IMAGING AND MTI PROCESSING BASED ON DUAL-FREQUENCIES DUAL-APERTURES SPACEBORNE SAR

Based on dual-frequencies dual-apertures spaceborne SAR （Synthetic Aperture Radar）, a new SAR system with four receiving channels and two operation modes is presented in this paper, SAR imaging and Moving Target Indication （MTI） are studied in this system. High resolution imaging with wide swath is implemented by the Mode Ⅰ, and MTI is completed by the Mode Ⅱ. High azimuth resolution is achieved by the Displaced Phase Center （DPC） multibeam technique. And the Coherent Accumulation （CA） method, which combines dual channels data of different carrier frequency, is used to enhance the range resolution. For the data of different carrier frequency, the two aperture interferometric processing is executed to implement clutter cancellation, respectively. And the couple of clutter suppressed data are employed to implement Dual Carrier Frequency Conjugate Processing （DCFCP）, then both slow and fast moving targets detection can be completed, followed by moving target imaging. The simulation results show the validity of the signal processing method of this new SAR system.

Understanding the spatial interaction of ultrasounds based on three-dimensional dual-frequency ultrasonic field numerical simulation

A transient 3D model was established to investigate the effect of spatial interaction of ultrasounds on the dual-frequency ultrasonic field in magnesium alloy melt.The effects of insertion depth and tip shape of the ultrasonic rods,input pressures and their ratio on the acoustic field distribution were discussed in detail.Additionally,the spacing,angle,and insertion depth of two ultrasonic rods significantly affect the interaction between distinct ultrasounds.As a result,various acoustic pressure distributions and cavitation regions are obtained.The spherical rods mitigate the longitudinal and transversal attenuation of acoustic pressure and expand the cavitation volume by 53.7%and 31.7%,respectively,compared to the plate and conical rods.Increasing the input pressure will enlarge the cavitation region but has no effect on the acoustic pressure distribution pattern.The acoustic pressure ratio significantly affects the pressure distribution and the cavitation region,and the best cavitation effect is obtained at the ratio of 2:1(P15:P20).

Degradation of 4-Chlorophenol Solution by Synergetic Effect of Dual-frequency Ultrasound with Fenton Reagent

4-Chlorophenol (4-CP) solution was treated by dual-frequency ultrasound inconjunction with Fenton reagent, and obvious improvement in the 4-CP degradation rate was observedin this advanced oxidation process. Experimental results showed that ultrasonic intensity,saturating gas and pH value affected greatly the 4-CP removal rate. Among four different saturatinggases (Ar, O_2, air and N_2), 4-CP degradation with Ar-saturated solution was the best. However, inthe view of practical wastewater treatment, using oxygen as the saturating gas would be moreeconomical. The addition of Fenton reagent followed the first-order kinetics and increased the 4-CPdegradation rate. The 4-CP removal rate increased by around 126% within 15 min treatment. Thesynergetic effect of dual-frequency ultrasound with Fenton reagent on 4-CP degradation was obviouslyobserved.

Design and preliminary test of a 105/140 GHz dual-frequency MW-level gyrotron

A dual-frequency(105/140 GHz)MW-level continuous-wave gyrotron was developed for fusion application at Institute of Applied Electronics,China Academy of Engineering Physics.This gyrotron employs a cylindrical cavity working in the TE18,7 mode at 105 GHz and the TE24,9 mode at 140 GHz.A triode magnetron injection gun and a built-in quasi-optical mode converter were designed to operate at these two frequencies.For the proof-test phase,the gyrotron was equipped with a single-disk boron nitride window to achieve radio frequency output with a power of~500 k W for a short-pulse duration.In the preliminary short-pulse proof-test in the first quarter of2021,the dual-frequency gyrotron achieved output powers of 300 k W at 105 GHz and 540 k W at140 GHz,respectively,under 5 Hz 1 ms continuous pulse-burst operations.Power upgrade and pulse-width extension were hampered by the limitation of the high-voltage power supply and output window.This gyrotron design was preliminarily validated.

Analytical Expressions of Dual-Frequency Plasma Diagnostic Theory

Based on the fundamental ideas concerning microwave attenuation in plasma, we obtain a new expression of transmission attenuation of microwaves as a function of the incident wave frequency. And with reasonable hypothesis, analytical forms of the electron density and the electron-neutral collision frequency are derived from the equations of the transmission attenuation of microwaves at two near frequencies. This method gives an effective and easy approach to diagnose the unmagnetized plasma.

