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second (6 knots) at the time of the poll.
2.1.11.3.2 Integrity Evaluation
The SPES wind data evaluation is based upon two separate, but related, tests.
These are:
• Wind speed
• Wind direction
2.1.11.3.2.1 Wind Speed Test
Wind speed data is collected from all the weather station sensors and a mean
value calculated. The ratio of variation with the mean value is then calculated
for each individual weather station.
The calculated variation ratios are stored in groups, based upon wind
direction. This enables the system to determine whether the variations in wind
speeds for a particular station are related to a few directions only, or all
directions.
Sensor status in relation to wind speed data is given by the following SPES
messages:
• Sheltering
• Frictional drag or sensor situated too low
• Wind channelling.
• Sensor situated too high
2.1.11.3.2.2 Wind Direction Test
Wind direction data is collected from all the weather station sensors and a
mean value calculated, similarly to wind speed. Wind directions are compared
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LLWAS
Low Level Windshear Alert System
to the mean wind direction for the network and the mean variation is stored.
The variations are stored in groups of similar variations for each station.
If the percentage of ratio differences is greater than a predetermined figure
over a certain time, it may be determined that there is a problem with the
sensor, or that the orientation of the station is not correct.
Sensor status in relation to wind direction data is given by the following SPES
messages:
• Sensor orientation incorrect
• Electrical grounding fault
• Loose mounting or sticky wind-vane bearings
2.1.11.3.3 SPES Screens
The SPES has two types of screen:
• A” Site Performance Evaluation System” screen
• (Shows the general status of all weather stations)
• A “Station Analysis” screen for each individual weather station in
• The LLWAS system
• (Provides more detailed information about each weather station)
2.1.11.3.3.1 “Site Performance Evaluation System” Screen
This screen shows the wind speed and wind direction status of each weather
station on the system.
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LLWAS
Low Level Windshear Alert System
2.1.11.3.3.2 “Station Analysis” Screen
By clicking on a Details button on the “Site Performance Evaluation System”
screen, specific information about a particular station can be viewed on its
Analysis screen. A typical Station Analysis screen is shown below.
Figure 2-16 SPES “Station Analysis” Screen
A station’s Analysis screen displays data integrity information about its wind
sensors on 3 graphs and a wind gauge. Each graph provides the user with a
different perspective on the data received from the weather station.
Each Analysis screen also has a Windrun gauge and Wind Rose display related
to that station. Although not actually part of the SPES, they provide useful
wind related information to the user.
The Windrun gauge plots the amount of wind in meters per second,
measured by the sensor, on a rotating time axis. The axis is configurable, but
set to 24 hours for Chaing Kai Shek airport.
The Wind Rose provides a real-time graphical indication of wind direction and
speed in accordance with the LLWAS system 10-second wind data.
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LLWAS
Low Level Windshear Alert System
Speed Ratio over Direction Graph
This graph shows the sensor performance as a ratio to the network mean
value, over the last large sample of polls. This ratio is represented on the bar
graph horizontal axis for every direction with a vertical incremental scale of
0.00 to 2.00.
In a “perfect” system a weather station would return a value that is the same
as the network mean, thus registering a ratio of unity. The graph would then
plot a value of 1.00 for all directions.
Difference from Mean (Wind Gauge)
The wind gauge displays the ratio shown on the “Speed Ratio over Direction
Graph” on a rotating axis. The gauge axis is the same as the graph, i.e. 0.00
to 2.00.
For the “perfect” system referred to above, the sensor would display a ratio of
1.00 in all directions and plot a circle halfway into the gauge. If a calculated
ratio is low, then the gauge plot will be nearer the centre for that direction. If
the ratio is high then the plot will be nearer the outside of the gauge for that
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