Navigation message authentication(NMA)technology has become one of the important development directions for defending against generative spoofing and protecting global navigation satellite system(GNSS)civilian signals.In view of the vulnerability that NMA is difficult to de-fend against the spoofing without changing navigation message,a symbol level attack(SLA)method for BeiDou NMA is proposed,and an improved maximum a posteriori(MAP)symbol esti-mation algorithm is designed.The mode of the MAP estimates over time is used as the new esti-mator to achieve rapid recovery of navigation message.A spoofing effectiveness evaluation experi-ment based on the BeiDou B1C signal NMA scheme is carried out.The results show that under the conditions of carrier-to-noise ratio greater than 45 dB·Hz and short estimator delay(less than 30 μs),SLA can achieve a sequential spoofing success rate of 90％for BeiDou B1C authentication message.Compared with security code estimation and replay(SCER),this method can signifi-cantly reduce the symbol error rate,shorten the estimator delay,and has better spoofing effective-ness,providing technical support for the design optimization of BeiDou NMA.
Symbol level attackNavigation message authenticationBeiDou navigation satellite systemMaximum a posteriori estimator
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