Computation of satellite clock-ephemeris augmentation parameters for dual-frequency multi-constellation satellite-based augmentation system

Dual-frequency multi-constellation(DFMC) satellitebased augmentation system(SBAS) does not broadcast fast correction, which is important in reducing range error in L1-only SBAS.Meanwhile, the integrity bound of a satellite at low elevation is so loose that the service availability is decreased near the boundary of the service area. Therefore, the computation of satellite clockephemeris(SCE) augmentation parameters needs improvement.We propose a method introducing SCE prediction to eliminate most of the SCE error resulting from global navigation satellite system GNSS broadcast message. Compared with the signal-inspace(SIS) after applying augmentation parameters broadcast by the wide area augmentation system(WAAS), SIS accuracy after applying augmentation parameters computed by the proposed algorithm is improved and SIS integrity is ensured. With global positioning system(GPS) only, the availability of category-I(CAT-I)with a vertical alert level of 15 m in continental United States is about 90%, while the availability in the other part of the WAAS service area is markedly improved. With measurements made by the stations from the crustal movement observation network of China,users in some part of China can obtain CAT-I(vertical alert limit is 15 m) service with GPS and global navigation satellite system(GLONASS).

Optical feedback characteristics in a dual-frequency laser during laser cavity tuning

The optical feedback characteristics in a Zeeman-birefringence dual-frequency laser are studied during the laser cavity tuning in three different kinds of optical feedback conditions： （i） only //-light is fed back; （ii） only ⊥-light is fed back; （iii） both lights are fed back. A compact displacement sensor is designed using the experimental result that there is a nearly 90 degrees phase delay between the two lights＇ cosine optical feedback signals when both lights are fed back into the laser cavity. The priority order that the two lights＇ intensity curves appear can be used for direction discrimination. The resolution of the displacement sensor is at least 79 rim, and the sensor can discriminate the target＇s moving direction easily.

DEFORMATION MEASUREMENT USING DUAL-FREQUENCY PROJECTION GRATING PHASE-SHIFT PROFILOMETRY

2π phase ambiguity problem is very important in phase measurement when a deformed object has a large out of plane displacement. The dual-frequency projection grating phaseshifting profilometry （PSP） can be used to solve such an issue. In the measurement, two properchosen frequency gratings are utilized to synthesize an equivalent wavelength grating which ensures the computed phase in a principal phase range. Thus, the error caused by the phase unwrapping process with the conventional phase reconstruct algorithm can be eliminated. Finally, experimental result of a specimen with large plastic deformation is given to prove that the proposed method is effective to handle the phase discontinuity.

EXPERIMENTAL STUDY ON SOIL MOISTURE USING DUAL-FREQUENCY MICROWAVE RADIOMETER

An experiment of measuring soil moisture was carried out by using dual-frequency microwave radiometer designed by the authors. The measured data were analyzed by using statistical regression method and the empirical regression model of retrieving soil moisture in L-band and C-band was developed. The soil moisture in a rainfall event was retrieved using the experiential regression model, which is consistent well with the field sampling value. The results show that when soil moisture is lower than 75%, the brightness temperature is linear with soil moisture. However, when soil moisture is higher than 75%, the brightness temperature is not linear with soil moisture, so it is difficult for microwave radiometer to measure the changes of soil moisture. The experiment verifies the effectiveness and feasibility of microwave remote sensing soil moisture. Although this method for linear regression based on the data measured with the radiometer is simple, and has strong adaptability, generally it has only local application value, and lacks universal applicability for different areas and different conditions.

The Performance of Dual-Frequency Polarimetric Scatterometer in Sea Surface Wind Retrieval

The wind retrieval performance of HY-2 A scanning scatterometer operating at Ku-band in HH and VV polarizations has been well evaluated in the wind speed range of 0–25 m s^-1.In order to obtain more accurate ocean wind field,a potential extension of dual-frequency(C-band and Ku-band)polarimetric measurements is investigated for both low and very high wind speeds,from 5 to 45 m s^-1.Based on the geophysical model functions of C-band and Ku-band,the simulation results show that the polarimetric measurements of Ku-band can improve the wind vector retrieval over the entire scatterometer swath,especially in nadir area,with the wind direction root-mean-square error(RMSE)less than 12?in the wind speed range of 5–25 m s^-1.Furthermore,the results also show that C-band cross-polarization plays a very important role in improving the wind speed retrieval,with the wind speed retrieval accuracy better than 2 m s^-1 for all wind conditions(0–45 m s^-1).For extreme winds,the C-band HH backscatter coefficients modeled by CMOD5.N(H)and the ocean co-polarization ratio model at large incidence are used to retrieve sea surface wind vector.This result reveals that there is a big decrease of wind direction retrieval RMSE for extreme wind fields,and the retrieved result of C-band HH polarization is nearly the same as that of C-band VV polarization for low-to-high wind speed(5–25 m s^-1).Thus,to improve the wind retrieval for all wind conditions,the dual-frequency polarimetric scatterometer with C-band and Ku-band horizontal polarization in inner beam,and C-band horizontal and Ku-band vertical polarization in outer beam,can be used to measure ocean winds.This study will contribute to the wind retrieval with merged satellites data and the future spaceborne scatterometer.

