Evaluation of the Vaisala FD12P 1.91S firmware with insect filtering

A so-called transmissometer (TMM) measures the extinction of a light beam over an atmospheric path between an emitter and a receiver. The atmospheric extinction is by definition directly related to the Meteorological Optical Range (MOR). Since a TMM measures extinction, its signal varies exponentially with MOR. As a result a TMM can only measure low MOR values very accurately, although a second receiver, placed at a larger distance from the emitter, is often used to increase the MOR range of a TMM. The MOR reported by a double baseline TMM is only used by KNMI up to values of 3 km and the reporting of runway visible range for aviation is restricted to values below 2 km. A so-called forward scatter sensor (FS) measures the amount of light scattered by a small measurement volume at an angle of about 33 °. Its signal varies linearly with MOR so that it can be used to measure higher MOR values. However a relation between the amount of forward scattering and the extinction of the scattering medium needs to be taken into account. KNMI uses the Vaisala FD12P forward scatter sensor for measurements of the MOR up to 50 km. At first the sensor was only used at synoptic weather stations, but after approval by ICAO the sensor is now also used for (runway) visibility measurements at airports. The advantages of a FS compared to a TMM are the lower procurement and installation costs (a double baseline TMM consists of 3 separate sensor units that need to be carefully aligned), lower maintenance due to lesser sensitivity of MOR measurements to contamination of lenses, the possibility to measure up to higher MOR values, and some FS sensors, like the FD12P, have extensions so that it can measure the so-called present weather, i.e. precipitation type and intensity. In fact, the FD12P and similar instruments are often called present weather sensors (PWS). The advantages of a TMM are that it measures MOR (extinction) directly and that it can be calibrated through neutral density filters with a known extinction. The calibration of a FS requires a field setup so that the MOR of a reference FS can be related to that of a calibrated TMM. The reference FS is then used to calibrate a scatter plate by which the calibration is transferred to other FS sensors (cf. Bloemink et al., 2010). Note that there is no field reference for high MOR values although sensor inter-comparisons and comparisons with human observations over the full MOR range have been performed (cf. e.g. Wauben, 2003). A TMM or FS can be equipped with a background luminance sensor, which is required to convert the observed MOR into the aeronautical (runway) visibility that is defined as the maximum distance at which a standard (or actual runway) lights can be distinguished form the background. The background luminance sensor measures the brightness of the sky to the North at an elevation of about 30 °