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美国人真的破解了北斗系统的PRN码?

已有 9168 次阅读 2010-10-30 21:03 |个人分类:科学研究|系统分类:科研笔记

Stanford的GPS实验室(http://waas.stanford.edu/index.html)破解了北斗系统的伪随机噪声(Pseudo Random Noise,PRN)码,其中一位关键的成员是来自清华大学的才女Grace Xingxin Gao,她的博士论文《Towards navigation based on 120 satellites analyzing the new signals》就是以PRN码的破解为主题的。她的主页:http://www.stanford.edu/~gracegao/

 

破解了PRN码,就可以跟踪上相应的卫星信号,也可以对该卫星信号实施干扰,从而使得通信完全中断。

在她博士论文的结论和贡献中有这样的描述:

Designed Algorithms for Deciphering Unknown PRN Codes Chips


Chapter 3 built a mathematical model for received Galileo and Compass signals that represented their components and parameters.

This model provided the foundation for the decoding process, which involved stripping all other components from the received signals, leaving only the PRN codes.

The fundamental technique to reveal the unknown PRN code chips was to raise the SNR level by coherently combining multiple periods of the received signal.

There were four main challenges in deciphering the codes.

First, the received Galileo and Compass signals were very weak due to the path loss. The received signal power was on the order of 1e-16 W, assuming an omnidirectional antenna.

Second, the signal structure was complicated and unknown.
Third, the data collection apparatus was asynchronous with respect to the satellite transmission.

Fourth, the Galileo and Compass test signals were subject to strong interfering pulses from nearby aeronautical radio systems.

对于卫星信号的截获确实很困难,但在先进科研设施的基础上,他们似乎做到了,而且做得很好。

 

而最重要的是,在第127页她提到:“Our Compass and GIOVE PRN code results have been implemented in software and hardware and independently verified by receiver companies. For example, Trimble Navigation, Ltd. tracked GIOVE-A in March 2007, and GIOVE-B in May 2008. Javad GNSS Inc. Have applied our decoding results in their receivers since May 2007. W. Dewilde et al. from Septentrio Satellite Navigation N.V., a Belgian company, were able to track Compass-M1 E2 and E5b signals in real-time through a hemispherical antenna using the codes we supplied.”

 

最后一句,一家比利时公司根据我们提供的PRN码成功实现了对北斗M1 E2和E5b信号的实时跟踪。为了表彰她的这一贡献,获得了2009年的William E. Jackson Award, Radio Technical Commission for Aeronautics (RTCA) (2009) 。

 

对于熟悉无线通信的人来说,即使是在很高的信噪比下,要破解PRN码也是非常困难的,除非我们具备某些先验知识。我没有很仔细地拜读Gao的博士论文,所以还不清楚破解的具体方法。但是我觉得,要验证破解的PRN码是否正确,尤其是针对一个完全未知的系统,应该是非常困难或者不可能的事情。

 

现在只有伽利略和北斗的设计人员才知道这件事情的真实程度,但事件本身是很遗憾的。

 



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