14 C.F.R. § 171.107
As used in this subpart:
SDF (simplified directional facility) means a directional aid facility providing only lateral guidance (front or back course) for approach from a final approach fix.
DDM (difference in depth of modulation) means the percentage modulation depth of the larger signal minus the percentage modulation depth of the smaller signal, divided by 100.
Angular displacement sensitivity means the ratio of measured DDM to the corresponding angular displacement from the appropriate reference line.
Back course sector means the course sector on the opposite end of the runway 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 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.
Displacement sensitivity means the ratio of measured DDM to the corresponding 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 distance of 4 nautical miles from the threshold.
Point A1 means a point on the front course in the approach direction a distance of 1 statute mile from the threshold.
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 approach must be abandoned, if the approach and subsequent landing cannot be safely completed by visual reference, whether or not the aircraft has descended to the minimum descent altitude.
14 C.F.R. § 171.107