Global Positioning System Reference
In-Depth Information
Other types of integrity anomalies can result in smaller ranging errors. An exam-
ple of this occurred on GPS SVN 19. After approximately 8 months on orbit, an
anomalous condition developed on the satellite that resulted in carrier leakage on
the observed L1 signal spectrum, which is normally carrier suppressed. In this case
no control segment problems were observed or user equipment problems reported,
so the SV was left to operate in the off-nominal mode. No incident reports or prob-
lems regarding the SVN 19 C/A code occurred until March 1993 during FAA field
tests using differential navigation for aided landings. The differential navigation
solution was corrupted with a 4-m bias [42].
The GPS ground-monitoring network currently does not provide coverage for
all satellites 24 hours a day [40]. Therefore, if an integrity problem were to occur, it
may not be detected immediately. An example of this occurred on July 28, 2001,
when SVN22 experienced a clock failure over the southern Pacific Ocean region
resulting in user range errors in excess of 200,000m. For about a half-hour, this was
undetectable by the GPS CS because the satellite was not in view of any CS monitor
stations [42].
Most MCS problems are due to hardware, software, or human error. Past prob-
lems have involved incorrect ionospheric correction database coefficients being
incorporated in the navigation message of all satellites. Single frequency receivers
may have experienced ranging errors of up to 16m before the problem was detected.
The MCS is continuously working to minimize integrity anomalies as much as
possible by installing redundant hardware, robust software, and providing training to
prevent human error. The best response time, however, may still be several minutes,
which is insufficient for aviation applications. There are methods, however, by which
the user is independently able to be notified of a satellite anomaly if it does occur.
7.5.3 Integrity Enhancement Techniques
The integrity problem is important for many applications, but crucial for aviation
since the user is traveling at high speeds and can quickly deviate from the flight path.
The integrity function becomes especially critical if GPS is to be used as a primary
navigation system. RTCA Special Committee 159 (SC-159), a federal advisory com-
mittee to the FAA, has devoted much effort to developing techniques to provide
integrity for airborne use of GPS. Three methods used for GPS integrity monitoring
are RAIM—one element of a set of airborne GPS enhancements defined by ICAO as
aircraft-based augmentation systems (ABAS)—SBAS, and GBAS.
This section primarily concentrates on RAIM, since SBAS and GBAS are differ-
ential GPS-based techniques discussed in more detail in Chapter 8.
7.5.3.1 RAIM and FDE
The use of standalone GPS or GPS in conjunction with use of ranging sources from
other satellites, such as geostationary satellites, GALILEO, and GLONASS, where
integrity is provided by RAIM and FDE, is referred to as an ABAS. The RAIM algo-
rithm is contained within the receiver, hence the term autonomous monitoring.
RAIM is a technique that uses an overdetermined solution to perform a consistency
check on the satellite measurements [43].
 
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