1. Department of Earth and Planetary Sciences and Department of Physics, Massachusetts Institute of Technology, Cambridge 02139
2. Haystack Observatory, Westford, Massachusetts 01886
3. Max Planck Institut für Radioastronomie, Bonn, Federal Republic of Germany
4. Jet Propulsion Laboratory, Pasadena, California 91125
5. NASA/Instituto Nacional de Tecnica Aeroespacial, Madrid, Spain