Federal Aviation Administration, DOT

Section 171.265

(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 detected audio tone in the airborne VHF
receiver of 1020 Hz Section50Hz.
(4) The depth of modulation must be
between the limits of 10 and 12 percent.
(5) The emissions carrying the identification signal must be vertically polarized.
(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 immediate 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 approximately seven words per minute, and
must be repeated at approximately
equal intervals, not less than six times
per minute, during which time the localizer is available for operational use.
When the localizer is not available for
transmission, the identification signal
must be suppressed.
Section 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 occurs:
(1) A shift of the mean course line of
the localizer from the runway centerline 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 identification transmissions.
(3) Changes of displacement sensitivity 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 malfunctioning of the element being monitored.
(b) Within 10 seconds of the occurrence of any of the conditions prescribed in paragraph (a) of this section,
including periods of zero radiation, localizer signal radiation must cease or
the navigation and identification components must be removed.
Section 171.265 Glide path performance requirements.
This section prescribes the performance requirements for glide path equipment components of the ISMLS. These
requirements are based on the assumption that the aircraft is heading directly 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 specified in subpart C of part 97 of this
chapter.
(b) The radiation from the glide path
antenna system must produce a composite field pattern which is pulse duration 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 provide a straight line descent path in the
vertical plane containing the centerline 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
1.752q. As used in this section theta (q),
denotes the nominal glide path angle.
The glide path angle must be adjusted
and maintained within 0.075q.
(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

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