Viavi 87339, Руководство по эксплуатации

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Principals of Operation
6 -22
6.1.7.A SBAS Systems
WAAS (W
ide
A
rea
A
ugmentation
S
ystem)
Developed and managed by the FAA, to augment GPS, with the goal of improving its
accuracy, integrity, and availability. WAAS is intended to enable aircraft to rely on GPS
for all phases of flight, including precision approaches to any airport within its coverage
area. The system communicates with several ground stations and provides atmospheric
corrections & early warning of GPS failures. The data rate is higher that L1 C/A code at
250 Hz and two geostationary satellites provide area coverage.
EGNOS ( E uropean G eostationary N avigation O verlay S ystem)
Managed by the European tripartite group. Corrections for GPS and GLONASS
Similarly to WAAS, EGNOS is mostly designed for aviation users which enjoy unperturbed
reception of direct signals from three geostationary satellites up to very high latitudes.
The use of EGNOS on the ground, especially in urban areas is limited due to relatively
low elevation of geostationary satellites: about 30° above horizon in central Europe and
much less in the North of Europe. To address this problem, ESA released in 2002
SISNeT, an Internet service designed for continuous delivery of EGNOS signals to ground
users. SV PRN 126 is used for test purposes at this time (2010).
MSAS ( M ulti-Functional S atellite A ugmentation S ystem
Managed by the Japanese Civil Aviation Bureau (JCAB), a satellite navigation system
which supports differential GPS, designed to supplement the GPS system by reporting
(then improving) on the reliability and accuracy of those signals. MSAS for aviation use
was commissioned on September 27, 2007. Two geostationary satellites provide area
coverage.
GAGAN ( G PS A ided G eo A ugmented N avigation)
The GAGAN system is a planned implementation of a regional Satellite Based
Augmentation System (SBAS) by the Indian government. It is a system to improve the
accuracy of a GNSS receiver by providing reference signals. Two geostationary satellites
will eventually provide area coverage. One SV is currently deployed, PRN 127.
6.1.7.B RAIM (Receiver Autonomous Integrity Monitoring System)
A unique aviation requirement of GPS avionics is RAIM. While GPS provides the user
with unparalleled levels of accuracy, one significant deficiency of GPS is integrity, or the
ability of the system to provide a timely warning if the navigation solution is inaccurate or
erroneous. Navigation systems prior to GPS, particularly aviation applications, provided a
means to warn the aircraft that the signal was outside certain limits. For example, a
Category I ILS provides this warning within six seconds.
The only means available for the GPS system itself to provide the user with a warning of
system unreliability is through the data message forming part of the GPS signal. The
“health” flag found in subframe 4 and 5 will alert the receiver to a failure of a GPS
satellite.
The time lag from the beginning of the failure to when it is incorporated in the health flag
(up to eight hours) represents an unacceptably long period of time for aviation.