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but many GPS receivers are designed to assume that SA
is still active. New receivers may take advantage of the
discontinuance of SA based on the performance values in
ICAO Annex 10, and do not need to be designed to operate
outside of that performance.
7-34
GPS Familiarization
Pilots should practice GPS approaches under visual
meteorological conditions (VMC) until thoroughly proficient
with all aspects of their equipment (receiver and installation)
prior to attempting flight by IFR in instrument meteorological
conditions (IMC). Some of the tasks which the pilot should
practice are:
1. Utilizing the receiver autonomous integrity monitoring
(RAIM) prediction function;
2. Inserting a DP into the flight plan, including setting
terminal CDI sensitivity, if required, and the conditions
under which terminal RAIM is available for departure
(some receivers are not DP or STAR capable);
3. Programming the destination airport;
4. Programming and flying the overlay approaches
(especially procedure turns and arcs);
5. Changing to another approach after selecting an
approach;
6. Programming and flying “direct” missed approaches;
7. Programming and f lying “routed” mi s sed
approaches;
8. Entering, flying, and exiting holding patterns,
particularly on overlay approaches with a second WP
in the holding pattern;
9. Programming and flying a “route” from a holding
pattern;
10. Programming and flying an approach with radar vectors
to the intermediate segment;
11. Indication of the actions required for RAIM failure
both before and after the FAWP; and
12. Programming a radial and distance from a VOR (often
used in departure instructions).
Differential Global Positioning Systems (DGPS)
Differential global positioning systems (DGPS) are designed
to improve the accuracy of global navigation satellite systems
(GNSS) by measuring changes in variables to provide
satellite positioning corrections.
Because multiple receivers receiving the same set of satellites
produce similar errors, a reference receiver placed at a known
location can compute its theoretical position accurately and
can compare that value to the measurements provided by the
navigation satellite signals. The difference in measurement
between the two signals is an error that can be corrected by
providing a reference signal correction.
As a result of this differential input accuracy of the
satellite system can be increased to meters. The Wide
Area Augmentation System (WAAS) and Local Area
Augmentation System (LAAS) are examples of differential
global positioning systems.
Wide Area Augmentation System (WAAS)
The WAAS is designed to improve the accuracy, integrity,
and availability of GPS signals. WAAS allows GPS to be
used, as the aviation navigation system, from takeoff through
Category I precision approaches. The International Civil
Aviation Organization (ICAO) has defined Standards for
satellite-based augmentation systems (SBAS), and Japan
and Europe are building similar systems that are planned
to be interoperable with WAAS: EGNOS, the European
Geostationary Navigation Overlay System, and MSAS,
the Japanese Multifunctional Transport Satellite (MTSAT)
Satellite-based Augmentation System. The result will be a
worldwide seamless navigation capability similar to GPS but
with greater accuracy, availability, and integrity.
Unlike traditional ground-based navigation aids, WAAS
will cover a more extensive service area in which surveyed
wide-area ground reference stations are linked to the WAAS
network. Signals from the GPS satellites are monitored by
these stations to determine satellite clock and ephemeris
corrections. Each station in the network relays the data to a
wide-area master station where the correction information is
computed. A correction message is prepared and uplinked to
a geostationary satellite (GEO) via a ground uplink and then
broadcast on the same frequency as GPS to WAAS receivers
within the broadcast coverage area. [Figure 7-30]
In addition to providing the correction signal, WAAS
provides an additional measurement to the aircraft receiver,
improving the availability of GPS by providing, in effect,
an additional GPS satellite in view. The integrity of GPS is
improved through real-time monitoring, and the accuracy
is improved by providing differential corrections to reduce
errors. [Figure 7-31] As a result, performance improvement
is sufficient to enable approach procedures with GPS/WAAS
glide paths. At this time the FAA has completed installation of
　
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