Difference-frequency ultrasound generation from microbubbles under dual-frequency excitation

The difference-frequency （DF） ultrasound generated by using parametric effect promises to improve detection depth owing to its low attenuation, which is beneficial for deep tissue imaging. With ultrasound contrast agents infusion, the harmonic components scattered from the microbubbles, including DF, can be generated due to the nonlinear vibration. A theoretical study on the DF generation from microbubbles under the dual-frequency excitation is proposed in formula based on the solution of the RPNNP equation. The optimisation of the DF generation is discussed associated with the applied acoustic pressure, frequency, and the microbubble size. Experiments are performed to validate the theoretical predictions by using a dual-frequency signal to excite microbubbles. Both the numerical and experimental results demonstrate that the optimised DF ultrasound can be achieved as the difference frequency is close to the resonance frequency of the microbubble and improve the contrast-to-tissue ratio in imaging.

Effect of Low-frequency Power on F, CF_2 Relative Density and F/CF_2 Ratio in Fluorocarbon Dual-Frequency Plasmas

Effect of low-frequency power on F, CF2 relative density and F/CF2 ratio, in C2F6, C4F8 and CHF3 dual-frequency capacitively couple discharge driven by the power of 13.56 MHz/2 MHz, was investigated by using optical emission spectroscopy. High F, CF2 relative density and high F/CF2 ratio were obtained in a CHF3 plasma. But for C2F6 and C4Fs plasmas, the F, CF2 relative density and F/CF2 ratio all decreased significantly due to the difference in both reactive paths and reactive energy. The increase of LF power caused simultaneous increase of F and CF2 radical relative densities in C4Fs and CHF3 plasmas, but led to increase of F with the decrease in CF2 relative densities in C2F6 plasma due to the increase of lower energy electrons and the decrease of higher energy electrons in electron energy distribution function （EEDF）.

Effect of Low-Frequency Power on Etching Characteristics of 6H-SiC in C4F8/Ar Dual-Frequency Capacitively Coupled Plasma

Dry etching of 6H silicon carbide （6H-SiC） wafers in a C4Fs/Ar dual-frequency capacitively coupled plasma （DF-CCP） was investigated. Atomic force microscopy （AFM） and X-ray photoelectron spectroscopy （XPS） were used to measure the SiC surface structure and compositions, respectively. Optical emission spectroscopy （OES） was used to measure the relative concentration of F radicals in the plasma. It was found that the roughness of the etched SiC surface and the etching rate are directly related to the power of low-frequency （LF） source. At lower LF power, a smaller surface roughness and a lower etching rate are obtained due to weak bombardment of low energy ions on the SiC wafers. At higher LF power the etching rate can be efficiently increased, but the surface roughness increases too. Compared with other plasma dry etching methods, the DF-CCP can effectively inhibit CχFγ films＇ deposition, and reduce surface residues.

Improved Double-Probe Technique for Spatially Resolved Diagnosis of Dual-Frequency Capacitive Plasmas

The conventional double-probe technique was improved with a combination of selfpowering and radio-frequency（RF） choking.RF perturbations in dual-frequency capacitively coupled discharge were effectively eliminated,as judged by the disappearance of self-bias on the probes.The improved technique was tested by spatially resolved measurements of the electron temperature and ion density in both the axial and radial directions of a dual-frequency capacitive plasma.The measured data in the axial direction were compared with simulation results,and they were excellently consistent with each other.The measured radial distributions of the ion density and electron temperature were influenced significantly by the lower frequency（LF） power.It was shown that superposition of the lower frequency to the higher frequency（HF） power shifted the maximum ion density from the radial center to the edge region,while the trend for the electron temperature profile was the opposite.The changing feature of the ion density distribution is qualitatively consistent with that of the optical emission intensity reported.

Investigation of Capacitively Coupled Argon Plasma Driven by Dual-Frequency with Different Frequency Configurations

Low pressure argon dual-frequency （DF） capacitively coupled plasma （CCP） is generated by using different frequency configurations, such as 13.56/2, 27/2, 41/2, and 60/2 MHz. Characteristics of the plasma are investigated by using a floating double electrical probe and optical emission spectroscopy （OES）. It is shown that in the DF-CCPs, the electron temperature Te decreases with the increase in exciting frequency, while the onset of 2 MHz induces a sudden increase in Te and the electron density increases basically with the increase in low frequency （LF） power. The intensity of 750.4 nm emission line increases with the LF power in the case of 13.56/2 MHz, while different tendencies of line intensity with the LF power appear for other configurations. The reason for this is also discussed.

