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842 

14 CFR Ch. I (1–1–24 Edition) 

§ 171.255 

§ 171.255 Requests for IFR procedures. 

(a) Each person who requests an IFR 

procedure based on an ISMLS facility 
that he owns must submit the fol-
lowing information with that request: 

(1) A description of the facility and 

evidence that the equipment meets the 
performance requirements of §§ 171.259, 
171.261, 171.263, 171.265, 171.267, and 
171.269, and is installed in accordance 
with § 171.271. 

(2) A proposed procedure for oper-

ating the facility. 

(3) A proposed maintenance organiza-

tion and a maintenance manual that 
meets the requirements of § 171.273. 

(4) A statement of intent to meet the 

requirements of this subpart. 

(5) A showing that the ISMLS facil-

ity has an acceptable level of oper-
ational reliability, maintainability and 
acceptable standard of performance. 
Previous equivalent operational experi-
ence with a facility with identical de-
sign and operational characteristics 
will be considered in showing compli-
ance with this paragraph. 

(b) After the FAA inspects and evalu-

ates the ISMLS facility, it advises the 
owner of the results and of any re-
quired changes in the ISMLS facility 
or in the maintenance manual or main-
tenance organization. The owner must 
then correct the deficiencies, if any, 
and operate the ISMLS facility for an 
inservice evaluation by the FAA. 

§ 171.257 Minimum requirements for 

approval. 

(a) The following are the minimum 

requirements that must be met before 
the FAA approves an IFR procedure for 
a non-Federal ISMLS facility: 

(1) The performance of the ISMLS fa-

cility, as determined by flight and 
ground inspection conducted by the 
FAA, must meet the requirements of 
§§ 171.259, 171.261, 171.263, 171.265, 171.267, 
and 171.269. 

(2) The installation of the equipment 

must meet the requirements of 
§ 171.271. 

(3) The owner must agree to operate 

and maintain the ISMLS facility in ac-
cordance with § 171.273. 

(4) The owner must agree to furnish 

periodic reports as set forth in § 171.275 
and agree to allow the FAA to inspect 

the facility and its operation whenever 
necessary. 

(5) The owner must assure the FAA 

that he will not withdraw the ISMLS 
facility from service without the per-
mission of the FAA. 

(6) The owner must bear all costs of 

meeting the requirements of this sec-
tion and of any flight or ground inspec-
tion made before the ISMLS facility is 
commissioned, except that the FAA 
may bear certain costs subject to budg-
etary limitations and policy estab-
lished by the Administrator. 

(b) If the applicant for approval 

meets the requirements of paragraph 
(a) of this section, the FAA approves 
the ISMLS facility for use in an IFR 
procedure. The approval is withdrawn 
at any time that the ISMLS facility 
does not continue to meet those re-
quirements. In addition, the ISMLS fa-
cility may be de-commissioned when-
ever the frequency channel is needed 
for higher priority common system 
service. 

§ 171.259 Performance requirements: 

General. 

(a) The ISMLS consists of the fol-

lowing basic components: 

(1) C-Band (5000 MHz–5030 MHz) local-

izer equipment, associated monitor 
system, and remote indicator equip-
ment; 

(2) C-Band (5220 MHz–5250 MHz) glide 

path equipment, associated monitor 
system, and remote indicator equip-
ment; 

(3) VHF marker beacons (75 MHz), as-

sociated monitor systems, and remote 
indicator equipment. 

(4) An ISMLS airborne receiver or a 

VHF/UHF ILS receiver modified to be 
capable of receiving the ISMLS signals. 
This modification requires the addition 
of a C-Band antenna, a converter unit, 
a microwave/ILS mode control, and a 
VHF/UHF receiver modification kit. 

(b) The electronic ground equipments 

in paragraph (a)(1), (2), and (3) of this 
section, must be designed to operate on 
a nominal 120/240 volt, 60 Hz, 3-wire sin-
gle phase AC power source. 

(c) ISMLS ground equipment must 

meet the following service conditions: 

(1) AC line parameters, DC voltage, 

elevation, and duty: 

120 V nominal value, 102 V to 138 V (

±

1 V).* 

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843 

Federal Aviation Administration, DOT 

§ 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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844 

14 CFR Ch. I (1–1–24 Edition) 

§ 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