Federal Aviation Administration, DOT

Section 171.109

Back course sector means the course
sector on the opposite end of the runway from the front course sector.
Course line means the locus of points
along the final approach course at
which the DDM is zero.
Course sector means a sector in a horizontal plane containing the course line
and limited by the loci of points nearest to the course line at which the
DDM is 0.155.
Displacement sensitivity means the
ratio of measured DDM to the corresponding lateral displacement from
the appropriate reference line.
Front course sector means the course
sector centered on the course line in
the direction from the runway in which
a normal final approach is made.
Half course sector means the sector in
a horizontal plane containing the
course line and limited by the loci of
points nearest to the course line, at
which the DDM is 0.0775.
Point A means a point on the front
course in the approach direction a distance of 4 nautical miles from the
threshold.
Point A1 means a point on the front
course in the approach direction a distance of 1 statute mile from the threshold.
Point A2 means a point on the front
course at the threshold.
Reference datum means a point at a
specified height located vertically
above the intersection of the course
and the threshold.
Missed approach point means the
point on the final approach course, not
farther from the final approach fix
than Point  - A2 - , at which the approach must be abandoned, if the approach and subsequent landing cannot
be safely completed by visual reference, whether or not the aircraft has
descended to the minimum descent altitude.
Section 171.109 Performance requirements.
(a) The Simplified Directional Facility must perform in accordance with
the following standards and practices:
(1) The radiation from the SDF antenna system must produce a composite field pattern which is amplitude
modulated by a 90 Hz and a 150 Hz tone.
The radiation field pattern must
produce a course sector with the 90 Hz

tone predominating on one side of the
course and with the 150 Hz tone predominating on the opposite side.
(2) When an observer faces the SDF
from the approach end of 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.
(3) All horizontal angles employed in
specifying the SDF field patterns must
originate from the center of the antenna system which provides the signals used in the front course sector.
(4) The SDF must operate on odd
tenths or odd tenths plus a twentieth
MHz within the frequency band 108.1
MHz to 111.95 MHz. The frequency tolerance of the radio frequency carrier
must not exceed plus or minus 0.002
percent.
(5) The radiated emission from the
SDF must be horizontally polarized.
The vertically polarized component of
the radiation on the course line must
not exceed that which corresponds to
an error one-twentieth of the course
sector width when an aircraft is positioned on the course line and is in a
roll attitude of 20Section from the horizontal.
(6) The SDF must provide signals sufficient to allow satisfactory operation
of a typical aircraft installation within
the sector which extends from the center of the SDF antenna system to distances of 18 nautical miles within a
plus or minus 10Section sector and 10 nautical
miles within the remainder of the coverage when alternative navigational facilities provide satisfactory coverage
within the intermediate approach area.
SDF signals must be receivable at the
distances specified at and above a
height of 1,000 feet above the elevation
of the threshold, or the lowest altitude
authorized for transition, whichever is
higher. Such signals must be receivable, to the distances specified, up to a
surface extending outward from the
SDF antenna and inclined at 7Section above
the horizontal.
(7) 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 direction within 20Section of
phase relative to the 150 Hz component,
every half cycle of the combined 90 Hz

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