The paper deals with a two-mode antenna and analytic algorithm f o r diagnostics of plasma parameters designed for experimental study and prediction offlight characteristics of plasma on the basis of the results obtained during bench tests and flight experiments and available in the data base.An experimental approach to the study of plasma parameters implies the development of methods and apparatus for direct measurement of the characteristics of a spacecraft-toearth radio communication system during a spacecraft descent, including plasma sheath parameters, with subsequent determination of the energy potential of the radio communication system using experimental results. The analysis of the existing methods for diagnostics of plasma parameters has shown that the method of a probe diagnostics and radiometric method are the most effective.The essence of the radiometric method for the diagnostics of equilibrium isothermal plasma and heat protection materials is founded on measurements of their own noise radiation (radiobrigthness temperature). The diagnostics of plasma parameters by this method is conventionally subdivided into three variants: 1 -spectral diagnostics when measurements are carried out on one or several frequencie -measurements of a noise radiation on one frequency at the different modes of a multimode waveguide, 3 -interferometric and multibeam measurements of the noise radiation of a medium on one or several frequencies with the help of several antennas separated in space.The key features of a radiometric method are the ambiguity of the solution of an inverse problem for some domains of plasma parameters, the necessity of multichannel measurements (spectral, multimode and multibeam measurements) and a priori data to select a computational model, the spatial resolution of plasma parameters is defined by the number of radiometric channels.For diagnostics of parameters averaged over a plasma layer and also for diagnostics of the profile of parameters distribution by a radiometric method using an antenna window placed in plasma we propose the scheme of diagnostics application which is part of the method of an experimental study and consists of the set of experimental and logical operations. The procedure of the realisation of studies consists of the following operations.1. The selection of the plasma model which defines a functional dependence between a noise temperature of the antenna and the radio brightness temperature of plasma.2. The selection of one or several frequencies on which measurements will be independent, and solutions of an inverse problem will be with minimal errors.3. The selection of the type of a measuring antenna, the creation of additional external channels for measurement of the radio brightness temperature of plasma. 4.The selection of the type of a radiometric receiver. 5. The measurements of an antenna noise temperature.

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