.0 for the PFE and PFN limits;
TABLE 13 - ELEVATION ACCURACIES AT THE
APPROACH REFERENCE DATUM
Angular error (degrees)
Error type

System

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

12

Ground subsystem

Airborne
subsystem 4

(3)
Section0.020

Section0.017
Section0.010

Section0.133

1Section0.050

Notes:
1 Includes errors due to ground and airborne equipment and
propagation effects.
2 The system PFN component must not exceed Section0.087 degree.
3 The mean (bias) error component contributed by the
ground equipment should not exceed Section0.067 degree.
4 The airborne subsystem angular errors are provided for information only.

(iv) With decreasing elevation angle
from + 3 degrees (or 60% of the minimum glide path angle, whichever is
less) to the coverage extreme, by a factor of 3 for the PFE, PFN and CMN
limits; and
(v) Maximum angular limits. the
CMN limits shall not exceed Section0.10 degree in any coverage region within Section10
degrees laterally of runway centerline
extended which is above the elevation
angle specified in (iv) above.
NOTE: It is desirable that the CMN not exceed Section0.10 degree throughout the coverage region above the elevation angle specified in
paragraph (d)(1)(iv) of this section.

(2) The system and ground subsystem
accuracies shown in Table 13 are to be
demonstrated at commissioning as
maximum error limits. Subsequent to
commissioning, the accuracies are to
be considered at 95% probability limits.
(e) Elevation antenna characteristics
are as follows:
(1) Drift. Any elevation angle as encoded by the scanning beam at any
point within the coverage sector must
not vary more than 0.04 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 elevation
angle as encoded by the scanning beam
at any point within the coverage sector

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