# 6.4.3 Calibration and trending

The wavelength calibration coefficients and the $G_{\rm RVS}$ zero point, are derived for every calibration unit, according to the calibration models (see Section 6.3). The duration of a CaU is 30 hours to ensure that most CaUs include RVS observations of auxiliary data, which the calibration requires. The trending consists of fitting a function with respect to time to each calibration coefficient. To have a trend function with respect to time means that it does not matter if the calibration in any one CaU does not produce coefficients, as long as this is not too often. Moreover, fitting a function with outlier rejection allows the trend functions to be more robust against noise and outliers.