| High frequency surface wave radar utilizes the physical characteristics of 3-30 MHz vertical polarization electromagnetic wave which can be diffracted by the coastal surface,realizes the detection of ships and low altitude aircraft beyond the horizon,and also has the function of sea state remote sensing.However,ionospheric clutter in radar echoes often occupies a large number of range-Doppler bins,and often leads to sea clutter broadening and raising the whole target detection base,which is the most important factor affecting the target detection performance of radar system and the accuracy of sea remote sensing.Therefore,many al gorithms based on radar signal processing have been studied to suppress ionospheric clutter.Ionospheric clutter is not only related to radar parameters,but also to the unique distribution characteristics of the ionosphere itself.To a large extent,ionos pheric clutter suppression technology depends on the understanding of its physical mechanism.Therefore,deepening and broadening the scientific understanding of the intrinsic mechanism of ionospheric clutter will contribute to a breakthrough in the research of ionospheric clutter suppression technology.In addition,ionospheric clutter originates from the ionospheric backscattering of electromagnetic waves,which naturally contains real-time ionospheric information.HFSWR has high Doppler resolution and can continuously observe the ionosphere for a long time,so it can be used to detect ionospheric distribution structure and disturbance state.In this paper,we intend to use ionospheric clutter as a useful source to explore the physical mechanism of ionospheric clutter,establish a mathematical model between ionospheric clutter and plasma irregularities,traveling ionospheric disturbances(TIDs),and then invert the characteristic parameters of the ionosphere,and exploratory study on ionospheric disturbance response during typhoons in the East China Sea and Bohai Sea.The main contents of this paper are as follows:1.Description of the most basic chemical reaction mechanism in the ionosphere and observation by HFSWR combined ionosonde.Firstly,the ionospheric layered structure,atmospheric composition and ionic chemical reaction are summarized.Especially,the reaction-diffusion partial differential equations describing the basic mechanism of ion loss process are studied.The space-time variations of electron density under different boundary conditions are simulated.After that,the software simulation of the actual receiving and receiving antenna of Weihai HFSWR shows that there is a correlation between the ionospheric clutter and the antenna system.Finally,based on the simultaneous observation of ionospheric clutter by HFSWR and ionosonde,the skywave propagation path,the distribution of O and X characteristic waves and the fluctuation of electron density of HFSWR ionospheric echoes are analyzed,which lays a foundation for the follow-up study of ionospheric echoes.2.The HFSWR ionospheric generalized distance equation based on the physical mechanism of irregular bodies is established.The ionospheric echo distance detection equation of HFSWR is established by taking the ionosphere as the surface and volume scattering distribution,and the ionospheric RCS is estimated based on the principle of high frequency coherent scattering.Then the power spectrum of ionospheric echo is simulated under different ionospheric state parameters and radar operating parameters,and the spatial distribution of irregularities is estimated.Finally,the electron density,plasma frequency and drift velocity of the irregularities in E layer of the vertical reflection path are estimated based on the radar measured data,and compared with the IRI model.3.Improve the mathematical model between FMCW/FMPCW system,backscattering propagation path ionospheric small-scale irregularities and HFSWR.Based on the electromagnetic scattering model proposed by Walsh,the irregularities model of the vertical HFSWR ionosphere in FMCW/FMPCW system is extended,and the ionospheric reflection coefficients are estimated respectively from the perspective of mathematical probability distribution and ionospheric physical mechanism.Secondly,the spectral density of ionospheric echo under oblique scattering path of FMCW/FMPCW is modeled,and the general expression of phase spectral density of irregularities in oblique scattering ionosphere at mid-latitude is derived.Through comprehensive and in-depth simulation,the key parameters that play a decisive role in HFSWR ionospheric echo intensity and Doppler frequency shift broadening are extracted.Finally,the average electron density fluctuation of the irregularities in F layer of oblique scattering propagation path is estimated by using the measured radar data combined with the ionosonde.4.Based on the measured HFSWR data,an exploratory study on the correlation mechanism between typhoon and ionosphere is carried out.Firstly,the theoretical model of large-scale TIDs and HFSWR in FMCW/FMPCW system is established.Then based on the measured data of HFSWR station during the strong typhoon " Megi " in the East China Sea in 2016 and typhoon " Rumbia " in Bohai Sea in 2018,the distribution characteristics of ionospheric echoes in the range,time and Doppler domains were studied,and the TIDs characteristic of quasi-periodic sinusoidal "S" in the ionospheric echoes was observed,which may be caused by the gravity waves induced by typhoon uploaded to the ionosphere.The physical mechanism of TIDs and the coupling effect between troposphere,stratosphere,mesosphere and ionosphere in atmospheric structure are consistent.It is also found that the Doppler characteristics of HFSWR ionospheric echoes during a typhoon are very significant,showing a "strip" uniform broadening distribution pattern in the Doppler bins,while the distribution pattern of ionospheric structural disturbances is not easily observed in the near distance of the typhoon.The research results in this paper are expected to deepen the scientific understanding of the physical mechanism of HFSWR ionospheric clutter,quantify the effects of ionospheric irregularities and TIDs on HFSWR,provide new theoretical support for the suppression of ionospheric clutter and the optimal frequency selection,provide new ionospheric detection techniques,and broaden the understanding of the scientific laws of geophysical phenomena.It has important practical engineering significance and higher value of natural science research. |
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