GPS中伪随机码的捕获技术研究
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摘要
全球定位系统(又称为GPS)是新一代的导航定位系统，它可以向广大用户提供精确的定时和定位服务。GPS卫星发射的信号，是将基带信息先经过伪随机码扩频，再对发射载波进行二进制相移键控(BPSK)而形成的。利用伪随机码进行扩频是GPS信号形成中的关键所在。本文主要研究GPS中伪随机码的快速捕获技术。
在各种扩频通信系统中，伪随机码都起着至关重要的作用。对伪随机码捕获技术的研究由来已久，方法也是多种多样。GPS中用到了两种伪随机码，它们分别是C/A码(粗测码)和P(Y)码(精测码)。
C/A码具有周期较短，复杂性较低的特点。由于载波多普勒频移的影响，所以对C/A码的捕获是同时对伪码相位和载波频率进行二维搜索的过程。本文采取PMF-FFT的方案来实现C/A码的快速捕获，此方案的主要思想是利用FFT变换来估计多普勒频移，然后用得到的频移估计值来调节载波NCO(数控振荡器)，直至完成码相位和载波频率的捕获。该方法有效地利用了FFT的特性，将对码相位和载波频率的二维搜索变为仅对码相位的一维搜索，大大减少了捕获时间，提高了C/A码接收机的实时性。此外，FFT前的PMF部分有效地降低了接收到的GPS数据的速率，使后面FFT的点数成倍地降低，大大节约了硬件资源。但PMF带来的一个显著缺点是随着多普勒频移的增加，FFT输出的相关峰值会出现下降，且下降的程度与PMF的长度X密切相关，而X的大小又直接影响着接下来FFT的点数，因此，应按照具体的要求来设置X的值。FFT也有一个缺点是给相关结果带来了扇贝损失，本文选择了加汉宁窗的方法对其做了改善，取得了比较令人满意的结果。
GPS中的P码及其加密后生成的Y码比C/A码的码速率更快、精确度更高、反欺骗容限更强。它在设计之初并不是用于直接捕获，而是通过已经捕获到的C/A码的转换来实现。如果C/A失效，则直接捕获P(Y)码就成了唯一的选择。P(Y)码的最大特点是具有长达一周的周期，如果采用传统的方法对其捕获，将会消耗很长的时间，这对于实时接收机来说是不能接受的。采用硬件并行相关技术可以缩短捕获时间，但代价非常昂贵。本文采用的叠加平均法是对直接平均法的一种改进。它一方面采取了数据平均的方法，不仅可以实现P(Y)码的快速捕获，而且其小点数的FFT让硬件的消耗大大降低，实现更加简单；另一方面，叠加平均法与直接平均法相比，最主要的改善就是它的相关峰值在一定范围内不受码相位偏移的影响，即使有噪声存在的情况下，它的这一特性仍然保持稳定。此外，作者又拿叠加平均法与一些经典的P(Y)码直接捕获方法的性能做了对比，发现它在某些方面的捕获性能要优于这些算法，如其抗噪声性能就优于XFAST算法，捕获时间短于直接法等等。
本文详细分析了GPS中两种伪随机码各自的特点，介绍了几种常用的伪随机码捕获方法，对传统GPS接收机上惯用的滑动相关法的原理作了比较详细的讲解。针对C/A码而采用的PMF-FFT捕获方案，详细介绍了其工作原理，对其捕获性能也作了深入的讨论，尤其是分别对PMF和FFT缺点的改善方法做了认真细致的分析。文中用MATLAB对PMF-FFT捕获结构的结果进行了仿真，并在捕获性能上与传统的捕获方法作了对比，最后又给出了其DSP的实现方案。对实现P(Y)码直接捕获的叠加平均法，主要讲述了该方法的思想，从理论上说明了其与直接平均法相比，在捕获性能方面的改善。另外，还与其它方法在捕获性能的某些方面作了认真的对比。为了有效地验证叠加平均法的优点，本文在分析算法原理的同时又给出其MATLAB的仿真图，并且为了方便比较，还对一些重要的数据用表格进行了归纳。
GPS(Global Position System) is a new generation Navigation and Position System which can supply many users with exact timing and positioning service. The signal that GPS satellites transmit is formed via the course: firstly the spectrum of base band signal is extended by PN(Pseudo-random Noise) code, and then the spread spectrum signal modulates the carrier wave by way of BPSK(Binary Phase-Shift Keying). We mainly discuss rapid acquisition of PN codes in GPS in this dissertation.
