Microwave spectral line model development: A retrospective in honor of Dr. Hans J. Liebe

The quantum mechanical basis for microwave spectral line models of oxygen and water vapor absorption as used in radio astronomy, Earth and planetary remote sensing, and telecommunications can be traced to the work of J.H. Van Vleck in 1934, and extended by Van Vleck and V.F. Weisskopf (1945), G. Herzberg (1950), then more fully studied by C.H. Townes and A.L. Schawlow (1955), M.W.P Strandberg (1954), and others. The extensive impact of spectral line models in radiowave propagation is ubiquitous, and seen in the design of virtually all microwave radar, radiometer, navigation, and communication, systems, as well as in many devices such as lasers, masers, fiber amplifiers, and atomic clocks used for precision timekeeping. However, the early spectral line models required a more complete empirical characterization of the absorptive and refractive line parameters for water vapor and oxygen than available using approximate solutions to the problem of interacting fields and distributions of gas molecules.