Based on the rich selection of wavelengths and small detection blind zone of LED light source radar, the atmospheric remote sensing lidar system with LED light source can achieve the observation of aerosols at near range and special wavelengths. To reduce the volume of radar system and realize portable observation of atmospheric condition, a compact and easy-to-use atmospheric lidar is designed, which receives echo signals from Fresnel lens. The geometric overlap factor is calculated and analyzed according to coaxial radar transmission and reception principles, and the detection blind zone less than 2 m can be confirmed. The system model is constructed by ZEMAX simulation, and the light intensity distribution and light energy utilization rate of the radar transmission system are obtained. At the same time, by calculating the signal-to-noise ratio, the aerosol detection capability is evaluated and the maximum detection distance is obtained. The simulation results show that three types of LED light source radars which single-pulse energies are 96 nJ, 30 nJ and 105 nJ, wavelengths are 475 nm, 530 nm and 625 nm respectively, can reach the detection distance of 480 m, 200 m and 280 m. It verifies that the compact and easy-to-use LED light source radar system has the ability to detect near-ground atmospheric aerosols.
In order to achieve a large area of deep soil moisture monitoring, a soil moisture sensor based on fiber Bragg grating was designed. The polyimide with high water sensitivity and high linear expansion coefficient was coated to the surface of FBG. The center wavelength of the FBG will change due to moisture absorption and expansion of the polyimide material when moisture changes. Using this basis principle, the soil moisture can be measured. A stainless steel tube with a hole is used as a sensor package and wrapped in polyurethane nets to maintain consistent with the external temperature and moisture environment and prevent soil from entering the sensor. A fiber Bragg grating without a PI coating layer was packaged in the same package structure with the soil moisture sensor as a reference fiber for temperature compensation. An experimental system was set up, calibration experiments were performed on the sensors, and various performances of the sensors were tested. The experimental results show that the measuring range of moisture is 15%RH~75%RH, the sensitivity is 12.6pm/%RH, and the accuracy of the sensor is ±10.26%.
In order to alleviate the ill-posed problem of the ill-conditioned equation, the regularization algorithm combined with averaging method is used to retrieve the aerosol particle size distribution. In this method, the selection of the optimal average interval is crucial. According to the spectral of LED, fourteen wavelengths for the LEDs are selected, and different optical parameter combinations are set. By the retrieval of multiple sets of original volume concentration distribution and error statistics, the optimal average interval of each optical parameter combination is obtained. The retrieval simulations of logarithmic-normal volume concentration distribution were completed. The simulation results show that when using a combination of ten backscatter and ten extinction coefficients, the reconstructed monomodal distribution has the best fitting effect, and the relative error of effective radius and volume are the smallest. And when using a combination of six backscatter and six extinction coefficients, the reconstructed bimodal distribution has the best fitting effect, and the relative error of effective radius and volume are the smallest.
A lysimeter weighing system based on fiber Bragg grating (FBG) sensor for measuring the soil water evaporation was presented in this paper. By the use of three mechanical levers and balance weight, the weight loaded on the FBG sensor was reduced K times (here, K was the ratio of levers). So the amount of water change in the soil container of tons can be weighted. A two-hole cantilever was selected as the elastomer structure of FBG weighing sensor, and an optimum design was carried on using the finite element method to meet the small-scaled design requirements. Using the matching fiber Bragg grating demodulation method based on LabVIEW, the demodulation system was easy to be implemented. Then the FBG center wavelength drift was converted into a time interval, and the weight can be obtained automatically through measuring the interval by computer. Preliminary experiment showed that this weighing system has the ability of measuring soil water evaporation accurately.
A LED lidar system program was proposed for atmospheric aerosol detection. Compared with the conventional lidar using the laser, this system employs LED as the detection light source. Due to the wealth wavelength of LED, LED lidar achieves aerosol detection at a specific wavelength, and it makes the detection of aerosol particle size distribution easier. This paper presents a pulse modulation system for high-power LED light source. The system consists of pulse generator, narrow pulse driving circuit, and high-power LED array. A FPGA system is designed to generate the pulse whose frequency is adjusted by remote control. Using avalanche switching characteristics, the avalanche transistor circuit produces ultra-narrow pulses to drive the high-power LED array. Using U.S. standard atmospheric model, the detection capability of LED lidar system was simulated. Simulation result shows that the designed LED pulse light source used for lidar can meet the requirements of lower atmospheric aerosol detection.
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