The artificial finite outer boundary present in Cauchy codes introduce two sources of error: The outer boundary condition and waveform extraction at an inner worldtube. The problem of proper boundary condition for a radiating system can be solved only by extension to Ι+ (conformal compactification). Cauchy Characteristic Extraction (CCE) offers a means to avoid these errors.
Babiuc, M. C. (2008, September). Towards a fully nonlinear Cauchy-characteristic extraction. Slides presented at the Numerical Relativity Group Seminars, NASA Goddard Space Flight Center, Greenbelt, MD.