A new method for checking the leveling of pyranometers

Accurate leveling of the pyranometers and the continuous control of the leveling are essential to obtain high-precision global solar radiation data. Yet, many automatic weather stations work without such control and provide erroneous data due to the accidental tilt of the pyranometer. Many historical data are used without the certainty of accurate leveling. This paper presents a new method capable of detecting a small tilt of the pyranometer. It can detect a tilt as small as 2-3° from one-year 10-min or hourly averaged global radiation data. Clear-sky global radiation is estimated from the data measured. Estimated values from each day are added up azimuth-wise. These totals are normalized with a similar quantity calculated from a horizontal clear-sky model. The variation of the normalized values is analyzed to detect the tilt. The following three conditions are required for the successful use of the method: (1) reference data for building a horizontal clear-sky model should be available; (2) the cloud climatology should allow enough clear-sky so that the clear-sky global radiation can be estimated via quantile regression; (3) the direction of the tilt of the tested pyranometer should not change during the evaluation period. The method is tested on horizontal and tilted global radiation calculated from direct, diffuse and reflected radiation measured in Budapest, Hungary. The method can be a useful operational tool for controlling the quality of global radiation data.