code: def _G_calc(zenith, a_coeff):
      return (
          np.cos(np.deg2rad(zenith))
          + (
              a_coeff[0]
              * (zenith ** a_coeff[1])
              * (a_coeff[2] - zenith) ** a_coeff[3]
          )
      ) ** -1
  
imports: from __future__ import annotations
  import logging
  from typing import Optional, Type, Union
  import dask.array
  import numpy
  import xarray
  from satpy.dataset.dataid import WavelengthRange
label_desc: Use image node_id 6 for calling a global function with example usage: _G_calc(zenith, a_coeff) and returns: unknown