869 

Federal Aviation Administration, DOT 

§ 171.313 

datum to the coverage limit, the PFE, 
PFN and CMN limits, expressed in an-
gular terms, shall be allowed to lin-
early 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 

±

0.10 degree for the CMN limits. 

(ii) With azimuth angle, by a factor 

of 1.5 at the 

±

40 degree and a factor of 

2.0 at the 

±

60 degree azimuth angles for 

the PFE, PFN and CMN limits. 

(iii) With elevation angle from + 9 de-

grees 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 

±

0.25 de-

gree in any coverage region below an 
elevation angle of + 9 degrees nor ex-
ceed 

±

0.50 degree in any coverage re-

gion above that elevation angle. The 
CMN limits shall not exceed 

±

0.10 de-

gree in any coverage region within 

±

10 

degrees of runway centerline extended 
nor exceed 

±

0.20 degree in any other re-

gion within coverage. 

N

OTE

: It is desirable that the CMN not ex-

ceed 

±

0.10 degree throughout the coverage. 

(f) Approach azimuth antenna char-

acteristics are as follows: 

(1) 

Drift. 

Any azimuth angle as en-

coded by the scanning beam at any 
point within the proportional coverage 
must not vary more than 

±

0.07 degree 

over the range of service conditions 
specified in § 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 

±

0.5 degree of the 

zero degree azimuth must not deviate 
from the true azimuth angle at that 
point by more than 

±

.05 degree. 

Multipath and drift effects are ex-
cluded from this requirement. 

T

ABLE

10—A

PPROACH

A

ZIMUTH

A

CCURACIES AT

 

THE

A

PPROACH

R

EFERENCE

D

ATUM

 

Error type 

System 

Angular error (degrees) 

Ground 

subsystem 

Airborne 

subsystem 

PFE ........

±

20 ft. (6.1m)

1 2

 

±

0.118

°

3

..

±

0.017

° 

CMN ......

±

10.5 ft. (3.2m)

1 2 4

 

±

0.030

°

....

±

0.050

° 

Notes: 

1

Includes errors due to ground and airborne equipment and 

propagation effects. 

2

The system PFN component must not exceed 

±

3.5 meters 

(11.5 feet). 

3

The mean (bias) error component contributed by the 

ground equipment should not exceed 

±

10 feet. 

4

The system control motion noise must not exceed 0.1 de-

gree. 

5

The airborne subsystem angular errors are provided for in-

formation 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 coinci-
dence with the approach reference 
datum (for centerline siting) with a 
maximum error of 0.02 degree. Addi-
tionally, the azimuth antenna bias ad-
justment must be electronically steer-
able 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 azi-

muth antenna mainlobe pattern must 
conform to Figure 10, and the beam-
width must be such that, in the in-
stalled environment, no significant lat-
eral reflections of the mainlobe exist 
along the approach course. In any case 
the beamwidth must not exceed three 
degrees. Anywhere within coverage the 

¥

3 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 de-
grees). The antenna mainlobe may be 
allowed to broaden from the value at 
boresight by a factor of 1/cos

q

, where 

q 

is the angle off boresight. 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 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 re-
moved 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 en-
vironment, the CMN contributed by 
sidelobe reflections will not exceed the 
angular equivalent of 9 feet at ap-
proach reference datum over the re-
quired range of aircraft approach 
speeds. 

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