Federal Aviation Administration, DOT

Section 171.261

(8) The minimum duty ratio detectable by a receiver located anywhere in
the coverage areas defined by this specification may not be less than 0.1. Detected 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 Cband unmodulated reference frequency
signal of sufficient strength to allow
satisfactory operation of an aircraft receiver within the specified localizer
and glide path coverage sectors. Pairing of this reference frequency with the
localizer and glide slope frequencies
must be in accordance with a frequency
plan approved by the FAA.
Section 171.261 Localizer performance requirements.
This section prescribes the performance requirements for localizer equipment 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 necessary to comply with the coverage requirements prescribed in paragraph (j)
of this section;
(4) Be located at a distance from the
stop end of the runway that is consistent with safe obstruction clearance
practices;
(5) Not obscure any light of the approach 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 centerline 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 determined 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 composite field pattern which is pulse duration modulated, the time average
equivalent to amplitude modulation by
a 90 Hz and 150 Hz tone. The localizer
station must transmit angular guidance information over a C-band microwave carrier on narrow, scanned antenna 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 opposite 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 provides 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 approved on an individual basis.
(g) The ISMLS localizer must operate
in the band 5000 MHz to 5030 MHz. The
frequency tolerance may not exceed
Section0.0001 percent.
(h) The emission from the localizer
must be vertically polarized. The horizontally polarized component of the radiation of the course line may not exceed that which corresponds to a DDM
error of 0.016 when an aircraft is positioned on the course line and is in a
roll attitude of 20 degrees from the horizontal.
(i) The localizer must provide signals
sufficient to allow satisfactory operation of a typical aircraft installation

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