836 

14 CFR Ch. I (1–1–19 Edition) 

§ 171.265 

procedures or to overcome an obstruc-
tion clearance problem, only in accord-
ance with the criteria specified in sub-
part C of part 97 of this chapter. 

(d) The downward extended straight 

portion of the ISMLS glide path must 
pass through the ISMLS reference 
datum at a height ensuring safe guid-
ance over obstructions and safe and ef-
ficient use of the runway served. The 
height of the ISMLS reference datum 
must be in accordance with subpart C 
of part 97 of this chapter. 

(e) The glide path equipment must 

operate in the band 5220 MHz to 5250 
MHz. The frequency tolerance may not 
exceed 

±

0.0001 percent. 

(f) The emission from the glide path 

equipment must be vertically polar-
ized. 

(g) The glide path equipment must 

provide signals sufficient to allow sat-
isfactory operation of a typical aircraft 
installation insectors of 8 degrees on 
each side of the centerline of the 
ISMLS glide path, to a distance of at 
least 10 nautical miles up to 1.75

q 

and 

down to 0.45

q 

above the horizontal or to 

such lower angle at which 0.22 DDM is 
realized. 

(h) To provide the coverage for glide 

path performance specified in para-
graph (g) of this section, the minimum 
peak field strength within this cov-
erage sector must be 

¥

82 dBW/m

2

. The 

peak field strength must be provided 
on the glide path down to a height of 30 
meters (100 feet) above the horizontal 
plane containing the threshold. 

(i) Bends in the glide path may not 

have amplitudes which exceed the fol-
lowing: 

Zone 

Amplitude 

(DDM) (95 

pct. prob-

ability) 

Outer limit of coverage to ISMLS point ‘‘C.’’ .....

0.035. 

The amplitude referred to is the DDM 
due to bends as realized on the mean 
ISMLS glide path correctly adjusted. 
In regions of the approach where 
ISMLS glide path curvature is signifi-
cant, bend amplitude is calculated 
from the mean curved path, and not 
the downward extended straight line. 

(j) Guidance modulation must be im-

pressed on the microwave carrier of the 
radiated glide slope signal in the form 

of a unique summation of 90 Hz and 150 
Hz sinusoidal modulation cor-
responding to the point direction of the 
particular beam which radiates the sig-
nal. Each of the effective beam posi-
tions must be illuminated in sequence 
for a short time interval. The scan rate 
must be synchronous with the 90 and 
150 Hz tone base. The modulation im-
pressed on each beam must be a sample 
of the combined 90 Hz and 150 Hz wave-
form appropriate for that particular 
beam direction and time slot. The ac-
tual modulation must be accomplished 
by appropriately varying the length of 
time the carrier is radiated during each 
beam illumination interval. 

(k) The nominal depth of modulation 

of the radio frequency carrier due to 
each of the 90 Hz and 150 Hz tones must 
be 40 percent along the ISMLS glide 
path. The depth of modulation may not 
deviate outside the limits of 37.5 per-
cent to 42.5 percent. 

(l) The following tolerances apply to 

the frequencies of the modulating 
tones: 

(1) The modulating tones must be 90 

Hz and 150 Hz within 2.5 percent. 

(2) The total harmonic content of the 

90 Hz tone may not exceed 10 percent. 

(3) The total harmonic content of the 

150 Hz tone may not exceed 10 percent. 

(m) At every half cycle of the com-

bined 90 Hz and 150 Hz wave form, the 
modulation must be phase-locked so 
that, within the ISMLS half glide path 
sector, the demodulated 90 Hz and 150 
Hz wave forms pass through zero in the 
same direction within 20 degrees of 
phase relative to the 150 Hz component. 
However, the phase need not be meas-
ured within the ISMLS half glide path 
sector. 

(n) The nominal angular displace-

ment sensitivity must correspond to a 
DDM of 0.0875 at an angular displace-
ment above and below the glide path of 
0.12

q

. The glide path angular displace-

ment sensitivity must be adjusted and 
maintained within 

±

25 percent of the 

nominal value selected. The upper and 
lower sectors must be as symmetrical 
as practicable within the limits pre-
scribed in this paragraph. 

(o) The DDM below the ISMLS glide 

path must increase smoothly for de-
creasing angle until a value of 0.22 
DDM is reached. This value must be 

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