Clouds influence precision and accuracy of ground-based spectroradiometers

The objectives of this study were to determine the precision and accuracy, under field conditions, of two commonly used ground-based spectroradiometers and to propose guidance on how to minimize system errors. Sunlight irradiance and reflected radiance were measured on calibration tarps (3.6% and 52% reflectance) on 6 days using a CropScan MSR 16 handheld multispectral radiometer and a Fieldspec model FR hyperspectral radiometer during 2002. Radiance and irradiance were corrected for temperature and sun angle and converted to percent reflectance. Analysis showed that variances of the reflectance values for both radiometers increased with cloud cover. These results were attributed to several factors. First, cloud cover produced atmospheric conditions that made irradiance highly variable. Under these conditions, if reflected light is calculated by dividing radiance from target by radiance from a known standard, which is only periodically measured, then the calculated reflectance value may contain errors. Second, the reduction of diffuse irradiance by increasing cloud cover may introduce errors into reflectance calibration. Third, the relationship between incident irradiance, reflection of surface, and sensor efficiency may not be linear, and therefore, calculated reflectance can be variable when incident irradiance is variable. Results from this study showed that 1) the field measurements must be conducted under similar conditions at a similar time, 2) both sensors must be calibrated before and after measurements with reference panel, with ample time for device warm up, 3) measured reflectance should be corrected with reflectance from a reference panel, and 4) for the FieldSpec, reflectance measurements can be improved by simultaneously measuring radiance from the target and a known standard.