Research Of Phase Coding Algorithm To Mitigate Range Ambiguity In Doppler Weather Radar

Because of Doppler weather radar transmitting uniformly spaced pulses the maximum unambiguous range and the maximum unambiguous velocity have a basic limitation, their product is a constant, so that the range and velocity ambiguity problems are coupled:trying to overcome one tends to worsen the other. When using a high PRF to observe the medium and large-scale severe weather, may be appear of range ambiguity. For different wavelengths of radar, the possibility of appearing range ambiguity is different. The shorter of the wavelength of radar, the more possibility of appearing range ambiguity. In order to expand the use of Doppler weather radar, demand to mitigate range ambiguity. This paper main researches and realizes the phase coding algorithm to mitigate range ambiguity in the Doppler weather radar.Firstly, this dissertation introduces the problem of Doppler dilemma and the background of the reseach of mitigating range ambiguity, and introduces two methods to mitigate range ambiguity:Batch mode and phase coding. Batch mode can separate the overlaid echoes in time domain, but this algorithm cannot recover the velocity from weaker trips and fails in regions where the overlaid trip powers in the short PRT scan are within TOVER of each other. There are two phase coding methods to mitigate range ambiguity:random phase method and system phase method. For random phase method, the power of non-expectant echoes are register as noises when restoring the expectant echoes, will raise the noise level, then lower the equivalent signal to noise ratio, thus will reduce the accuracy of spectral moment estimation; For system phase, the spectrum of non-expectant echoes are distributed within the Nyquist interval evenly when receiving the expectant echoes synchronously, will not affect the spectral moments’ estimate of the expectant echoes. Among the many coding methods, focused on the SZ(n/M) code, and thus the introduction of SZ(8/64) code. In this technique, the transmitted pulses and the received echo samples are phase shifted to cohere the stronger signal and modulate the weaker signal. Then the stronger signal is filtered, the weaker signal is recovered using the remaining components of weak echo spectral. In the initial SZ phase coding algorithm, radar transmits short PRT pulses that are phase coded, separates overlaid echoes in frequency domain, then estimates the spectral moments of overlaid echoes respectively. The algorithm can recover the overlaid echo effectively, when the ratio of echo power and spectrum is larger than a certain value, the error of recovered spectral moments of weak echo is great. Recovery of strong and weak trip signals can proceed with the aid of an extra scan using a long PRT. This requires an additional scan similarly to the split cuts. The long PRT data provide the power free of range ambiguities, the short PRT data are used to compute Doppler velocities associated with the two strongest overlaid signals. Modified SZ phase coding algorithm has better performance than the initial SZ phase coding algorithm. Using Matlab to simulate weather echo of Gauss to verify the effectiveness of the modified SZ phase coding algorithm, results show that:this algorithm can recover the overlaid echo accurately.Finally, in the Doppler weather radar system realized the technique of phase coding, generated the SZ coding, and by increasing the corresponding signal processing software realized the algorithm of mitigating range ambiguity. Using the S-band Doppler weather radar of Dalian, the C-band Doppler weather radar of Guiyang, the S-band Doppler weather radar of Ganzhou, the S-band Doppler weather radar of Linyi, the X-band Doppler weather radar of Chengdu verified the phase coding algorithm of mitigating range ambiguity. These tests are executed in the specific scan mode.Experiment results show that:modified SZ phase coding algorithm meets the need of real-time processing, and can remove the overlaid echoes, recover the parameters of overlaid echoes accurately, and its performance is better than traditional batch method obviously.