KNMI Visibility Standard for Calibration of Scatterometers

Scatterometers (or forward scatter meters) are used to measure visibility. This type of instrument has certain advantages compared to a transmissometer: it is easy to install, it requires relatively little maintenance (cleaning) and it is not too expensive. However, calibration of scatterometers is not trivial. Calibration of visibility instruments is of particular importance if they are used for aeronautical purposes such as for Runway Visual Range (RVR) measurements. For civil aviation, the International Civil Aviation Authority 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.”
Calibration of transmissometers is relatively easy. It can be performed in the laboratory. Neutral density filters are used to calibrate the instrument over the entire relevant range of visibility. However, such a straight-forward method is not possible for calibration of scatterometers.
Scatterometers do not measure visibility directly, but they measure the amount of 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 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 of the instrument, produces a known amount of scattering. When the visibilities measured by the two reference 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 scatterometer.
In this contribution, the KNMI visibility standard will be presented. The instrument set-up will be shown, and the chain of calibration will be described, starting with the neutral density filters used to calibrate the reference transmissometer and ending with the final calibration of scatterometers in the field. Results are shown.