In each kind of spread spectrum communication systems, PN codes are playing very important role. Research on acquisition of PN codes began several ten years ago and a lot of methods came forth. Two kind of PN codes are used in GPS, they are C/A(coarse acqusition) code and P(Y)(precision) code respectively.
C/A code has short period and low complexity. Because of the fluence of carrier frequence Doppler shifts, acqusition of C/A code is a two-dimensional search process which includes searching carrier frequence Doppler shifts and the phase of C/A code.A scheme called PMF-FFT(Patial Matched Flter-Fast Fourier Transform) is presented in this paper. C/A code can be captured in virtue of estimating. Doppler frequence shifts with FFT,then the estimation is used to adjust NCO(Numerically Controlled Oscillator) until C/A code is captured. The characteristic of FFT is fully utilized in this scheme and the former two-dimensional search process becomes a one-dimensional search processs. It reduces the acqusition time greatly and improves Real-time performance of C/A code receivers, beside that, application of the PMF in the front of FFT decreases the rate of received GPS signal and reduces the number of data which is used to implement FFT efficiently, so a mass of hardware resource is saved. However, it has a remarkable shortcoming that the correlation peak of FFT output will decrease along with the increment of Doppler frequence shifts and the degree of decrement is related with the value of X(the length of PMF). What the value of X is has direct effect on the size of FFT, so it should be setted according actual instance. FFT also has a shortcoming that it can bring Scalloping Loss. It was improved by adding Hanning Window in the dissertation and the results were satisfied.
Compared to C/A code, P-Code and Y-Code which forms by encrypting P-Code in GPS have faster chipping rate, higher precision capability and better anti-jamming property. Traditionally, P(Y)-Code is acquired depending on handover from C/A code rather than direct acquisition. The most important characteristic of P(Y)-Code is it has even one week long period, so using traditional methods to acquire it will spend too much time, which can not be accepted for Real-time receivers. Adoption massive physical correlators in parallel can shorten the acqusition time, but it will result in costly expense. A new overlop average method presented in this dissertation is an improvement on direct average method. Firstly, it introduces data average method to achieve fast direct P(Y)-Code acquisition and the small size FFT decreases hardware resource enormously, which can be realized easily. On the other hand, compared to direct average method, the most important advantage of overlop average method is it's correlation peak has no relation with code phase shifts, which is also same even though noise is available. The author compared overlop average method with other classical methods which were used to acquire P(Y)-Code and found it was superior to these classical methods in some aspects of acquisition perfomance, e.g. it had better anti-jamming than direct average method and shorter acquisition time than direct method.
We labor their respective characteristic of the two different PN codes in GPS and describe several common methods for acquiring PN codes, and then expound the principle of slippage correlation method which is usually utilized in old receivers. For the scheme of PMF-FFT which is used to acquire C/A code, we analyse it's work principle and disscusse it's acquisition performance penetratingly, especially seriously analyses the improved technique of PMF and FFT respectively. Simulation of the results of PMF-FFT scheme with MATLAB and comparison with old methods in acquisition performance are presented. At last, actualization framework of PMF-FFT with DSP(Digital Signal Processing) is given. For the overlop averaged method which is used to acquire P(Y)-Code, we mainly introduce it's arithmetic process and explain why it is superior to direct averaged method in this dissertation, moreover, we compare it with other methods in some aspects of acquisition performance seriously. To effectively demonstrate the advantage of overlop average method, both it's arithmetic principle and simulation with MATLAB are included, simultaneously some concernful data is tabulated in order to compare their acquisition performance among different methods.
