817 

Federal Aviation Administration, DOT 

§ 171.109 

Back course sector 

means the course 

sector on the opposite end of the run-
way 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 hori-

zontal plane containing the course line 
and limited by the loci of points near-
est to the course line at which the 
DDM is 0.155. 

Displacement sensitivity 

means the 

ratio of measured DDM to the cor-
responding 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 dis-
tance of 4 nautical miles from the 
threshold. 

Point A1 

means a point on the front 

course in the approach direction a dis-
tance of 1 statute mile from the thresh-
old. 

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 ap-
proach must be abandoned, if the ap-
proach and subsequent landing cannot 
be safely completed by visual ref-
erence, whether or not the aircraft has 
descended to the minimum descent al-
titude. 

§ 171.109

Performance requirements. 

(a) The Simplified Directional Facil-

ity must perform in accordance with 
the following standards and practices: 

(1) The radiation from the SDF an-

tenna system must produce a com-
posite 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 pre-
dominating on the opposite side. 

(2) When an observer faces the SDF 

from the approach end of runway, the 
depth of modulation of the radio fre-
quency 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 an-
tenna system which provides the sig-
nals 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 tol-
erance 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 posi-
tioned on the course line and is in a 
roll attitude of 20

° 

from the horizontal. 

(6) The SDF must provide signals suf-

ficient to allow satisfactory operation 
of a typical aircraft installation within 
the sector which extends from the cen-
ter of the SDF antenna system to dis-
tances of 18 nautical miles within a 
plus or minus 10

° 

sector and 10 nautical 

miles within the remainder of the cov-
erage when alternative navigational fa-
cilities 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 receiv-
able, to the distances specified, up to a 
surface extending outward from the 
SDF antenna and inclined at 7

° 

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 20

° 

of 

phase relative to the 150 Hz component, 
every half cycle of the combined 90 Hz 

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