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describe these fixes.
18.10.3Unnamed waypoints in the database will be
uniquely identified for each airport but may be
repeated for another airport (e.g., RW36 will be used
at each airport with a runway 36 but will be at the
same location for all approaches at a given airport).
18.10.4The runway threshold waypoint, which is
normally the MAWP, may have a five letter identifier
(e.g., SNEEZ) or be coded as RW## (e.g., RW36,
RW36L). Those thresholds which are coded as five
letter identifiers are being changed to the RW##
designation. This may cause the approach chart and
database to differ until all changes are complete. The
runway threshold waypoint is also used as the center
of the MSA on most GPS approaches. MAWPs not
located at the threshold will have a five letter
identifier.
AIP ENR 4.1−31
United States of America 15 MAR 07
Federal Aviation Administration Nineteenth Edition
18.11Position Orientation
18.11.1As with most RNAV systems, pilots should
pay particular attention to position orientation while
using GPS. Distance and track information are
provided to the next active waypoint, not to a fixed
navigation aid. Receivers may sequence when the
pilot is not flying along an active route, such as when
being vectored or deviating for weather, due to the
proximity to another waypoint in the route. This can
be prevented by placing the receiver in the
nonsequencing mode. When the receiver is in the
nonsequencing mode, bearing and distance are
provided to the selected waypoint, and the receiver
will not sequence to the next waypoint in the route
until placed back in the auto sequence mode or the
pilot selects a different waypoint. On overlay
approaches, the pilot may have to compute the along
track distance to stepdown fixes and other points due
to the receiver showing along track distance to the
next waypoint rather than DME to the VOR or ILS
ground station.
18.12Conventional Versus GPS Navigation Data
18.12.1There may be slight differences between the
course information portrayed on navigational charts
and a GPS navigation display when flying authorized
GPS instrument procedures or along an airway. All
magnetic tracks defined by any conventional
navigation aids are determined by the application of
the station magnetic variation. In contrast, GPS
RNAV systems may use an algorithm, which applies
the local magnetic variation and may produce small
differences in the displayed course. However, both
methods of navigation should produce the same
desired ground track when using approved, IFR
navigation system. Should significant differences
between the approach chart and the GPS avionics’
application of the navigation database arise, the
published approach chart, supplemented by
NOTAMs, holds precedence.
18.12.2Due to the GPS avionics’ computation of
great circle courses, and the variations in magnetic
variation, the bearing to the next waypoint and the
course from the last waypoint (if available) may not
be exactly 180 apart when long distances are
involved. Variations in distances will occur since
GPS distance−to−waypoint values are along−track
distances (ATD) computed to the next waypoint and
the DME values published on underlying procedures
are slant−range distances measured to the station.
This difference increases with aircraft altitude and
proximity to the NAVAID
18.13Departures and Instrument Departure
Procedures (DPs)
18.13.1The GPS receiver must be set to terminal
(±1 NM) course deviation indicator (CDI) sensitivity
and the navigation routes contained in the
database in order to fly published IFR charted
departures and DPs. Terminal RAIM should be
automatically provided by the receiver. (Terminal
RAIM for departure may not be available unless the
waypoints are part of the active flight plan rather than
proceeding direct to the first destination.) Certain
segments of a DP may require some manual
intervention by the pilot, especially when radar
vectored to a course or required to intercept a specific
course to a waypoint. The database may not contain
all of the transitions or departures from all runways
and some GPS receivers do not contain DPs in the
database. It is necessary that helicopter procedures be
flown at 70 knots or less since helicopter departure
procedures and missed approaches use a 20:1
obstacle clearance surface (OCS), which is double the
fixed−wing OCS, and turning areas are based on this
speed as well.
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