834 

14 CFR Ch. I (1–1–19 Edition) 

§ 171.261 

within the localizer and glide path cov-
erage sectors. The localizer coverage 
sector must extend from the center of 
the localizer antenna system to dis-
tances of 18 nautical miles minimum 
within 

±

10 degrees from the front 

course line, and 10 nautical miles min-
imum between 

±

10 degrees and 

±

35 de-

grees from the front course line. The 
ISMLS localizer signals must be re-
ceivable at the distances specified up 
from a surface extending outward from 
the localizer antenna and within a sec-
tor in the elevation plane from 0.300 to 
1.750 of the established glide path angle 
(

q

). 

(j) Except as provided in paragraph 

(k) of this section, in all parts of the 
coverage volume specified in paragraph 
(i) of this section, the peak field 
strength may not be less than 

¥

87 

dBW/m

2

, and must permit satisfactory 

operational usage of ISMLS localizer 
facilities. 

(k) The minimum peak field strength 

on the ISMLS glide path and within 
the localizer course sector from a dis-
tance of 10 nautical miles to a height of 
100 feet (30 meters) above the hori-
zontal plane containing the threshold, 
may not be less than + 87 dBW/m

2

. 

(l) Above 16 degrees, the ISMLS lo-

calizer signals must be reduced to as 
low a value as practicable. 

(m) Bends in the course line may not 

have amplitudes which exceed the fol-
lowing: 

Zone 

Amplitude (DDM) 

(95 pct. probability) 

Outer limit of coverage to: 

ISMLS point ‘‘A’’ .............

0.031. 

ISMLS point ‘‘A’’ to 
ISMLS point ‘‘B’’.

0.031 at ISMLS point ‘‘A’’ de-

creasing at linear rate to 
0.015 at ISMLS point ‘‘B’’. 

ISMLS point ‘‘B’’ to 
ISMLS point ‘‘C’’.

0.015. 

(n) The amplitudes referred to in 

paragraph (m) of this section are the 
DDMs due to bends as realized on the 
mean course line, when correctly ad-
justed. 

(o) The radio frequency carrier must 

meet the following requirements: 

(1) The nominal depth of modulation 

of the radio frequency carrier due to 
each of the 90 Hz and 150 Hz tones must 
be 20 percent along the course line. 

(2) The depth of modulation of the 

radio frequency carrier due to each of 

the 90 Hz and 150 Hz tones must be be-
tween 18 and 22 percent. 

(3) The frequency tolerance of the 90 

Hz and 150 Hz modulated tones must be 
within 

±

25 percent. 

(4) Total harmonic content of the 90 

Hz tone may not exceed 10 percent. 

(5) Total harmonic content of the 150 

Hz tone may not exceed 10 percent. 
However, a 300 Hz tone may be trans-
mitted for identification purposes. 

(6) At every half cycle of the com-

bined 90 Hz and 150 Hz wave form, the 
modulation tones must be phase-locked 
so that within the half course sector, 
the demodulated 90 Hz and 150 Hz wave 
forms pass through zero in the same di-
rection within 20 degrees with phase 
relative to the 150 Hz component. How-
ever, the phase need not be measured 
within the half course sector. 

(p) The mean course line must be ad-

justed and maintained within 

±

.015DDM from the runway centerline 

at the ISMLS reference datum. 

(q) The nominal displacement sensi-

tivity within the half course sector at 
the ISMLS reference datum, must be 
0.00145 DDM/meter (0.00044DDM/foot). 
However, where the specified nominal 
displacement sensitivity cannot be 
met, the displacement sensitivity must 
be adjusted as near as possible to that 
value. 

(r) The lateral displacement sensi-

tivity must be adjusted and maintained 
within 17 percent of the nominal value. 
Nominal sector width at the ISMLS 
reference datum is 210 meters (700 feet). 

(s) The increase of DDM must be sub-

stantially linear with respect to angu-
lar displacement from the front course 
line where DDM is zero, up to angle on 
either side of the front course line 
where the DDM is 0.180. From that 
angle to 

±

10 degrees, the DDM may not 

be less than 0.180. From 

±

10 degrees to 

±

35 degrees, the DDM may not be less 

than 0.155. 

(t) The localizer must provide for the 

simultaneous transmission of an iden-
tification signal which meets the fol-
lowing: 

(1) It must be specific to the runway 

and approach direction, on the same 
radio frequency carrier, as used for the 
localizer function. 

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