833 

Federal Aviation Administration, DOT 

§ 171.261 

(8) The minimum duty ratio detect-

able by a receiver located anywhere in 
the coverage areas defined by this spec-
ification may not be less than 0.1. De-
tected duty ratio means the ratio of 
the average energy per scan detected at 
a point in space to the average energy 
per scan transmitted in all directions 
through the transmitting antenna. 

(9) The localizer must produce a C- 

band unmodulated reference frequency 
signal of sufficient strength to allow 
satisfactory operation of an aircraft re-
ceiver within the specified localizer 
and glide path coverage sectors. Pair-
ing of this reference frequency with the 
localizer and glide slope frequencies 
must be in accordance with a frequency 
plan approved by the FAA. 

§ 171.261

Localizer performance re-

quirements. 

This section prescribes the perform-

ance requirements for localizer equip-
ment components of the ISMLS. 

(a) The localizer antenna system 

must: 

(1) Be located on the extension of the 

centerline of the runway at the stop 
end; 

(2) Be adjusted so that the course line 

be on a vertical plane containing the 
centerline of the runway served; 

(3) Have the minimum height nec-

essary to comply with the coverage re-
quirements prescribed in paragraph (j) 
of this section; 

(4) Be located at a distance from the 

stop end of the runway that is con-
sistent with safe obstruction clearance 
practices; 

(5) Not obscure any light of the ap-

proach landing system; and 

(6) Be installed on frangible mounts 

or beyond the 1000

′ 

light bar. 

(b) On runways where limited terrain 

prevents the localizer antennae from 
being positioned on the runway center-
line extended, and the cost of the land 
fill or a tall tower antenna support is 
prohibitive, the localizer antenna array 
may be offset, including a collocated 
ground station, so that the course 
intercepts the centerline at a point de-
termined by the amount of the angular 
offset and the glide path angle. If other 
than a runway centerline localizer is 
used, the criteria in subpart C of part 
97 of this chapter is applicable. 

(c) At locations where two separate 

ISMLS facilities serve opposite ends of 
of a single runway, an interlock must 
ensure that only the facility serving 
the approach direction being used will 
radiate. 

(d) The radiation from the localizer 

antenna system must produce a com-
posite field pattern which is pulse du-
ration modulated, the time average 
equivalent to amplitude modulation by 
a 90 Hz and 150 Hz tone. The localizer 
station must transmit angular guid-
ance information over a C-band micro-
wave carrier on narrow, scanned an-
tenna beams that are encoded to 
produce a modulation in space which, 
after averaging over several beam 
scans, is equivalent to the modulation 
used for conventional ILS as specified 
in subpart C of this part. The radiation 
field pattern must produce a course 
sector with one tone predominating on 
one side of the course and with the 
other tone predominating on the oppo-
site side. When an observer faces the 
localizer from the approach end of the 
runway, the depth of modulation of the 
radio frequency carrier due to the 150 
Hz tone must predominate on his right 
hand and that due to the 90 Hz tone 
must predominate on his left hand. 

(e) All horizontal angles employed in 

specifying the localizer field patterns 
must originate from the center of the 
localizer antenna system which pro-
vides the signals used in the front 
course sector. 

(f) The ISMLS course sector angle 

must be adjustable between 3 degrees 
and 9 degrees. The applicable course 
sector angle will be established and ap-
proved on an individual basis. 

(g) The ISMLS localizer must operate 

in the band 5000 MHz to 5030 MHz. The 
frequency tolerance may not exceed 

±

0.0001 percent. 

(h) The emission from the localizer 

must be vertically polarized. The hori-
zontally polarized component of the ra-
diation of the course line may not ex-
ceed that which corresponds to a DDM 
error of 0.016 when an aircraft is posi-
tioned on the course line and is in a 
roll attitude of 20 degrees from the hor-
izontal. 

(i) The localizer must provide signals 

sufficient to allow satisfactory oper-
ation of a typical aircraft installation 

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