VOR distance-measuring equipment (DME) RNAV approach procedures that use collocated VOR and DME information to construct RNAV approaches are named “VOR/DME RNAV RWY XX,” where XX stands for the runway number for which the approach provides guidance. Sometimes referred to as “station mover” approaches, these procedures were the first RNAV approaches issued by the FAA. They enable specific VOR/DME RNAV equipment to create waypoints on the final approach path by virtually “moving” the VOR a specific DME distance along a charted radial. [Figure 5-8]

GPS overlay procedures that are based on pre-existing nonprecision approaches contain the wording “or GPS” in the title. For instance, the title “VOR/DME or GPS A” denotes that throughout the GPS approach, the underlying ground-based NAVAIDs are not required to 

Figure 5-8.VOR/DME RNAV Approach Chart.

Figure 5-9.VOR/DME or GPS A Approach.

be operational and associated aircraft avionics need not be installed, operational, turned on, or monitored. [Figure 5-9] Monitoring of the underlying approach is suggested when equipment is available and functional. The procedure can be used as a GPS approach or as a traditional VOR/DME approach and may be requested using “GPS” or “VOR/DME,” such as “GPS A” for the VOR/DME or GPS A. As previously mentioned, the “A” in the title shows that this is a circling approach without straight-in minimums. Many GPS overlay procedures have been replaced by stand-alone GPS or RNAV (GPS) procedures.

Stand-alone GPS procedures are not based on any other procedures, but they may replace other procedures. The naming convention used for stand-alone GPS approaches is “GPS RWY XX.” The coding for the approach in the database does not accommodate multisensor FMSs because these procedures are designed only to accommodate aircraft using GPS equipment. These procedures will eventually be converted to RNAV (GPS) approaches. [Figure 5-10 on page 5-12]

RNAV (GPS) approach procedures have been developed in an effort to accommodate all RNAV systems, including multi-sensor FMSs used by airlines and corporate operators. RNAV (GPS) IAPs are authorized as stand-alone approaches for aircraft equipped with RNAV systems that contain an airborne navigation database and are certified for instrument approaches. GPS systems require that the coding for a GPS approach activate the receiver autonomous integrity monitoring (RAIM) function, which is not a requirement for other RNAV equipment. The RNAV procedures are coded with both the identifier for a GPS approach and the identifier for an RNAV approach so that both systems can be used. In addition, so that the chart name, air traffic control (ATC) clearance, and database record all match, the charted title of these procedures uses both “RNAV” and “(GPS),” with GPS in parentheses. “GPS” is not included in the ATC approach clearance for these procedures.

RNP, a refinement of RNAV, is part of a collaborative effort by the FAA and the aviation industry to develop performance- based procedures. RNP is a statement of the navigation performance necessary for operation within defined airspace. RNP includes both performance and functional requirements, and is indicated by the RNP value. The RNP value designates the lateral performance requirement associated with a procedure. A key feature of

Figure 5-10. GPS Stand-alone Approach.

RNP is the concept of on-board monitoring and alerting. This means the navigation equipment is accurate enough to keep the aircraft in a specific volume of airspace, which moves along with the aircraft. The aircraft is expected to remain within this volume of airspace for at least 95 percent of the flight time, and the integrity of the system ensures the aircraft will do so. The aircraft avionics also continuously monitor sensor inputs, and through complex filtering, generate an indication in the level of confidence in the navigation performance sometimes referred to as actual navigation performance (ANP). An essential function required for RNP operations is the ability of the system to alert the pilot when the ANP exceeds the requisite RNP value.

Navigation performance for a particular RNP type is expressed numerically. Depending on the capability of each aircraft's system, RNP values can be as low as 0.1 of a nautical mile. A performance value of RNP 0.3, for example assures that the aircraft has the capability of remaining within 0.3 of a nautical mile to the right or left side of the centerline 95 percent of the time.