def parallaxErrorSkyAvg(G, vmini, extension=0.0):
  """
  Calculate the sky averaged parallax error from G and (V-I).

  Parameters
  ----------

  G     - Value(s) of G-band magnitude.
  vmini - Value(s) of (V-I) colour.

  Keywords
  --------

  extension - Add this amount of years to the mission lifetime and scale the errors accordingly.

  Returns
  -------

  The parallax error in micro-arcseconds.
  """
  factor = errorScalingMissionLength(extension, -0.5)
  z=calcZ(G)
  return sqrt(-1.631 + 680.766*z + 32.732*z*z)*(0.986 + (1.0 - 0.986)*vmini)*factor
Calculate the sky averaged parallax error from G and (V-I).

  Parameters
  ----------

  G     - Value(s) of G-band magnitude.
  vmini - Value(s) of (V-I) colour.

  Keywords
  --------

  extension - Add this amount of years to the mission lifetime and scale the errors accordingly.

  Returns
  -------

  The parallax error in micro-arcseconds.