Estimation of a characteristic of a user's physiological signals can be improved using one or more calibration relationships that may be dependent on a characteristic of the optical sensor. For example, different calibration relationships can be used that are dependent on a spatial characteristic and/or that are dependent on a wavelength characteristic of the light emitting component(s) of the respective emitter of a channel. In some examples, a unique calibration relationship can be used for each channel. In some examples, a common calibration relationship can be used for multiple channels with shared distance and/or wavelength characteristics. Utilizing distance-dependent and/or wavelength-dependent calibration relationships can improve robustness of pulse oximetry measurements.