  Collisionless shocks are known to induce turbulence via upstream ion
reflection within the precursor region. This study elucidates the properties of
electrostatic and electromagnetic instabilities, exploring their role over the
transrelativistic range. Notably, the growth of oblique Buneman waves and
previously overlooked, secondary small-scale ion-electron instabilities is
observed to be particularly promoted in the relativistic regime. Furthermore,
the growth of large-scale electromagnetic modes encounters severe limitations
in any cold, baryon-loaded precursor. It is argued that electron pre-heating in
electrostatic modes may alleviate these constraints, facilitating the
subsequent growth of large-scale filamentation modes in the precursor of
transrelativistic shocks.
