Research On Several Key Techniques In Signal Processing Of Agile Daul Auroral Radar Network

The ionosphere is the primary medium for radio wave propagation and the most critical region for spacecraft operation.Ionospheric disturbances will have a severe impact on the propagation of radio waves,especially on various platforms based on the ionosphere,such as radio communications,broadcasting,and navigation satellites.Meanwhile,the ionosphere is also the tie between the near-Earth atmosphere and outer space,and it is an important part and key link in the entire solar-terrestrial space.Therefore,the detection and research of the ionosphere have essential scientific significance and application value.The Super Dual Auroral Radar Network(SuperDARN)is an international co-operative organization.It comprises over 30 similar ground-based coherent-scatter radars that operate in the high-frequency(HF)band and whose fields-of-view combine to cover extensive regions of both the northern and southern hemisphere polar ionospheres.These radars are deployed to detect backscatter from plasma density irregularities in the mid-and high-latitude ionosphere and obtain the global ionospheric convection pattern.The accuracy of the ionospheric convection pattern is closely related to the geolocation of ionospheric backscatter.Effectively reducing system imbalances and improving radar target positioning are significant guarantees for the ionospheric convection patterns.However,due to the limited techniques of traditional SuperDARN radars,it is difficult to eliminate the inconsistency between channels.It is also difficult to effectively improve the accuracy of angle measurement in the azimuth and elevation angles.With the support of the National High-Tech Research and Development Program,an Agile Dual Auroral Radar Network(AgileDARN)has been developed as an all-digital phased array radar by the National Space Science Center,Chinese Academy of Sciences.The AgileDARN radar has the capabilities of distributed digital signal processing,which can independently control and process each channel,obtaining higher performance and flexibility.Based on the design of a distributed digital signal processing system and calibration circuit,the AgileDARN radar significantly improves the geolocation accuracy through system calibration(including internal calibration and external calibration),multiple digital beamforming,and multi-baseline elevation angle determination.These techniques enable higher accuracy of the ionospheric convection pattern.Based on the AgileDARN radar,this paper focuses on several key techniques in signal processing,including the calibration method on the large-scale phased array radar system,digital multiple beamforming technique,and multi-baseline elevation determination algorithm.The specific research content is as follows:(1)The research contents of the AgileDARN radar calibration method mainly includes:(a)The analysis of the inconsistency.According to the AgileDARN radar system's characteristics,this part establishes an inconsistent error transfer model and analyzes its influence on the array pattern;(b)The design and implementation of internal calibration.This part introduces the implementation method of the internal calibration,and the measured data are used to verify the effectiveness of the internal calibration method.(c)Research on the external calibration method.In this part,the principle and implementation method of the external calibration is introduced,and the measured meteor trails echoes are used as the calibration source to implement external calibration.Combined with internal calibration,the influence of inconsistencies between the entire transmitter/receiver chain is eliminated.After internal calibration and external calibration,the amplitude imbalances among the channels of the radar system are within 0.5d B,and the phase imbalances are within ±3°,which effectively reduces the system error and shows the effectiveness of the method.(2)The digital beamforming technique in AgileDARN radar.This part introduces the principle,improvement,implementation and the measured result of multi-beam forming in detail.The improvement of digital beamforming in AgileDARN radar includes:(a)Simultaneously generate seven receiving sub-beams within the transmitting beam pattern to determine the azimuth geolocation of echoes more accurately,which is increased from 3.25° to 0.46°.Meanwhile,the sum beam and delta-azimuth beam angle measurement method based on digital multi-beam forming is introduced,and the angle measurement accuracy can reach about 2% of the half-power beam-width(HPBW);(b)And the Chebyshev window with 30 d B sidelobe suppression is used to reduce the effect of interference.Finally,the performance of digital multi-beam forming is verified by meteor echoes and small-scale ionospheric field-aligned irregularities echoes.(3)The multi-baseline elevation angle determination algorithm for AgileDARN radar.This section first introduces the two most commonly used elevation angle measurements in the SuperDARN radars: single sub-array interferometry and double sub-arrays interferometry.Aiming at the problem of elevation angle measurement,we proposed a new antenna array layout and multi-baseline interferometric elevation angle measurement algorithm,and provided a reference for the location selection and gain requirements of the new antenna and the factors that may affect the accuracy of elevation angle measurement based on a large number of simulations.Finally,the measured meteor echoes are exploited to verify the validation of the proposed elevation determination algorithm.