GPS软件接收机宽载噪比载波频率精确捕获策略

摘要

To provide accurate initial condition for the tracking loop in a GPS software receiver, the acquired carrier frequency should be within dozens of Hz. Therefore, a method that can precisely and quickly measure Doppler frequency shift should be acquired. An estimation method for fine carrier frequency is proposed, in which the phase measurement method is used under high CNR, and the long coherent processing method is used under low CNR. Then the method is tested based on the intermediate frequency data generated by Matlab simulation. The results show that, when CNR of the input signal is higher than 35.5 dB·Hz, the error of Doppler frequency obtained by phase measurement algorithm is kept within 10 Hz. For weak signals, the frequency error will decrease from 3 Hz to 0.5 Hz when the time of coherent processing is extended from 200 ms to 600 ms. Moreover, compared with the traditional FFT method, the proposed method’s computations of addition and multiplication are reduced by 96.2% and 35% respectively. The test results show that this algorithm is effective and superiority.%为给GPS软件接收机的跟踪环提供精确的初始条件，捕获后得到的载波频率应在几十Hz范围内，所以必须寻找一种既能精确测量载波多普勒频移，又能有较快运算速度的方法。针对这一特点，提出了一种载噪比较高时采用相位测量和较低时采用长相干处理的载波频率精确估计策略。利用Matlab 仿真产生的卫星中频数据作为数据源对该策略进行验证，结果表明当输入信号的载噪比大于35.5 dB·Hz的时候，相位测量算法得到的多普勒频率值的误差保持在约10 Hz之内。对于微弱信号的捕获，如果将相干处理的时间从200 ms扩展到600 ms，捕获频率的误差从3 Hz减小到0.5 Hz。此外，与传统的FFT方法相比，该方法的加法和乘法运算量分别降低了96.2%和35%。测试结果体现了该算法的有效性和优越性。