Air Traffic Management Concept Baseline Definition - The Boeing ...

Systems (AWOS). These systems are complementary, but perform somewhat different
functions. The ASOS was intended to be a complete surface meteorological observing
station that would replace human observers. ASOS systems are deployed at airports but
also in a much broader weather observing network around the country. Problems have
emerged with some of the ASOS sensors, particularly the visibility package, which have
prevented the ASOS from achieving the goal to eliminate human observers. Work
continues on these problems, but the likelihood of their success is not known at this time.
Human observations of some critical aviation impact variables will likely be needed for the
foreseeable future (National Research Council, 1995). The AWOS is designed strictly as a
terminal weather information system. It was not intended to eliminate human observers,
but it does provide certified observations of ceiling, visibility, altimeter setting, wind
speed, and wind direction. It too has been criticized for providing misleading aviation
weather information, especially ceiling observations.
Specific information on the performance of the sensors on the ASOS and AWOS was not
available at the time of this writing. However, it is reasonable to expect that the accuracy
of the sensors is adequate for most current and expected analytical and modeling
applications. The notable exception is that the visibility and present weather sensors have
been criticized for giving inaccurate and misleading information under some circumstances
(NRC, 1995). An important issue for the success of future improvements in aviation
weather information is likely to be increasing the spatial density of measurements to
provide improved coverage of key weather parameters. When completed, the ASOS
network will consist of over 850 units and the AWOS network will consist of 160 units
located at airports that do not otherwise provide certified weather information. (Some
state governments have also purchased AWOS systems.) These two surface monitoring
systems will likely go forward for a decade or longer as the primary surface observing
systems used for aviation weather.
Other sources of surface data are used for aviation weather, primarily to support tactical
decision making. For example, the FAA operates sensors that measure runway visual
range (RVR). Errors in automated RVR systems (and ASOS and AWOS visibility
measurements) deployed to date suggest that near term improvements in visibility
measurement technologies could improve the efficiency of airport operations. The FAA
also operates the LLWAS, a network of tower-mounted anemometers that is supposed to
detect potentially hazardous wind shear and microburst conditions at the airport.
However, concerns over the efficacy of LLWAS data have sometimes lead controllers to
ignore LLWAS warnings. This was apparently the case during the 1994 crash of a USAir
MD-80 at Charlotte-Douglas airport (NRC, 1995) during a microburst event. In the near
term, improvements in wind shear algorithms and/or the use of more Doppler radar
information could improve safety conditions in the terminal area. There is also a national
network of lightning detection sensors that show where cloud-to-ground lightning strikes
are occurring.
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