These instruments also share a common drawback: They are difficult to calibrate. To measure the radiance received in a given frequency band each instrument must have some absolute standard of comparison. Thus they generally carry onboard calibration sources and employ other strategies, such as observing cold sky. Ultimately, however, their temperature and moisture measurements must be corrected to some reliable reference standard, such as weather balloons, which can compromise their independence. In the end, each instrument has its own distinct and varying biases.