Well-posedness of the scale-invariant tetrad formulation of the vacuum   Einstein equations
  We show that with a small modification, the formulation of the Einstein equations of Uggla et al, which uses tetrad variables normalised by the expansion, is a mixed symmetric hyperbolic/parabolic system. Well-posedness of the Cauchy problem follows from a standard theorem. 
David Garfinkle and Carsten Gundlach