#### § 171.303 Definitions.

As used in this subpart:

*Auxiliary data* means data transmitted in addition to basic data that provide ground equipment siting information for use in refining airborne position calculations and other supplementary information.

*Basic data* means data transmitted by the ground equipment that are associated directly with the operation of the landing guidance system.

*Beam center* means the midpoint between the −3 dB points on the leading and trailing edges of the scanning beam main lobe.

*Beamwidth* means the width of the scanning beam main lobe measured at the −3 dB points and defined in angular units on the boresight, in the horizontal plane for the azimuth function and in the vertical plane for the elevation function.

*Clearance guidance sector* means the volume of airspace, inside the coverage sector, within which the azimuth guidance information provided is not proportional to the angular displacement of the aircraft, but is a constant fly-left or fly-right indication of the direction relative to the approach course the aircraft should proceed in order to enter the proportional guidance sector.

*Control Motion Noise (CMN)* means those fluctuations in the guidance which affect aircraft attitude, control surface motion, column motion, and wheel motion. Control motion noise is evaluated by filtering the flight error record with a band-pass filter which has corner frequencies at 0.3 radian/sec and 10 radians/sec for azimuth data and 0.5 radian/sec and 10 radians/sec for elevation data.

*Data rate* means the average number of times per second that transmissions occur for a given function.

*Differential Phase Shift Keying (DPSK)* means differential phase modulation of the radio frequency carrier with relative phase states of 0 degree or 180 degrees.

*Failure* means the inability of an item to perform within previously specified limits.

*Guard time* means an unused period of time provided in the transmitted signal format to allow for equipment tolerances.

*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 technician reaches the facility.

*Mean course error* means the mean value of the azimuth error along a specified radial of the azimuth function.

*Mean glide path error* means the mean value of the elevation error along a specified glidepath of the elevation function.

*Mean-time-between-failures (MTBF)* means the average time between equipment failures over a given period.

*Microwave Landing System (MLS)* means the MLS selected by ICAO for international standardization.

*Minimum glidepath* means the lowest angle of descent along the zero degree azimuth that is consistent with published approach procedures and obstacle clearance criteria.

*MLS Approach Reference Datum* is a point at a specified height located vertically above the intersection of the runway centerline and the threshold.

*MLS back azimuth reference datum* means a point 15 meters (50 feet) above the runway centerline at the runway midpoint.

*MLS datum point* means a point defined by the intersection of the runway centerline with a vertical plane perpendicular to the centerline and passing through the elevation antenna phase center.

*Out of coverage indication (OCI)* means a signal radiated into areas outside the intended coverage sector, where required, to specifically prevent invalid removal of an airborne warning indication in the presence of misleading guidance information.

*Path Following Error (PFE)* means the guidance perturbations which could cause aircraft displacement from the desired course or glidepath. It is composed of the path following noise and of the mean course error in the case of azimuth functions, or the mean glidepath error in the case of elevation functions. Path following errors are evaluated by filtering the flight error record with a second order low pass filter which has a corner frequency at 0.5 radian/sec for azimuth data or 1.5 radians/sec for elevation data.

*Path following noise (PFN)* means that portion of the guidance signal error which could cause displacement from the actual mean course line or mean glidepath as appropriate.

*Split-site ground station* means the type of ground station in which the azimuth portion of the ground station is located near the stop end of the runway, and the elevation portion is located near the approach end.

*Time division multiplex (TDM)* means that each function is transmitted on the same frequency in time sequence, with a distinct preamble preceding each function transmission.