在各种扩频通信系统中，伪随机码都起着至关重要的作用。对伪随机码捕获技术的研究由来已久，方法也是多种多样。GPS中用到了两种伪随机码，它们分别是C/A码(粗测码)和P(Y)码(精测码)。
C/A码具有周期较短，复杂性较低的特点。由于载波多普勒频移的影响，所以对C/A码的捕获是同时对伪码相位和载波频率进行二维搜索的过程。本文采取PMF-FFT的方案来实现C/A码的快速捕获，此方案的主要思想是利用FFT变换来估计多普勒频移，然后用得到的频移估计值来调节载波NCO(数控振荡器)，直至完成码相位和载波频率的捕获。该方法有效地利用了FFT的特性，将对码相位和载波频率的二维搜索变为仅对码相位的一维搜索，大大减少了捕获时间，提高了C/A码接收机的实时性。此外，FFT前的PMF部分有效地降低了接收到的GPS数据的速率，使后面FFT的点数成倍地降低，大大节约了硬件资源。但PMF带来的一个显著缺点是随着多普勒频移的增加，FFT输出的相关峰值会出现下降，且下降的程度与PMF的长度X密切相关，而X的大小又直接影响着接下来FFT的点数，因此，应按照具体的要求来设置X的值。FFT也有一个缺点是给相关结果带来了扇贝损失，本文选择了加汉宁窗的方法对其做了改善，取得了比较令人满意的结果。
GPS中的P码及其加密后生成的Y码比C/A码的码速率更快、精确度更高、反欺骗容限更强。它在设计之初并不是用于直接捕获，而是通过已经捕获到的C/A码的转换来实现。如果C/A失效，则直接捕获P(Y)码就成了唯一的选择。P(Y)码的最大特点是具有长达一周的周期，如果采用传统的方法对其捕获，将会消耗很长的时间，这对于实时接收机来说是不能接受的。采用硬件并行相关技术可以缩短捕获时间，但代价非常昂贵。本文采用的叠加平均法是对直接平均法的一种改进。它一方面采取了数据平均的方法，不仅可以实现P(Y)码的快速捕获，而且其小点数的FFT让硬件的消耗大大降低，实现更加简单；另一方面，叠加平均法与直接平均法相比，最主要的改善就是它的相关峰值在一定范围内不受码相位偏移的影响，即使有噪声存在的情况下，它的这一特性仍然保持稳定。此外，作者又拿叠加平均法与一些经典的P(Y)码直接捕获方法的性能做了对比，发现它在某些方面的捕获性能要优于这些算法，如其抗噪声性能就优于XFAST算法，捕获时间短于直接法等等。
本文详细分析了GPS中两种伪随机码各自的特点，介绍了几种常用的伪随机码捕获方法，对传统GPS接收机上惯用的滑动相关法的原理作了比较详细的讲解。针对C/A码而采用的PMF-FFT捕获方案，详细介绍了其工作原理，对其捕获性能也作了深入的讨论，尤其是分别对PMF和FFT缺点的改善方法做了认真细致的分析。文中用MATLAB对PMF-FFT捕获结构的结果进行了仿真，并在捕获性能上与传统的捕获方法作了对比，最后又给出了其DSP的实现方案。对实现P(Y)码直接捕获的叠加平均法，主要讲述了该方法的思想，从理论上说明了其与直接平均法相比，在捕获性能方面的改善。另外，还与其它方法在捕获性能的某些方面作了认真的对比。为了有效地验证叠加平均法的优点，本文在分析算法原理的同时又给出其MATLAB的仿真图，并且为了方便比较，还对一些重要的数据用表格进行了归纳。
GPS(Global Position System) is a new generation Navigation and Position System which can supply many users with exact timing and positioning service. The signal that GPS satellites transmit is formed via the course: firstly the spectrum of base band signal is extended by PN(Pseudo-random Noise) code, and then the spread spectrum signal modulates the carrier wave by way of BPSK(Binary Phase-Shift Keying). We mainly discuss rapid acquisition of PN codes in GPS in this dissertation.
