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实验使用的激光测风雷达系统[15]的工作波长1550 nm,采样率500 MHz,中频80 MHz,脉宽200 ns,频率测量范围40~120 MHz,对应风速范围−31~31 m/s (见表1)。
Parameters Value Wavelength/nm 1550 Sample rate/MHz 500 Range resolution/m 30/50/75/100 Maximum detection range/km 10 Intermediate frequency/MHz 80 Pulse width/ns 200 Wind speed range/m·s−1 −31~31 Dimension/mm3 ≤φ420×700 Pointing precision/(°) 0.1 Table 1. Parameters of wind lidar system
生成频率100、40、80、20、60 MHz,信噪比−1 dB的仿真变频信号,使用自适应距离门方法和固定距离门方法(距离分辨率设置30 m)进行距离门划分(见图3)。
Figure 3. Results of range-gate division. (a) Results of fixed range-gate division; (b) Results of adaptive range-gate division
仿真信号距离门划分结果表明,固定距离门方法划分的距离门与信号频率变化无关;自适应距离门方法能够识别出频率突变点t1、t2、t3和t4,并对区间[0, t1]、[t1, t2]、[t2, t3]、[t3, t4]进行了等间隔整周期划分。
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为验证自适应距离门方法能够在提高距离分辨率的同时满足测量误差要求,使用自适应距离门方法和固定距离门方法处理范围信号频率40~120 MHz,信噪比−5~5 dB的仿真信号,进行5000次独立计算,对比分析频率计算结果的误差(见图4)。
结果表明,仿真信号信噪比大于1 dB时,自适应距离门方法与固定距离门方法的频率估计结果误差基本一致,约为0.2%。当信噪比小于1 dB时,随着信噪比的降低,两种方法的频率估计误差均增大,自适应距离门方法频率估计误差更小;信噪比小于−1 dB时,自适应距离门方法的频率误差是固定距离门方法频率误差的38%~62%。
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距离分辨率指相邻两个距离门中心的距离间隔,固定距离门方法的距离分辨率与距离门的长度相同;自适应距离门的距离门长度与信号频率相关,距离分辨率不固定。
单个自适应距离门内包含的整周期数量对频率估计误差和距离分辨率均会产生影响。通过仿真分析整周期的数量与频率估计误差的关系(见图5)以及自适应距离门长度与整周期数量、信号频率之间的关系(见图6)。
由仿真结果知,单个距离门内整周期数量取5以下时,频率估计误差开始明显增加,由0.5%以下增加到1%以上,最大增加到4.1%。整周期个数取5及以上时,频率估计误差基本保持不变,在0.2%左右。整周期数取5时,对应距离分辨率范围6.25~18.75 m。
在划分自适应距离门时,需要综合考虑距离分辨率和风速误差,选择合适的整周期数量,令距离分辨率和风速误差同时达到最优。
Research on high resolution range-gate adaptive technology of coherent wind lidar
doi: 10.3788/IRLA20210187
- Received Date: 2021-02-26
- Rev Recd Date: 2021-05-06
- Available Online: 2022-01-06
- Publish Date: 2021-12-31
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Key words:
- wind lidar /
- full cycle search /
- adaptive range-gate /
- signal processing
Abstract: The signal processing of pulse coherent wind lidar usually uses fixed range-gate to divide the time domain signal, and perform frequency spectrum calculation for each range-gate to obtain wind speed information. The time domain signal division of the fixed range-gate has the problem of non-periodic truncation of the intermediate frequency signal, which leads to spectrum leakage during spectrum calculation, resulting in errors and reduced signal-to-noise ratio. An adaptive range-gate division method based on full cycle search was proposed. The length of the range-gate was adaptive to the frequency of the intermediate frequency signal, which could realize the full cycle division of the signal, avoide the problem of spectrum leakage and improve frequency estimation accuracy. The two processing methods were simulated and analyzed by adding noise signal. The results show that the adaptive range-gate method can realize the adaptation of the range-gate length and the intermediate frequency signal. When the signal-to-noise ratio was less than 1 dB, the intermediate frequency estimation error obtained by this method was 38%-62% of the fixed range-gate method. The adaptive range-gate division method was used to process the turntable and wind field echo signals obtained by the laser wind measurement radar system, and the results were compared with the wind lidar equipped with the fixed range-gate method. The results show that the root mean square error of the adaptive range-gate method for the speed measurement of the turntable is 0.19 m/s, and the range resolution of atmospheric wind speed measurement varies from 7 to 11 m, which is better than that of fixed range-gate method, and improves the range resolution and measurement accuracy of the wind lidar.