Effect of Discharge Parameters on Properties of Diamond-Like Carbon Films Prepared by Dual-Frequency Capacitively Coupled Plasma Source

Diamond-like carbon （DLC） films were prepared with CH4-Ar using a capacitively coupled plasma enhanced chemical vapor deposition （CCP-CVD） method driven by dual-frequency of 41 MHz and 13.56 MHz in combination. Due to a coupling via bulk plasma, the self-bias voltage depended not only on the radiofrequency （RF） power of the corresponding electrode but also on another RF power of the counter electrode. The influence of the discharge parameters on the deposition rate, optical and Raman properties of the deposited films was investigated. The optical band decreased basically with the increase in the input power of both the low frequency and high frequency. Raman measurements show that the deposited films have a maximal sp3 content with an applied negative self-bias voltage of -150 V, while high frequency power causes a continuous increase in the sp3 content. The measurement of atomic force microscope （AFM） shows that the surface of the deposited films under ion-bombardment becomes smoother than those with non-intended self-bias voltage.

Deposition of organosilicone thin film from hexamethyldisiloxane(HMDSO)with 50kHz/33MHz dual-frequency atmospheric-pressure plasma jet

The deposition of organosilicone thin films from hexamethyldisiloxane（HMDSO） by using a dual-frequency （50 kHz/33 MHz） atmospheric-pressure micro-plasma jet with an admixture of a small volume of HMDSO and Ar was investigated.The topography was measured by using scanning electron microscopy.The chemical bond and composition of these films were analyzed by Fourier transform infrared spectroscopy （FTIR） and x-ray photoelectron spectroscopy.The results indicated that the as-deposited film was constituted by silicon,carbon,and oxygen elements,and FTIR suggested the films are organosilicon with the organic component （-CHx） and hydroxyl functional group（-OH） connected to the Si-O-Si backbone.Thin-film hardness was recorded by an MH-5-VM Digital Micro-Hardness Tester.Radio frequency power had a strong impact on film hardness and the hardness increased with increasing power.

Study of numerical simulation on dual-frequency IP method with FEM

Using the finite element method and Cole-Cole model for dual-frequency IP method to research numerical simulation, the authors introduced the fundamental principle of the dual-frequency IP method and the boundary value problem and variational equations, then replaced the complex resistivity of the model with the Cole-Cole model's parameters under ignoring the EM effect. Through solving the last linear equations, electric potentials of all the model's points were obtained. With changing model's parameters, the authors got different curves of the Fs and phases. According to the results of the simulation, the algorithm is proved to be correct and adaptable.

Optimization of Nonlinear Energy Sink for Vibration Suppression of Systems Under Dual-Frequency Excitation

Aiming to decrease the vibration of wing induced by dual-rotor civil turbofan engines,the dynamic models of a single-degree of freedom (DOF) linear main oscillator coupled with single-DOF and two-DOF nonlinear energy sink (NES) are established.According to the related energy criteria for the optimization of the dynamic vibration absorber,focusing on the effects of external excitation on the kinetic energy of the primary mass and total system energy,the vibration suppression effects of single-DOF,two-DOF serial and parallel NES on the main oscillator system are studied.Under the condition that the characteristic parameters of the main oscillator system and additional total mass of the vibration absorber remain unchanged,results show that the two-DOF parallel NES has the best vibration energy suppression effects,which can provide data reference for the optimal design of NES vibration suppression under dual-frequency excitation.

A Physics-Based Dual-Frequency Approach for Altimeter Wind Speed Retrieval

The altimeter normalized radar cross section(NRCS) has been used to retrieve the sea surface wind speed for decades, and more than a dozen of wind speed retrieval algorithms have been proposed. Despite the continuing efforts to improve the wind speed measurements, a bias dependence on wave state persists in all wind algorithms. On the basis of recent evidence that short waves are essentially modulated by local winds and much less affected by wave state, we proposed a physics-based approach to retrieve the wind speed from the dual-frequency difference in terms of the mean square slope of short waves. A collocated dataset of coincident altimeter/buoy measurements were used to develop and validate the approach. Validation against buoy measurements indicates that the approach is almost unbiased and has an overall root mean square error of 1.24 m s-1, which is 5.3% lower than the single-parameter algorithm in operational use(Witter and Chelton, 1991) and 2.4% lower than another dual-frequency approach(Chen et al., 2002). Furthermore, the results indicate that the new approach significantly improves the wave-dependent bias compared to the single-parameter algorithm. The capacity of altimeter to retrieve sea surface wind speed appears to be limited for the case of winds below 3 m s-1. The validity of the approach at high winds needs to be further examined in the future study.