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
845
Federal Aviation Administration, DOT
§ 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.
846
14 CFR Ch. I (1–1–24 Edition)
§ 171.263
(2) Transmission of the identification
signal may not interfere in any way
with the basic localizer function.
(3) The signal must be produced by
pulse duration modulation of the radio
frequency carrier resulting in a de-
tected audio tone in the airborne VHF
receiver of 1020 Hz
±
50Hz.
(4) The depth of modulation must be
between the limits of 10 and 12 percent.
(5) The emissions carrying the identi-
fication signal must be vertically po-
larized.
(6) The identification signal must
employ the International Morse Code
and consist of three letters. It must be
preceded by the International Morse
Code signal of the letter ‘‘M’’ followed
by a short pause where it is necessary
to distinguish the ISMLS facility from
other navigational facilities in the im-
mediate area. At airports where both
an ISMLS and an ILS are in operation,
each facility must have a different
identification call sign.
(7) The signal must be transmitted at
a speed corresponding to approxi-
mately seven words per minute, and
must be repeated at approximately
equal intervals, not less than six times
per minute, during which time the lo-
calizer is available for operational use.
When the localizer is not available for
transmission, the identification signal
must be suppressed.
§ 171.263 Localizer automatic monitor
system.
(a) The ISMLS localizer equipment
must provide an automatic monitor
system that transmits a warning to
designated local and remote control
points when any of the following oc-
curs:
(1) A shift of the mean course line of
the localizer from the runway center-
line equivalent to more than 0.015 DDM
at the ISMLS reference datum.
(2) For localizers in which the basic
functions are provided by the use of a
single-frequency system, a reduction of
power output to less than 50 percent of
normal or a loss of ground station iden-
tification transmissions.
(3) Changes of displacement sensi-
tivity to a value differing by more than
17 percent from nominal value for the
localizer.
(4) Failure of any part of the monitor
itself. Such failure must automatically
produce the same results as the mal-
functioning of the element being mon-
itored.
(b) Within 10 seconds of the occur-
rence of any of the conditions pre-
scribed in paragraph (a) of this section,
including periods of zero radiation, lo-
calizer signal radiation must cease or
the navigation and identification com-
ponents must be removed.
§ 171.265 Glide path performance re-
quirements.
This section prescribes the perform-
ance requirements for glide path equip-
ment components of the ISMLS. These
requirements are based on the assump-
tion that the aircraft is heading di-
rectly toward the facility.
(a) The glide slope antenna system
must be located near the approach end
of the runway, and the equipment must
be adjusted so that the vertical path
line will be in a sloping horizontal
plane containing the centerline of the
runway being served, and satisfy the
coverage requirements prescribed in
paragraph (g) of this section. For the
purpose of obstacle clearance, location
of the glide slope antenna system must
be in accordance with the criteria spec-
ified in subpart C of part 97 of this
chapter.
(b) The radiation from the glide path
antenna system must produce a com-
posite field pattern which is pulse du-
ration modulated by a 90 Hz and a 150
Hz tone, which is the time average
equivalent to amplitude modulation.
The pattern must be arranged to pro-
vide a straight line descent path in the
vertical plane containing the center-
line of the runway, with the 150 Hz tone
predominating below the path and the
90 Hz tone predominating above the
path to at least an angle equal to
1.752
q
. As used in this section theta (
q
),
denotes the nominal glide path angle.
The glide path angle must be adjusted
and maintained within 0.075
q
.
(c) The glide path equipment must be
capable of producing a radiated glide
path from 3 to 9 degrees with respect to
the horizontal. However, ISMLS glide
path angles in excess of 3 degrees may
be used to satisfy instrument approach