  A study of archival RXTE, Swift, and Suzaku pointed observations of the
transient high mass X-ray binary GRO J1008-57 is presented. A new orbital
ephemeris based on pulse arrival timing shows times of maximum luminosities
during outbursts of GRO J1008-57 to be close to periastron at orbital phase
-0.03. This makes the source one of a few for which outburst dates can be
predicted with very high precision.
  Spectra of the source in 2005, 2007, and 2011 can be well described by a
simple power law with high energy cutoff and an additional black body at lower
energies. The photon index of the power law and the black body flux only depend
on the 15-50 keV source flux. No apparent hysteresis effects are seen. These
correlations allow to predict the evolution of the pulsar's X-ray spectral
shape over all outbursts as a function of just one parameter, the source's
flux. If modified by an additional soft component, this prediction even holds
during GRO J1008-57's 2012 type II outburst.
