KNMI Visibility Standard for Calibration of Scatterometers

Scatterometers are used to measure visibility, but their calibration is not trivial. Calibration is of particular importance if the instruments are used for aeronautical purposes such as for Runway Visual Range (RVR). The International Civil Aviation Organization states: “The calibration of a forward-scatter meter has to be traceable and verifiable to a transmissometer standard, the accuracy of which has been verified over the intended operational range.”
Scatterometers measure scatter of a light source from a small measuring volume of atmosphere. This scatter is related to visibility by comparing it with the measurements of a transmissometer, the results of which are stored in the instrument’s software. Equally, the calibration of scatterometers also requires a comparison with a well-calibrated transmissometer. This means that it cannot take place in the laboratory, but requires a setup in the field which exists of a reference transmissometer and a reference scatterometer.
For the scatterometer used (a Vaisala FD12P), a calibration device exists which, when placed in the measuring volume, produces a known amount of scatter. When the visibilities measured by the two instruments of the standard are compared, this may result in a deviation of the scatterometer from the transmissometer. This can be corrected for by adjusting the calibration device. So in this way the standard is used to check and if necessary correct the calibration device, which can then be used to calibrate other FD12Ps. The comparison of the transmissometer and scatterometer is also used to check the linearity of the FD12P.
The KNMI visibility standard is be presented. The instrument setup is shown, and the chain of calibration is be described, starting with the neutral density filters used to calibrate the reference transmissometer and ending with the calibration of scatterometers in the field. An error analysis is included and results are shown.