Global Positioning System Reference
In-Depth Information
Table 8.3
ICAO GNSS Signal-in-Space Performance Requirements
Horizontal/
Vertical
Accuracy
(95%)
Horizontal/
Vertical Alert
Limit
Integrity
Level
Time
to Alert
Operation
Continuity
Availability
10
−4
/hour to
3.7 km
N/A
3.7-7.4 km
N/A
1-1
×
0.99 to
0.99999
1-1×10
−7
/hour
En-route
5 minutes
1-1
×
10
−8
/hour
0.74 km
N/A
1.85 km
N/A
10
−4
/hour to
1-1
×
0.999 to
0.99999
1-1×10
−7
/hour
Terminal
15 seconds
1-1
×
10
−8
/hour
220m
N/A
556m
N/A
Nonprecision
approach
1-1
×
10
−4
/hour to
0.99 to
0.99999
1-1×10
−7
/hour
10 seconds
10
−8
/hour
1-1
×
16m
20m
40m
50m
1-8
10
−6
in any
15 seconds
×
0.99 to
0.99999
1-2×10
−7
/approach
APV-I
10 seconds
16m
8m
40m
20m
1-8
10
−6
in any
15 seconds
×
0.99 to
0.99999
APV-II
1-2×10
−7
/approach
6 seconds
16m
4-6m
40m
10-15m
1-8
10
−6
in any
15 seconds
×
0.99 to
0.99999
1-2×10
−7
/approach
Category I
6 seconds
Source:
[48].
vertical guidance (APV)-I and vertical guidance (APV)-II have been defined to
enable the full utility of the performance that near-term SBASs can provide.
SBAS Architecture and Functionality
All SBAS systems are comprised of four subelements: monitoring receivers, central
processing facilities, satellite uplink facilities, and one or more geostationary satel-
lites. Unfortunately, the terminology for these subelements is not consistent among
the specific implementations. Within the U.S. WAAS, the monitors are referred to as
wide-area reference stations (WRSs), the central processing facilities are known as
wide area master stations (WMSs), and the uplink facilities as ground Earth stations
(GESs) (see Figure 8.26). Within EGNOS, these elements are referred to as ranging
and integrity monitoring stations (RIMS), mission control centers (MCCs), and
navigation land Earth stations (NLES), respectively (see Figure 8.27). Within
MSAS, the respective terms are ground monitoring stations (GMSs), MCSs, and
GESs. MSAS also includes additional elements referred to as monitor and ranging
stations (MRSs) that are similar to GMSs but are specifically dedicated to orbit
determination. MSAS will be implemented in two phases. The initial MSAS con-
tains 4 GMSs, 2 MRSs, and 2 MCSs (see Figure 8.28). For GAGAN, the terms
Indian Reference Station (INRES), Indian Master Control Centre (INMCC) and
Indian Land Uplink Station (INLUS) are used (see Figure 8.29).
The functionality provided by the SBAS subelements is summarized in Figure
8.30. As observed in Figure 8.30, users receive navigation signals transmitted from
GPS and, in some instances, GLONASS satellites. These signals are also received by
monitoring networks operated by the SBAS service providers. Each site within the
monitoring networks generally includes a number of GNSS receivers (for redun-
dancy) that provide L1 C/A code and L2 P(Y) code pseudorange and carrier-phase
data (using semicodeless processing techniques for the L2 measurements—see Sec-
tion 5.14) to the central processing facilities. At each central processing facility, the
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