Section 171.109

14 CFR Ch. I (1-1-19 Edition)

and 150 Hz wave form. However, the
phase need not be measured within the
half course sector.
(8) The angle of convergence of the
final approach course and the extended
runway centerline must not exceed 30Section.
The final approach course must be
aligned to intersect the extended runway centerline between points A1 and
the runway threshold. When an operational advantage can be achieved, a
final approach course that does not
intersect the runway or that intersects
it at a distance greater than point A1
from the threshold, may be established,
if that course lies within 500 feet laterally of the extended runway centerline at a point 3,000 feet outward from
the runway threshold. The mean course
line must be maintained within Section10 percent of the course sector width.
(9) The nominal displacement sensitivity within the half course sector
must be 50 microamperes/degree. The
nominal course sector width must be
6Section. When an operational advantage can
be achieved, a nominal displacement
sensitivity of 25 microamperes/degree
may be established, with a nominal
course sector width of 12Section with proportional displacement sensitivity. The
lateral displacement sensitivity must
be adjusted and maintained within the
limits of plus or minus 17 percent of
the nominal value.
(10) The off-course (clearance) signal
must increase at a substantially linear
rate with respect to the angular displacement from the course line up to
an angle on either side of the course
line where 175 microamperes of deflection is obtained. From that angle to
Section10Section, the off-course deflection must not
be less than 175 microamperes. From
Section10Section to Section35Section the off-course deflection
must not be less than 150 microamperes. With the course adjusted to
cause any of several monitor alarm
conditions, the aforementioned values
of 175 microamperes in the sector 10Section
each side of course and 150 microamperes in the sector Section10Section to Section35Section may
be reduced to 160 microamperes and 135
microamperes, respectively. These conditions must be met at a distance of 18
nautical miles from the SDF antenna
within the sector 10Section each side of
course line and 10 nautical miles from

the SDF antenna within the sector Section10Section
to Section35Section each side of course line.
(11) The SDF may provide a groundto-air radiotelephone communication
channel to be operated simultaneously
with the navigation and identification
signals, if that operation does not
interfere with the basic function. If a
channel is provided, it must conform
with the following standards:
(i) The channel must be on the same
radio frequency carrier or carriers as
used for the SDF function, and the radiation must be horizontally polarized.
Where two carriers are modulated with
speech, the relative phases of the modulations on the two carriers must avoid
the occurrence of nulls within the coverage of the SDF.
(ii) On centerline, the peak modulation depth of the carrier or carriers due
to the radiotelephone communications
must not exceed 50 percent but must be
adjusted so that the ratio of peak modulation depth due to the radiotelephone communications to that due
to the identification signal is approximately 9:1.
(iii) The audio frequency characteristics of the radiotelephone channel
must be flat to within 3 db relative to
the level at 1,000 Hz over the range
from 300 Hz to 3,000 Hz.
(12)(i) The SDF must provide for the
simultaneous transmission of an identification signal, specific to the runway
and approach direction, on the same
radio frequency carrier or carriers as
used for the SDF function. The transmission of the identification signal
must not interfere in any way with the
basic SDF function.
(ii) The identification signal must be
produced by Class A2 modulation of the
radio frequency carrier or carriers
using a modulation tone of 1020 Hz
within Section50 Hz. The depth of modulation
must be between the limits of 5 and 15
percent except that, where a radiotelephone communication channel is
provided, the depth of modulation must
be adjusted so that the ratio of peak
modulation depth due to radiotelephone communications to that due
to the identification signal modulation
is approximately 9:1. The emissions
carrying the identification signal must
be horizontally polarized.

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