The Research and Modification of The Cascaded FFT Acquisition Algorithm
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摘要: 新一代卫星导航信号体制采用的复合扩频码信号，给信号的捕获带来不利影响。本文深入研究了用于复合扩频码捕获的级联FFT伪码捕获算法，首次推导出算法的数学原理，为算法提供了理论依据。依据算法的数学原理，定量分析了载波多普勒和码多普勒对算法捕获性能的影响。载波多普勒的影响是两级多普勒补偿后的剩余多普勒共同作用的结果，码多普勒将导致算法在弱信号条件下的捕获失效。根据码多普勒的产生机理，结合算法的特点，提出分段码多普勒补偿改进算法，减小捕获性能损失，增加捕获时的能量累加时间。最后对分析结果进行仿真验证，仿真结果表明，算法的性能损失与理论分析结果相互吻合，改进算法能有效性的减小码多普勒影响，实现对微弱导航信号的捕获。Abstract: The tiered spreading code, which is widely adopted in the new generation of satellite navigation system, brings an obstruction for signal acquisition. This paper investigates the cascade FFT(Fast Fourier Transform) algorithm applied to the joint acquisition of tiered spreading code signal. The mathematic representation of the algorithm is derived for the first time and provided as theory foundation for the algorithm. The acquisition performance affected by carrier Doppler effect and code Doppler shifts are demonstrated via analysis in detail, according to the mathematic representation. The influence caused by carrier Doppler Effect is a combined action of the residual Doppler, which results from the two steps of carrier Doppler compensation. The influence caused by code Doppler shifts leads the acquisition unsuccessful in the weak signal condition. A modified method using segment code Doppler correction is proposed, which is according to the characteristics of the code Doppler shifts and cascade FFT algorithm. The modified method can extend the total accumulation interval and reduce the integral loss. Simulations results are provided at last in the paper, which are in accordance with the performance obtained by mathematic analysis. In addition, simulations results shows that the proposed modified techniques can work effectively in the weak signal condition.
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Keywords:
- Acquisition /
- Cascaded FFT /
- Doppler /
- Weak signal
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