A. A. Vasenina, Yu. K. Sveshnikov

Sounding radio-engineering systems with a pulse signal are replaced by ionosondes using chirp signals. The most considerable advantages of a chirp ionosonde are as follows: higher noise immunity of a probing radio link, low powers of signal radiation, considerably smaller dimensions and weight of the equipment. The use of low power, compact chirp ionosondes allows solving the problem of the optimal operating frequencies (OOF) automatic short-term forecasting and gives the possibility to extrapolate OOF values for the future. The problem of constructing the distance-frequency characteristic (DFC) based on HF link backscatter ionospheric sounding data is considered. The practical importance to backscatter ionospheric sounding approach is given by the definition of the maximum usable frequency (MUF). The paper presents an ionogram received on the proving ground of Irkutsk, on which in the range from 2 up to 6 MHz characteristic image for vertical sounding (VS) is observed, and in the range from 6 up to 11 MHz - experimental data based on backscatter ionospheric sounding. Such combined ionogram is obtained in the case of the narrow-beam focused on an azimuth receiving antenna which on the elevation angle is focused in zenith by a sidelobe. The model of the flat Earth at a parabolic reflecting layer of ionosphere is considered. For the given case the equivalent length of the full way of a beam and the corresponding distance in terms of the ground is calculated. From the presented expressions it is visible that the minimum distance defined in terms of the beam and describing the DFC depends linearly on frequency, i.e. in the case of the flat Earth the DFC is a straight line and it is one of the features of the DFC. This feature defines a choice of approximation of empirical data for the case of the flat Earth. The figure simultaneously displays empirical data of the backscatter ionospheric sounding, extrapolation of empirical data and result of theoretical calculations of the minimum distance in terms of the ground based on VS for model of the flat Earth and Ionosphere. From the presented results it is possible to draw the following conclusions: at insufficient range to the reflection zone of the backscatter ionospheric sounding at the assignment of the OOF it is possible to extrapolate the approximated line up to the demanded range; correction of the MUF in the range up to 2000 km based on VS by the magnitude not less than 10% on frequency is expected; distance deviation from the theoretically calculated DFC does not exceed 15 % for the ranges up to 2000 km at the offered approximation and subsequent extrapolation. The line of approximation of the reflected signals array is located close to the theoretical one and corresponds to MUF depending on the range. This conclusion follows from the method of detection of the reflected signals, namely rank detection in terms of the maximum amplitudes, and it is checked experimentally that HF link amplitude-frequency response has the abruptly falling front, approximately 6 dB per 10% on frequency towards the increase relative to the maximum. Novelty of the stated approach consists in simultaneous use of results based on vertical sounding and backscatter ionospheric sounding for the definition of parameters of the HF radio link, such as MUF and OOF. The practical value of the approach is the possibility to assign the OOF from the permitted operating frequencies list not on the monthly forecast of the propagation having a very big error on MUF (up to 30%) but on the basis of a tool method of measurement of the MUF at the reflection point at backscatter ionospheric sounding on the interval of forecasting of 5 minutes. Therefore, there is a possibility to have only one active ionosonde in a service zone of the regional center. Thus, switching HF narrow-beam receiving/transmitting antennas it is possible to receive a full picture of the propagation over several, depending on the number of antennas, azimuths in the communication sector.