AIM Text
- The SDF provides a final approach course similar to that of the ILS localizer. It does not provide glide slope information. A clear understanding of the ILS localizer and the additional factors listed below completely describe the operational characteristics and use of the SDF.
- The SDF transmits signals within the range of 108.10 to 111.95 MHz.
- The approach techniques and procedures used in an SDF instrument approach are essentially the same as those employed in executing a standard localizer approach except the SDF course may not be aligned with the runway and the course may be wider, resulting in less precision.
- Usable off-course indications are limited to 35 degrees either side of the course centerline. Instrument indications received beyond 35 degrees should be disregarded.
- The SDF antenna may be offset from the runway centerline. Because of this, the angle of convergence between the final approach course and the runway bearing should be determined by reference to the instrument approach procedure chart. This angle is generally not more than 3 degrees. However, it should be noted that inasmuch as the approach course originates at the antenna site, an approach which is continued beyond the runway threshold will lead the aircraft to the SDF offset position rather than along the runway centerline.
- The SDF signal is fixed at either 6 degrees or 12 degrees as necessary to provide maximum flyability and optimum course quality.
- Identification consists of a three-letter identifier transmitted in Morse Code on the SDF frequency. The appropriate instrument approach chart will indicate the identifier used at a particular airport.
FIG 1-1-8
FAA Instrument Landing Systems
1-1-11. NAVAID Identifier Removal During Maintenance
During periods of routine or emergency maintenance, coded identification (or code and voice, where applicable) is removed from certain FAA NAVAIDs. Removal of identification serves as a warning to pilots that the facility is officially off the air for tune-up or repair and may be unreliable even though intermittent or constant signals are received.
Source: FAA Aeronautical Information Manual · current edition · paragraph 1-1-10.
Research Notes
AIM 1-1-10 covers DME (Distance Measuring Equipment) — the distance-to-station component of conventional navigation.
DME characteristics:
- Operates on UHF (962-1213 MHz)
- Paired with VOR or TACAN at most facilities
- Provides slant-range distance to the station (not horizontal)
- Range: typically 100+ NM for high-altitude operations
Slant-range vs horizontal distance: DME measures the actual line-of-sight distance from aircraft to station. At high altitudes, slant range is significantly larger than horizontal distance.
GPS replacement: Most modern aircraft use GPS for distance, which provides horizontal distance directly. DME remains as backup and for VOR-based approaches that don't have GPS alternatives.
Reference: AIM 1-1-10.