V. E. Kupriyanov - Ph.D. (Eng.), Associate Professor, Russian Public Service Academy (Vladimir branch). Е-mail: vecvladub@mail.ru

Currently in practice, development, design and manufacturing of modern unmanned aerial vehicles (UAVs ) for special purposes ( CH ) increasingly passive navigation methods based on the use of so-called anomalous geophysical fields the earth's surface . These fields include, for example , and the field of radio , which are used for high-precision positioning and CH UAV based on the so-called reference maps of the physical fields. Comparison of the observed geophysical field with its benchmark performed on-board navigation system by finding the extremum correlation functional , and that was the assignment of such systems, called correlation-extremal navigation systems ( KESN ). Radiothermal field underlying the earth's surface , in practice, very often are very informative fields and allow a passive manner , which is very important for the UAV CH remove current maps underlying the earth's surface that are required to correct the flight path of the UAV KESN CH . Sensors information on the underlying ground surface in CH UAV using Radiothermal fields are radiometers ( RM) , which , like any technically complex system has its advantages and disadvantages. One measure of restricting the ability of RM in KESN UAV CH, is the range of heights at which it can operate reliably , in particular its lower altitudinal limit of application. Therefore, this paper proposes a method of estimating the lower altitudinal limit of application of radiometric KESN UAV CH and presents the main results of mathematical transformations to obtain the expression of the normalized correlation function of the received signal at the output of radio optimum radiometric detector.