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have been replaced by stand-alone GPS or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
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.