877 

Federal Aviation Administration, DOT 

§ 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; 

T

ABLE

13—E

LEVATION

A

CCURACIES AT THE

 

A

PPROACH

R

EFERENCE

D

ATUM

 

Error type 

System 

Angular error (degrees) 

Ground sub-

system 

Airborne 

subsystem

4

 

PFE ....................

1 2

±

0.133 (

3

) 

±

0.017 

CMN ..................

1

±

0.050 

±

0.020 

±

0.010 

Notes: 

1

Includes errors due to ground and airborne equipment and 

propagation effects. 

2

The system PFN component must not exceed 

±

0.087 de-

gree. 

3

The mean (bias) error component contributed by the 

ground equipment should not exceed 

±

0.067 degree. 

4

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 

±

10 

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

N

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 

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