| Medium frequency (MF) radar is used to explore the mesosphere-lower thermosphere (MLT) region, which can provide some important data about the wind fields and electron densities of that region. At present, more than thirty MF radars which mainly locate at North America, Australia, Japan and Antarctic continent are running well all over the world.The data obtained from MF radar have been successfully applied to establishing the atmospheric model, researching the atmospheric dynamics, and providing the meteorological supports for aerospace engineering.Wuhan MF radar is the first one developed independently by Chinese researchers. It consists of all solid-state transmitters, the direct digital radar receivers, and a Y-shape antenna array with inverted V antenna elements. The radar can accomplish continuous observation under the remote control and supply the real-time data about horizontal wind field and electron densities ranged from 60 to 100km. Wuhan MF radar will be important for us to acquire some useful data about the MLT region. Besides, it could accumulate helpful design experience which is used to develop our observing instrument suitable for the middle and upper atmosphere.The general design and key technologies of Wuhan MF radar are discussed in detail. The results in this dissertation can be mainly outlined as the following:(Ⅰ) According the design requirements of Wuhan MF radar, the general design scheme, main subsystems design scheme, and some key technical indexes are proposed.After reviewing the related literature from foreign countries, technical characteristics of MF radar is analyzed carefully. Combined the results of dada analysis, the more reasonable and flexible parameters are proposed for Wuhan MF radar. This design will bring the risk reduction and the stable exploring quality, because it can flexibly adjust the operation parameters of radar according to the characteristics of atmospheric echoes and environment.The mixed programming is used for the design of subsystem of data-processing, which means that the data processing software and radar control software will be exploited on the separate platforms. It has two advantages as following:(1) ensure the stability of system by using the mature system in radar software, (2) keep the advancement of radar in a long time by developing some novel algorithms effectively.The flexibility and reliability of system are emphasized while designing the hardware system of radar. Considering the requirement of adjusting the parameters online, lots of digital equipments such as the digital radar receivers and the programmable DDS excitation source are used. Besides, more devices are applied for ensuring the long-time continuous running, including all solid-state transmitters, the full-distributed power combination, built in test equipment (BITE), and the locale replaceable units (LRU).(Ⅱ) Design and parameters analysis of antenna array in Wuhan MF radar.The antenna array of Wuhan MF radar is a Y shape with inverted V antenna elements. Some critical factors about the antenna array are discussed in detail and designed, such as Triangle Size Effect (TSE), pattern of antenna array, antenna impedance, coupling degree among the antenna elements, antenna polarization and so on.Two facts have been considered when analyzing the TSE:(1) the relationship of the scale of antenna arrays, the calibration error of cross-correlation function and wind estimate bias; (2) the effect of the scale of antenna arrays on the acceptance rate. The results indicate that in order to improve the acceptance rate of data, the size of antenna array must be smaller than the size of diffraction patterns. Under this limitation, the larger the size of antenna array, the more accurate the result of wind measurement. When the size of antenna array is 190m, the above condition can be satisfied in 95 percent of all examples. So the size of antenna array of Wuhan MF radar is advised to be 190m.The analyses on the pattern of antenna array, antenna impedance and the coupling degree among the antennas are accomplished by FEKO. The results show that the higher the central and auxiliary antenna mast, the higher the antenna gain. However, the height difference of central and auxiliary antenna mast can not be over tall otherwise the vertical lobe of antenna array will split. What is more, the imaginary parts of antenna impedance will be affected by the length of antenna elements. So the proper parameters of antenna array are recommended as following,20m for the central antenna mast,6m for the auxiliary antenna mast, slightly less thanλ/2 for the length of antenna element. The average coupling degree of Y-shaped antenna array with inverted V antenna elements is-22.2945dB, which is not serious.Comparing with the traditional triangle antenna array and Y-shaped dipole antenna array, the Y-shaped antenna array with inverted V antenna elements is proved to have many merits, such as higher gain, narrower lobe and lower coupling degree, etc. It can not only satisfy the requirement of exploration, but also can be easily erected and maintained, which will reduce cost.(Ⅲ) Related algorithms for exploring the atmospheric wind in Wuhan MF radarSpaced antenna (SA) is used to measure the atmospheric wind in Wuhan MF radar. The related algorithms include the FCA and SCA.According to the Holdsworth’s Scattering Model, it can be found that the spatial-temporal correlation function in the FCA algorithm will vary from one condition to another. And then, based on the hypothesis of spatial-temporal correlation function, the difference between apparent velocity and true velocity is analyzed. The results suggest that the distribution of spatial-temporal correlation function will be influenced by the wind velocity, random disturbance intensity, and distribution of scatterer. At the same time, the last two are the important reasons why the apparent velocity is not equal to the true velocity.After analyzing the effect of data length, SNR and the size of antenna array on FCA algorithm, it is advised that in Wuhan MF radar the data length should be 512 and the threshold of SNR should be-3dB.(Ⅳ) The estimation of electron densities in Wuhan MF radarMF radars often use the circular polarization wave to replace the elliptical polarization wave in O_ and X_ mode. It is inevitable to elicit the DAE errors. The DAE errors are discussed by theoretical analysis and simulation computation in this dissertation. The results show that the replacement will bring the aliasing between the O_ and X_ wave, which will make the estimated value of electron densities increase obviously above 70km. It is proposed that the DAE errors can be eliminated by modifying the amplitude ratio of echo. The simulation results confirm the validity of modification.(V) The FCA, SCA and DAE software used in Wuhan MF radar are verified by contrast calculation. Comparing with the corresponding data of the same-type foreign radars, these algorithms software are proved to be valid. |
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