1. For a comprehensive review, including an extensive set of references to earlier work, seeP. L. Hanst, ?Advances in Environmental Science and Technology?, Volume 2, edited by James N. Pitts and Robert L. Metcalf (John Wiley, New York, 1971) chapter 4.
2. E. D. Hinkley andP. L. Kelley,Science 171 (1971) 635?639.
3. E. D. Hinkley andR. H. Kingston, Proceedings of the Joint Conference on Sensing of Environmental Pollutants, Palo Alto, California, 8?10 November, 1971.
4. H. Kildal andR. L. Byer,Proc. IEEE 59 (1971) 1644?1663.
5. I. Melngailis,IEEE Trans. on Geoscience Electronics GE-10 (1972) 7?17.