#### § 171.253 Definitions.

As used in this subpart:

*Angular displacement sensitivity* (Glide Slope) means the ratio of measured DDM to the corresponding angular displacement from the appropriate reference line.

*Collocated ground station* means the type of ground station which transmits two or more guidance signals simultaneously 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 modulation 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 coverage limit of the ISMLS to the point at which the localizer course line intersects 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* (θ) means the angle between a straight line which represents the mean of the ISMLS glide path and the horizontal.

*Glide path sector (full)* means the sector 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 radiated 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 sector 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 operates with the signal format and tolerances 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 average time required to correct an equipment failure over a given period, after a service man reaches the facility.

*Mean time between failures* means the average time between equipment failure 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 different locations, with the azimuth portion of the ground station located at the stop end of the runway, and the elevation guidance near the approach end of the runway.