Global Positioning System Reference
In-Depth Information
Table 7.9
RAIM/FDE Availability with a 5º Mask Angle and SA Off
Nonprecision
Approach
RAIM/FDE Function
En Route
Terminal
Fault detection
99.998%
99.990%
99.903%
Fault detection with
baro aiding
100%
100%
99.998%
Fault detection and
exclusion with baro
aiding
99.923%
99.643%
99.100%
Table 7.10
Maximum Duration of RAIM/FDE Outages with 5º Mask
Angle and SA Off
Nonprecision
Approach
RAIM/FDE Function
En Route
Terminal
Fault detection
5 minutes
10 minutes
30 minutes
Fault detection with
baro aiding
0 minutes
0 minutes
5 minutes
Fault detection and
exclusion with baro a
iding
10 minutes
35 minutes
60 minutes
As shown in Figure 7.24, outages can last up to 60 minutes in several locations,
but there is virtually 100% coverage near the equator. This high availability of FDE
near the equator is due to the increased number of visible satellites.
Another method for improving availability of RAIM and FDE is to lower the
mask angle so that more satellites are visible to the user equipment. However, as
mentioned previously, low elevation satellites will have higher atmospheric errors.
These satellites are deweighted in the solution according to (7.73). As demonstrated
in Tables 7.11 and 7.12, availability of the fault detection function is very high, even
without baro aiding. For FDE with baro aiding, outages remain, but the number of
occurrences and duration is shortened.
Satellite-Based Augmentation Systems
As discussed in the previous section, one of the limitations of the RAIM and FDE
algorithms is that they do not always have enough ranging sources with sufficient
geometry to meet availability requirements. Even with the availability improvement
obtained with SA off and employing a 2º mask angle, outages of up to 30 minutes
can occur for the nonprecision approach phase of flight with all 24 satellites opera-
tional. Satellites occasionally are taken out of service for maintenance, further
reducing the availability of RAIM and FDE.
Therefore, aviation authorities are developing augmentation systems to GPS.
One such augmentation is the SBAS. The U.S. version of SBAS is known as the
WAAS. Other SBAS systems under development are the EGNOS, the Japanese
MSAS, and the Indian GAGAN system.
SBAS systems consist of widely dispersed reference stations that monitor and
gather data on the GPS satellites. These data are forwarded to the SBAS master sta-
tions for processing to determine the integrity and differential corrections for each
monitored satellite. The integrity information and differential corrections are then
Search WWH ::
Custom Search