We propose a new and simple procedure to overcome the ambiguity in the determination of optical constants of thin absorbing films from spectroscopic reflectance and transmittance measurements. The basis for the proposed method is an error analysis with the help of an error simulation technique and an error variation technique. We show that in practice (owing to experimental errors) it is not possible to overcome the problem of ambiguity by normal-incidence spectroscopic measurements alone. At least one oblique-incidence measurement is necessary for unambiguously determining the optical constants of the film. We discuss the consequences of experimental errors of the measured transmittance and reflectance values for the determination of the optical constants.

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A proposal is made for improvement of J. A. Pierce's composite wave technique for the reduction of diurnal and solar flare effects on OMEGA navigation signals. Based on calculations of a fictitious free‐space transmission time Tc and multi‐frequency phase measurements, a general formalism is presented for the compensation of phase shifts due to changes in the earth‐ionosphere waveguide parameters. In principle, phase measurements at (n + 1) discrete frequencies are necessary to compensate for variations in n parameters such as reference height, ionospheric conductivity gradient, etc. For n = 1 (compensation of height changes), explicit expressions for Tc are derived from various models of the earth‐ionosphere waveguide using phase measurements recorded at the basic OMEGA frequencies of 10.2 and 13.6 kHz. For a flat earth with perfectly conducting boundaries, the results are very similar to those of Pierce. More realistic waveguide models (spherical earth with exponential ionosphere) result in a considerable reduction of diurnal shifts and anomalies due to solar disturbances. It is also shown that no further improvements result from the addition of a third signal because of unrealistic accuracy requirements for phase measurements.

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