This research is partially supported by ARO Grant DAAL03-89-C-0031 including participation by the U.S. Army Research Laboratory (Aberdeen), Natick Laboratory, the Army Research Institute, and NASA Ames Research Center; U.S. Air Force DEPTH contract through Hughes Missile Systems F33615-91-C-0001; DARPA through General Electric Government Electronic Systems Division; and NSF CISE Grant CDA88-22719.