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
847
Federal Aviation Administration, DOT
§ 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
848
14 CFR Ch. I (1–1–24 Edition)
§ 171.267
achieved at an angle not less than 0.30
q
above the horizontal. However, if it is
achieved at an angle above 0.45
q
, the
DDM value may not be less than 0.22 at
least down to an angle of 0.45
q
.
[Doc. No. 14120, 40 FR 36110, Aug. 19, 1975; 40
FR 41093, Sept. 5, 1975; 40 FR 43719, Sept. 23,
1975]
§ 171.267 Glide path automatic mon-
itor system.
(a) The ISMLS glide path 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 ISMLS glide
path angle equivalent to more than
0.075
q
.
(2) For glide paths 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.
(3) A change of the angle between the
glide path and the line below the glide
path (150 Hz predominating), at which a
DDM of 0.0875 is realized by more than
±
0.0375
q
.
(4) Lowering of the line beneath the
ISMLS glide path at which a DDM of
0.0875 is realized to less than 0.75
q
from
the horizontal.
(5) 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) At glide path facilities where the
selected nominal angular displacement
sensitivity corresponds to an angle
below the ISMLS glide path, which is
close to or at the maximum limits
specified, an adjustment to the mon-
itor operating limits may be made to
protect against sector deviations below
0.75
q
from the horizontal.
(c) 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,
glide path signal radiation must cease.
§ 171.269 Marker beacon performance
requirements.
ISMLS marker beacon equipment
must meet the performance require-
ments prescribed in subpart H of this
part.
§ 171.271 Installation requirements.
(a) The ISMLS facility must be per-
manent in nature, located, con-
structed, and installed according to ac-
cepted good engineering practices, ap-
plicable electric and safety codes, FCC
licensing requirements, and paragraphs
(a) and (c) of § 171.261.
(b) The ISMLS facility must have a
reliable source of suitable primary
power, either from a power distribution
system or locally generated. Adequate
power capacity must be provided for
the operation of test and working
equipment of the ISMLS.
(c) The ISMLS facility must have a
continuously engaged or floating bat-
tery power source for the ground sta-
tion for continued normal operation if
the primary power fails. A trickle
charge must be supplied to recharge
the batteries during the period of avail-
able primary power. Upon loss and sub-
sequent restoration of power, the bat-
teries must be restored to full charge
within 24 hours. When primary power is
applied, the state of the battery charge
may not affect the operation of the
ISMLS ground station. The battery
must permit continuation of normal
operation for at least two hours under
the normal operating conditions. The
equipment must meet all specification
requirements with or without batteries
installed.
(d) There must be a means for deter-
mining, from the ground, the perform-
ance of the equipment including anten-
nae, both initially and periodically.
(e) The facility must have, or be sup-
plemented by, ground-air or landline
communications services. At facilities
within or immediately adjacent to con-
trolled airspace and that are intended
for use as instrument approach aids for
an airport, there must be ground-air
communications or reliable commu-
nications (at least a landline tele-
phone) from the airport to the nearest
Federal Aviation Administration air
traffic control or communication facil-
ity. Compliance with this paragraph
need not be shown at airports where an
adjacent Federal Aviation Administra-
tion facility can communicate with
aircraft on the ground at the airport
and during the entire proposed instru-
ment approach procedure. In addition,
at low traffic density airports within