Manuscript received November 28, 1978; revised February 28, 1980. This work was supported in part by the National Science Foundation under Grant ENG 76-23334, in part by the Stanford Research Institute under International Contract D/&C-15-C-0187, and in part by the Joint Scientific Enaineerina Program under Contracts NO001475-C-0601 and F44620-76-C&01. This paper was presented at the 1979 IEEE ...