docs: def _get_reference_band(self, data):
          """ Extract the NDVI band from time-series
  
          :param data: 4D array from which to compute the NDVI reference band
          :type data: numpy array
          :return: 3D array containing the NDVI reference band
          """
          nir = data[..., self.nir_idx].astype("float32")
          red = data[..., self.red_idx].astype("float32")
          return (nir - red) / (nir + red)