def _get_reference_band(self, data):
        """ Extract the NDWI band from time-series

        :param data: 4D array from which to compute the NDWI reference band
        :type data: numpy array
        :return: 3D array containing the NDWI reference band
        """
        nir = data[..., self.nir_idx].astype("float32")
        swir1 = data[..., self.swir1_idx].astype("float32")
        return (nir - swir1) / (nir + swir1)