1. Abrami, A.: 1966,Mem. Soc. Astr. Ital. 37, 369?385.
2. Bryson, A. R. Jr. and Ho, Y. C.: 1969,Applied Optimal Control, Blaisdell Publishing Co., Waltham, Mass.
3. Bucy, R. S. and Joseph, P. D.: 1968,Filtering for Stochastic Processes wtih Applications to Guidance, Interscience Publishers, New York.
4. Catalano, S. and Rodon�, M.: 1968,Mem. Soc. Astr. Ital. 39, 617?623.
5. Catalano, S. and Rodon�, M.: 1970,Astron. Astrophys. 4, 173?179.