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Missed approach routings in which the first track is via a
course rather than direct to the next WP require additional
action by the pilot to set the course. Being familiar with all
of the required inputs is especially critical during this phase
of flight.
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 provided automatically by the
receiver. (Terminal RAIM for departure may not be available
unless the WPs 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 WP. 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. Turning areas are based
on this speed also. Missed approach routings in which the
first track is via a course rather than direct to the next WP
require additional action by the pilot to set the course. Being
familiar with all of the required inputs is especially critical
during this phase of flight.
GPS Errors
Normally, with 30 satellites in operation, the GPS
constellation is expected to be available continuously
worldwide. Whenever there are fewer than 24 operational
satellites, GPS navigational capability may not be available
at certain geographic locations. Loss of signals may also
occur in valleys surrounded by high terrain, and any time
the aircraft’s GPS antenna is “shadowed” by the aircraft’s
structure (e.g., when the aircraft is banked).
Certain receivers, transceivers, mobile radios, and portable
receivers can cause signal interference. Some VHF
transmissions may cause “harmonic interference.” Pilots
can isolate the interference by relocating nearby portable
receivers, changing frequencies, or turning off suspected
causes of the interference while monitoring the receiver’s
signal quality data page.
GPS position data can be affected by equipment characteristics
and various geometric factors, which typically cause errors
of less than 100 feet. Satellite atomic clock inaccuracies,
receiver/processors, signals reflected from hard objects
(multi-path), ionospheric and tropospheric delays, and
satellite data transmission errors may cause small position
errors or momentary loss of the GPS signal.
System Status
The status of GPS satellites is broadcast as part of the data
message transmitted by the GPS satellites. GPS status
information is also available by means of the United States
Coast Guard navigation information service: (703) 313-
5907, or on the internet at http://www.navcen.uscg.gov/.
Additionally, satellite status is available through the Notice
to Airmen (NOTAM) system.
The GPS receiver verifies the integrity (usability) of the
signals received from the GPS constellation through receiver
autonomous integrity monitoring (RAIM) to determine if
a satellite is providing corrupted information. At least one
satellite, in addition to those required for navigation, must
be in view for the receiver to perform the RAIM function;
thus, RAIM needs a minimum of five satellites in view, or
four satellites and a barometric altimeter (baro-aiding) to
detect an integrity anomaly. For receivers capable of doing
so, RAIM needs six satellites in view (or five satellites with
baro-aiding) to isolate the corrupt satellite signal and remove
it from the navigation solution.
RAIM messages vary somewhat between receivers; however,
there are two most commonly used types. One type indicates
that there are not enough satellites available to provide
RAIM integrity monitoring and another type indicates that
the RAIM integrity monitor has detected a potential error
that exceeds the limit for the current phase of flight. Without
RAIM capability, the pilot has no assurance of the accuracy
of the GPS position.
Selective Availability. Selective Availability (SA) is a method
by which the accuracy of GPS is intentionally degraded.
This feature is designed to deny hostile use of precise GPS
positioning data. SA was discontinued on May 1, 2000,
　
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