832 

14 CFR Ch. I (1–1–19 Edition) 

§ 171.259 

208 V nominal value, 177 V to 239 V (

±

2 V).* 

240 V nominal value, 204 V to 276 V (

±

0.2 

V).* 

AC line frequency (60 Hz), 57 Hz to 63 Hz 

(

±

0.2 Hz).* 

DC voltage (48 V), 44 V to 52 V (

±

0.5 V).* 

* N

OTE

: Where discrete values of the above 

frequency or voltages are specified for test-
ing purposes, the tolerances given in paren-
theses indicated by an asterisk apply to the 
test instruments used to measure these pa-
rameters. 

Elevation, 0 to 10,000 ft. above sea level. 
Duty, continuous, unattended. 

(2) Ambient conditions for localizer 

and glide path equipment: 

Temperature, 

¥

10 

°

C to + 50 

°

C. 

Relative humidity, 5% to 90%. 

(3) Ambient conditions for marker 

beacon facilities and all other equip-
ment installed outdoors (for example, 
antennae, field detectors, and shelters): 

Temperature, 

¥

50 

°

C. to + 70 

°

C. 

Relative humidity, 5% to 100%. 

(4) All equipment installed outdoors 

must operate satisfactorily under the 
following conditions: 

Wind velocity, 0–100 MPH (not including 

gusts). 

Hail stones, 

1

⁄

2

″ 

diameter. 

Rain, provide coverage through a distance 

of 5 nautical miles with rain falling at a rate 
of 50 millimeters per hour, and with rain fall-
ing at the rate of 25 millimeters per hour for 
the additional design performance range of 
the system. 

Ice loading, encased in 

1

⁄

2

″ 

radial thickness 

of clear ice. 

(d) The ISMLS must perform in ac-

cordance with the following standards 
and practices for Facility Performance 
Category I operation: 

(1) The ISMLS must be constructed 

and adjusted so that, at a specified dis-
tance from the threshold, similar in-
strumental indications in the aircraft 
represent similar displacements from 
the course line or ISMLS glide path, as 
appropriate, regardless of the par-
ticular ground installation in use. 

(2) The localizer and glide path com-

ponents listed in paragraphs (a)(1) and 
(a)(2) of this section which form part of 
an ISMLS, must comply at least with 
the standard performance requirements 
specified herein. The marker beacon 
components listed in paragraph (a)(3) 
of this section which form part of an 
ISMLS, must comply at least with the 

standard performance requirements 
specified in subpart H of this part. 

(3) The ISMLS must be so designed 

and maintained that the probability of 
operation is within the performance re-
quirements specified in § 171.273(k). 

(e) The signal format and pairing of 

the runway localizer and glide path 
transmitter frequencies of an ISMLS 
must be in accordance with the fre-
quency plan approved by the FAA, and 
must meet the following signal format 
requirements: 

(1) The localizer and glide slope sta-

tions must transmit angular guidance 
information on a C-band microwave 
carrier on narrow, scanned antenna 
beams that are encoded to produce a 
modulation in space which, after aver-
aging over several beam scans, is 
equivalent to the modulation used for 
conventional ILS as specified in sub-
part C of this part, except that the fre-
quency tolerance may not exceed 

±

0.0001 percent. 

(2) Guidance modulation must be im-

pressed on the microwave carrier of the 
radiated signal in the form of a sum-
mation of 90 Hz and 150 Hz sinusoidal 
modulation corresponding to the point-
ing direction of the particular beam 
which radiates the signal. 

(3) Each of the effective beam posi-

tions must be illuminated in a par-
ticular sequence for a short time inter-
val. The modulation impressed on each 
beam must be a sample of the com-
bined 90 Hz and 150 Hz waveform appro-
priate for that particular beam direc-
tion and time slot, and must be accom-
plished by appropriately varying the 
length of time the carrier is radiated 
during each beam illumination inter-
val. 

(4) For those cases where the scan-

ning beam fills the coverage space in 
steps, the incremental step must not 
exceed 0.6 times the beam width where 
the beam is in the proportional guid-
ance sector. In the clearance region, 
the step may not exceed 0.8 times the 
beam width. 

(5) At least one pulse duration modu-

lation (pdm) sample pulse per beam 
width of scan must be provided. 

(6) The minimum pulse duration 

must be 40 microseconds. 

(7) The minimum beam scan cycle 

must be 600 Hz. 

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