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distance, to describe these fixes.
3. Unnamed 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).
4. The 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 Minimum Safe Altitude (MSA) on most GPS
approaches. MAWPs not located at the threshold will
have a five letter identifier.
k. Position Orientation
As 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.
l. Conventional Versus GPS Navigation Data
There 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.
Due 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.
AIM 2/14/08
1-1-34 Navigation Aids
m. Departures and Instrument Departure
Procedures (DPs)
The GPS receiver must be set to terminal (±1 NM)
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 20:1obstacle
clearance surface (OCS), which is double the
fixed-wing OCS, and turning areas are based on this
speed as well.
n. Flying GPS Approaches
1. Determining which area of the TAA the
aircraft will enter when flying a “T” with a TAA must
be accomplished using the bearing and distance to the
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