830 

14 CFR Ch. I (1–1–19 Edition) 

§ 171.253 

two or more guidance signals simulta-
neously from a common location. 

Course line 

means the locus of points 

nearest to the runway centerline in 
any horizontal plane at which the DDM 
is zero. 

Course sector (full) 

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. 

Course sector (half) 

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 DDM is 0.0775. 

DDM 

means difference in depth of 

modulation. The percentage modula-
tion depth of the larger signal minus 
the percentage modulation depth of the 
smaller signal, divided by 100. 

Displacement sensitivity 

(Localizer) 

means the ratio of measured DDM to 
the corresponding lateral displacement 
from the appropriate reference line. 

Facility Performance Category I— 

ISMLS 

means an ISMLS which provides 

guidance information from the cov-
erage limit of the ISMLS to the point 
at which the localizer course line inter-
sects the ISMLS glide path at a height 
of 200 feet or less above the horizontal 
plane containing the threshold. 

Glide path 

means that locus of points 

in the vertical plane containing the 
runway center line at which the DDM 
is zero, which, of all such loci, is the 
closest to the horizontal plane. 

Glide path angle 

(

q

) means the angle 

between a straight line which rep-
resents the mean of the ISMLS glide 
path and the horizontal. 

Glide path sector (full) 

means the sec-

tor in the vertical plane containing the 
ISMLS glide path and limited by the 
loci of points nearest to the glide path 
at which the DDM is 0.175. The ISMLS 
glide path sector is located in the 
vertical plane containing the runway 
centerline, and is divided by the radi-
ated glide path in two parts called 
upper sector and lower sector, referring 
respectively to the sectors above and 
below the glide path. 

Glide path sector (half) 

means the sec-

tor in the vertical plane containing the 
ISMLS glide path and limited by the 
loci of points nearest to the glide path 
at which the DDM is 0.0875. 

ISMLS Point ‘A’ 

means an imaginary 

point on the glide path/localizer course 
measured along the runway centerline 
extended, in the approach direction, 
four nautical miles from the runway 
threshold. 

ISMLS Point ‘B’ 

means an imaginary 

point on the glide path/localizer course 
measured along the runway centerline 
extended, in the approach direction, 
3500 feet from the runway threshold. 

ISMLS Point ‘C’ 

means a point 

through which the downward extended 
straight portion of the glide path (at 
the commissioned angle) passes at a 
height of 100 feet above the horizontal 
plane containing the runway threshold. 

Interim standard microwave landing 

system 

(ISMLS) means a ground station 

which transmits azimuth and elevation 
angle information which, when decoded 
and processed by the airborne unit, 
provides signal performance capable of 
supporting approach minima for V/ 
STOL and CTOL operations and oper-
ates with the signal format and toler-
ances specified in §§ 171.259, 171.261, 
171.263, 171.265, and 171.267. 

Integrity 

means that quality which 

relates to the trust which can be placed 
in the correctness of the information 
supplied by the facility. 

Mean corrective time 

means the aver-

age time required to correct an equip-
ment failure over a given period, after 
a service man reaches the facility. 

Mean time between failures 

means the 

average time between equipment fail-
ure over a given period. 

Reference datum 

means a point at a 

specified height located vertically 
above the intersection of the runway 
centerline and the threshold and 
through which the downward extended 
straight portion of the ISMLS glide 
path passes. 

Split type ground station 

means the 

type of ground station in which the 
electronic components for the azimuth 
and elevation guidance are contained 
in separate housings or shelters at dif-
ferent locations, with the azimuth por-
tion of the ground station located at 
the stop end of the runway, and the 
elevation guidance near the approach 
end of the runway. 

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