title-14-part-171-sec171-317

886

14 CFR Ch. I (1–1–24 Edition)

§ 171.317

neous guidance information is radiated
must not exceed the periods specified
in § 171.315(a). If the fault is not cleared
within the time allowed, the ground
equipment must be shut down. After
shutdown, no attempt must be made to
restore service until a period of 20 sec-
onds has elapsed.

§ 171.317 Approach elevation perform-

ance requirements.

This section prescribes the perform-

ance requirements for the elevation
equipment components of the MLS as
follows:

(a)

Elevation coverage requirements.

The approach elevation facility must
provide proportional guidance informa-
tion in at least the following volume of
space (see Figure 13):

(1) Laterally within a sector origi-

nating at the datum point which is at
least equal to the proportional guid-
ance sector provided by the approach
azimuth ground equipment.

(2) Longitudinally from 75 meters (250

feet) from the datum point to 20 nau-
tical miles from threshold in the direc-
tion of the approach.

(3) Vertically within the sector

bounded by:

(i) A surface which is the locus of

points 2.5 meters (8 feet) above the run-
way surface;

(ii) A conical surface originating at

the datum point and inclined 0.9 degree
above the horizontal and,

(iii) A conical surface originating at

the datum point and inclined at 15.0 de-
grees above the horizontal up to a
height of 6000 meters (20,000 feet).

888

14 CFR Ch. I (1–1–24 Edition)

§ 171.317

(4) Within the elevation coverage sec-

tor 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

1.5 dB.

(ii) Rain loss of

2.2 dB at the cov-

erage extremes.

(b)

Elevation siting requirements.

The

Elevation Antenna System must:

(1) Be located as close to runway cen-

terline as possible (without violating
obstacle clearance criteria).

(2) Be located near runway threshold

such that the asymptote of the min-
imum glidepath crosses the threshold
of the runway at the Approach Ref-
erence 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 Ap-

proach Reference Datum and ILS Ref-
erence 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 accura-

cies 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 in-
crease 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

0.10

degree for the CMN limits;

(ii) With azimuth angle, from runway

centerline extended to the coverage ex-
treme, 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 lim-
its;

ABLE

13—E

LEVATION

CCURACIES AT THE

PPROACH

EFERENCE

ATUM

Error type

System

Angular error (degrees)

Ground sub-

system

Airborne

subsystem

PFE ....................

0.133

(

)

0.017

CMN ..................

0.050

0.020

0.010

Notes:

Includes errors due to ground and airborne equipment and

propagation effects.

The system PFN component must not exceed

0.087 de-

gree.

The mean (bias) error component contributed by the

ground equipment should not exceed

0.067 degree.

The airborne subsystem angular errors are provided for in-

formation only.

(iv) With decreasing elevation angle

from + 3 degrees (or 60% of the min-
imum glide path angle, whichever is
less) to the coverage extreme, by a fac-
tor of 3 for the PFE, PFN and CMN
limits; and

(v) Maximum angular limits. the

CMN limits shall not exceed

0.10 de-

gree in any coverage region within

degrees laterally of runway centerline
extended which is above the elevation
angle specified in (iv) above.

OTE

: It is desirable that the CMN not ex-

ceed

0.10 degree throughout the coverage re-

gion 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 en-

coded 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
§ 171.309(d) without the use of internal
environmental controls. Multipath ef-
fects are excluded from this require-
ment.

(2)

Beam pointing errors.

The elevation

angle as encoded by the scanning beam
at any point within the coverage sector

889

Federal Aviation Administration, DOT

§ 171.319

must not deviate from the true ele-
vation angle at that point by more
than

0.04 degree for elevation angles

from 2.5

to 3.5

. Above 3.5

these errors

may linearly increase to

0.1 degree at

7.5

. Multipath and drift effects are ex-

cluded 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 max-
imum error of 0.01 degree. Addition-
ally, the elevation antenna bias adjust-
ment 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 operation-

ally required glidepath, the antenna
mainlobe pattern must conform to Fig-
ure 10, and the beamwidth must be
such that in the installed environment,
no significant ground reflections of the
mainlobe exist. In any case, the beam-
width must not exceed 2 degrees. The
antenna mainlobe may be allowed to
broaden from the value at boresight by
a factor of 1/cos

, where

is the angle

of boresight. Anywhere within cov-
erage, the

3 dB width of the antenna

mainlobe, while scanning normally,
must not be less than 25 microseconds
(0.5 degrees) or greater than 250 micro-
seconds (5 degrees). The sidelobe levels
must be as follows:

(i)

Dynamic sidelobe levels.

With the

antenna scanning normally, the dy-
namic sidelobe level that is detected by
a receiver at any point within the pro-
portional 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 de-
grees.

(5)

Antenna far field pattern in the hor-

izontal 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 de-
emphasis.

(6)

Data antenna.

The data antenna

must have horizontal and vertical pat-
terns as required for its function.

(f)

False guidance.

False courses

which can be acquired and tracked by
an aircraft shall not exist anywhere ei-
ther inside or outside of the MLS cov-
erage sector. False courses which exist
outside of the minimum coverage sec-
tor may be suppressed by the use of
OCI.

OTE

: False courses may be due to (but not

limited to) MLS airborne receiver acquisi-
tion of the following types of false guidance:
reflections of the scanning beam and scan-
ning beam antenna sidelobes and grating
lobes.

§ 171.319 Approach elevation monitor

system requirements.

(a) The monitor system must act to

ensure that any of the following condi-
tions 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 glide-
path exceeds the limits specified in
§ 171.317(d) for a period of more than
one second.

OTE

: The above requirement and the re-

quirement to limit the ground equipment
mean error to

0.067 degree can be satisfied

by the following procedure. The integral
monitor alarm limit should be set to

0.067

degree. This will limit the electrical compo-
nent of mean glidepath error to

0.067 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 radi-

ated power to a level not less than that
specified in § 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.