In each kind of spread spectrum communication systems, PN codes are playing very important role. Research on acquisition of PN codes began several ten years ago and a lot of methods came forth. Two kind of PN codes are used in GPS, they are C/A(coarse acqusition) code and P(Y)(precision) code respectively.
C/A code has short period and low complexity. Because of the fluence of carrier frequence Doppler shifts, acqusition of C/A code is a two-dimensional search process which includes searching carrier frequence Doppler shifts and the phase of C/A code.A scheme called PMF-FFT(Patial Matched Flter-Fast Fourier Transform) is presented in this paper. C/A code can be captured in virtue of estimating. Doppler frequence shifts with FFT,then the estimation is used to adjust NCO(Numerically Controlled Oscillator) until C/A code is captured. The characteristic of FFT is fully utilized in this scheme and the former two-dimensional search process becomes a one-dimensional search processs. It reduces the acqusition time greatly and improves Real-time performance of C/A code receivers, beside that, application of the PMF in the front of FFT decreases the rate of received GPS signal and reduces the number of data which is used to implement FFT efficiently, so a mass of hardware resource is saved. However, it has a remarkable shortcoming that the correlation peak of FFT output will decrease along with the increment of Doppler frequence shifts and the degree of decrement is related with the value of X(the length of PMF). What the value of X is has direct effect on the size of FFT, so it should be setted according actual instance. FFT also has a shortcoming that it can bring Scalloping Loss. It was improved by adding Hanning Window in the dissertation and the results were satisfied.
Compared to C/A code, P-Code and Y-Code which forms by encrypting P-Code in GPS have faster chipping rate, higher precision capability and better anti-jamming property. Traditionally, P(Y)-Code is acquired depending on handover from C/A code rather than direct acquisition. The most important characteristic of P(Y)-Code is it has even one week long period, so using traditional methods to acquire it will spend too much time, which can not be accepted for Real-time receivers. Adoption massive physical correlators in parallel can shorten the acqusition time, but it will result in costly expense. A new overlop average method presented in this dissertation is an improvement on direct average method. Firstly, it introduces data average method to achieve fast direct P(Y)-Code acquisition and the small size FFT decreases hardware resource enormously, which can be realized easily. On the other hand, compared to direct average method, the most important advantage of overlop average method is it's correlation peak has no relation with code phase shifts, which is also same even though noise is available. The author compared overlop average method with other classical methods which were used to acquire P(Y)-Code and found it was superior to these classical methods in some aspects of acquisition perfomance, e.g. it had better anti-jamming than direct average method and shorter acquisition time than direct method.
We labor their respective characteristic of the two different PN codes in GPS and describe several common methods for acquiring PN codes, and then expound the principle of slippage correlation method which is usually utilized in old receivers. For the scheme of PMF-FFT which is used to acquire C/A code, we analyse it's work principle and disscusse it's acquisition performance penetratingly, especially seriously analyses the improved technique of PMF and FFT respectively. Simulation of the results of PMF-FFT scheme with MATLAB and comparison with old methods in acquisition performance are presented. At last, actualization framework of PMF-FFT with DSP(Digital Signal Processing) is given. For the overlop averaged method which is used to acquire P(Y)-Code, we mainly introduce it's arithmetic process and explain why it is superior to direct averaged method in this dissertation, moreover, we compare it with other methods in some aspects of acquisition performance seriously. To effectively demonstrate the advantage of overlop average method, both it's arithmetic principle and simulation with MATLAB are included, simultaneously some concernful data is tabulated in order to compare their acquisition performance among different methods.
引文
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[2]熊志昂，李红瑞，赖顺香．GPS技术与工程应用[M]．北京：国防工业出版社，2005：8-18．
[3]杨俊，武奇生．GPS基本原理及其Matlab仿真[M]．西安：西安电子科技大学出版社，2006：142-146．
[4]朱近康．CDMA通信技术[M]．北京：人民邮电出版社，2001：2-4，121—130．
[5]卢晓春，陈清刚，胡永辉．卫星导航定位系统中伪随机码的研究[J]．时间频率学报，2004，27(1)：23-31．
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