818 

14 CFR Ch. I (1–1–19 Edition) 

§ 171.109 

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 30

°

. 

The final approach course must be 
aligned to intersect the extended run-
way centerline between points A1 and 
the runway threshold. When an oper-
ational 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 lat-
erally of the extended runway center-
line at a point 3,000 feet outward from 
the runway threshold. The mean course 
line must be maintained within 

±

10 per-

cent of the course sector width. 

(9) The nominal displacement sensi-

tivity within the half course sector 
must be 50 microamperes/degree. The 
nominal course sector width must be 
6

°

. 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 12

° 

with propor-

tional 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 dis-
placement from the course line up to 
an angle on either side of the course 
line where 175 microamperes of deflec-
tion is obtained. From that angle to 

±

10

°

, the off-course deflection must not 

be less than 175 microamperes. From 

±

10

° 

to 

±

35

° 

the off-course deflection 

must not be less than 150 micro-
amperes. With the course adjusted to 
cause any of several monitor alarm 
conditions, the aforementioned values 
of 175 microamperes in the sector 10

° 

each side of course and 150 micro-
amperes in the sector 

±

10

° 

to 

±

35

° 

may 

be reduced to 160 microamperes and 135 
microamperes, respectively. These con-
ditions must be met at a distance of 18 
nautical miles from the SDF antenna 
within the sector 10

° 

each side of 

course line and 10 nautical miles from 

the SDF antenna within the sector 

±

10

° 

to 

±

35

° 

each side of course line. 

(11) The SDF may provide a ground- 

to-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 ra-
diation must be horizontally polarized. 
Where two carriers are modulated with 
speech, the relative phases of the mod-
ulations on the two carriers must avoid 
the occurrence of nulls within the cov-
erage of the SDF. 

(ii) On centerline, the peak modula-

tion 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 mod-
ulation depth due to the radio-
telephone communications to that due 
to the identification signal is approxi-
mately 9:1. 

(iii) The audio frequency characteris-

tics 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 iden-
tification signal, specific to the runway 
and approach direction, on the same 
radio frequency carrier or carriers as 
used for the SDF function. The trans-
mission 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 

±

50 Hz. The depth of modulation 

must be between the limits of 5 and 15 
percent except that, where a radio-
telephone communication channel is 
provided, the depth of modulation must 
be adjusted so that the ratio of peak 
modulation depth due to radio-
telephone 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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