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Federal Aviation Administration, DOT 

§ 171.111 

(iii) The identification signal must 

employ the International Morse Code 
and consist of three letters. 

(iv) The identification signal must be 

transmitted at a speed corresponding 
to approximately seven words per 
minute, and must be repeated at ap-
proximately equal intervals, not less 
than six times per minute. When SDF 
transmission is not available for oper-
ational use, including periods of re-
moval of navigational components or 
during maintenance or test trans-
missions, the identification signal 
must be suppressed. 

(b) It must be shown during ground 

inspection of the design features of the 
equipment that there will not be condi-
tions that will allow unsafe operations 
because of component failure or dete-

(c) The monitor must be checked pe-

riodically during the in-service test 
evaluation period for calibration and 
stability. These tests, and ground 
checks of SDF radiation characteris-
tics must be conducted in accordance 
with the maintenance manual required 
by § 171.115(c) and must meet the stand-
ards and tolerances contained in 
§ 171.111(j). 

(d) The monitor system must provide 

a warning to the designated control 
point(s) when any of the conditions of 
§ 171.111(j) occur, within the time peri-
ods specified in that paragraph. 

(e) Flight inspection to determine 

the adequacy of the facility’s oper-
ational performance and compliance 
with applicable performance require-
ments must be conducted in accord-
ance with the ‘‘U.S. Standard Flight 
Inspection Manual.’’ Tolerances con-
tained in the U.S. Standard Flight In-
spection Manual, section 217, must be 
complied with except as stated in para-
graph (f) of this section. 

(f) Flight inspection tolerances speci-

fied in section 217 of the ‘‘U.S. Stand-
ard Flight Inspection Manual’’ must be 
complied with except as follows: 

(1)  Course sector width. The nominal 

course sector width must be 6°. When 
an operational advantage can be 
achieved, a nominal course sector 
width of 12° may be established. Course 
sector width must be adjusted and 
maintained within the limits of ±17 
percent of the nominal value. 

(2) Course alignment. The mean course 

line must be adjusted and maintained 
within the limits of ±10 percent of the 
nominal course sector width. 

(3) Course structure. Course deviations 

due to roughness, scalloping, or bends 
must be within the following limita-

(i)  Front course. (a) Course structure 

from 18 miles from runway threshold to 
Point A must not exceed ±40 micro-

(b) Point A to Point A–1—linear de-

crease from not more than ±40 micro-
amperes at Point A to not more than 
±20 microamperes at Point A–1; 

(c) Point A–1 to Missed Approach 

Point—not more than ±20 micro-

(d) Monitor tolerances: width ±17 per-

cent of nominal; alignment—±10 per-
cent of nominal course sector width. 

(ii)  Back course. (a) Course structure 

18 miles from runway threshold to 4 
miles from runway threshold must not 
exceed  ±40 microamperes. Four miles 
to 1 mile from R/W must not exceed ±40 
microamperes decreasing to not more 
than  ±20 microamperes, at a linear 

(b) Monitor tolerances: width—±17 

percent of nominal; alignment—±10 
percent of nominal course sector width. 

[Doc. No. 10116, 35 FR 12711, Aug. 11, 1970, as 
amended by Amdt. 171–9, 38 FR 28557, Oct. 15, 

§ 171.111

Ground standards and toler-


Compliance with this section must be 

shown as a condition to approval and 
must be maintained during operation 
of the SDF. 

(a) Frequency. (1) The SDF must oper-

ate on odd tenths or odd tenths plus a 
twentieth MHz within the frequency 
band 108.1 MHz to 111.95 MHz. The fre-
quency tolerance of the radio fre-
quency carrier must not exceed plus or 
minus 0.002 percent. 

(2) The modulating tones must be 90 

Hz and 150 Hz within ±2.5 percent. 

(3) The identification signal must be 

1020 Hz within ±50 Hz. 

(4) The total harmonic content of the 

90 Hz tone must not exceed 10 percent. 

(5) The total harmonic content of the 

150 Hz tone must not exceed 10 percent. 

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