Comparison on Measurements by Airport Visibility Automatic Observation Instruments in Low Visiblity Weather

The runway visual range measurements form January 2013 to August 2014 by atmospheric transmission meter and forward scattering meter installed on the same end of Xi'an Xianyang international airport south runway are compared. Conclusions are as follows: When the visual range is greater than 400 m, measurements by forward scattering meter are likely to be greater than those by atmospheric transmission meter, while when the runway visual range is less than or equal to 400 m, measurements by atmospheric transmission meter are likely to be greater. Overall, when the runway visual range is less than or equal to 600 m, both two sets of equipment can be replaced by each other as their bias is accord with requirements of Civil Aviation Observation and Measurement. However, when the runway visual range is between 600 m and 1000 m, whether observations from two sets of equipment can be replaced mainly depends on weather conditions. When the runway visual range is greater than 1000 m, differences between two sets of equipment exceeds requirements of Civil Aviation Observation and Measurement.
    In fog, freezing fog, haze or big rain weather, observations show very good consistency and can be replaced with each other when runway visual range is less than or equal to 1000 m. In snow, smoke or floating sand weathers, measurements by forward scattering meter are much greater; while in big rain weather, it is found that measurements by atmospheric transmission meter are greater than that by forward scattering meter. In fog weather and haze weathers, it is found that measurements by atmospheric transmission meter are greater when the runway visual range is less than or equal to 1000 m, but less than data measured by forward scattering meter when visual range is greater than 1000 m.