The Facility has been operating a Vaisala LD40 laser ceilometer at the MST radar site near Aberystwyth since 8th August 2005.

The LD40 transmits 75 ns duration pulses at a repetition frequency of 6494 Hz. The temperature of the laser diode is maintained within a narrow range in order to ensure high stability of the emitted wavelength. This allows a very narrow band optical filter to be used at the input to the receiver. The receiver signal amplitude is sampled at time intervals of 50 ns, corresponding to altitude intervals of 7.5 m. Owing to the low power of the transmitter, the receiver signal amplitude for a single pulse backscattered from a high-altitude cloud is not sufficiently large to be distinguished above the noise level. Nevertheless the contribution from cloud backscatter remains coherent from pulse to pulse, whereas that from noise varies randomly. Consequently the detectability of the desired signal can be greatly improved by summing the receiver signal amplitude (separately for each altitude interval) over all pulses within a 15 s interval. Attention is restricted to altitudes below 13.05 km (corresponding to maximum range of detection of 13.00 km - the instrument is at 50 m above mean sea level) since, even after summation, the presence of any clouds which might exist at higher altitudes can only be determined with difficulty.

The instrument manufacturer's cloud detection algorithm is applied to the 15 s receiver signal amplitude profiles. It is designed to detect up to three possible cloud layers. For each of these, the altitude (above mean sea level) of the cloud base and the depth to which the laser light penetrates is determined. If the laser light is not able to penetrate the entire depth of a particular cloud layer, missing datum values are given for subsequent layers. If no cloud layer is detected, missing datum values are given for all three layers. In order to distinguish between cloud-free conditions and a lack of observations, attention should be paid to instrument status flag. An estimate is also given of the maximum possible altitude of cloud detection and of the visibility for a human observer looking upwards. The maximum range of cloud detection defaults to 13.05 km under clear sky conditions. A lower value implies that no information may be given on any cloud layers which may exist beyond this altitude. Under such conditions there is also a limit to the visibility for a human observer. Otherwise a missing datum value is given. Finally, an estimate is given of the precipitation rate based on the characteristics of the backscatter profile. A value of 0 implies no precipitation and a value of 3 suggests heavy precipitation.