Research Of The Near-far Effect And Mitigation Techniques In The Direct Ranging Pseudolite Applications

Demanding for the all space,all time and high precision positioning and navigation is always on the way and various types of terrestrial augmentation systems burst out fulfilling these requirements.The direct ranging pseudolite plays an important role in the terrestrial augmentation system,which can meet the positioning and navigation requirements where GNSS signal is denied.However,the near-far effect gets in the way of the application of the direct ranging pseudolite for the demanding of high precision positioning.This paper aims at the near-far effect mitigation techniques in the high precision positioning direct ranging pseudolite application and the study of this paper contains the following aspects:First of all,the impact of the near-far effect to performance of the receiver is studied.The signal model affected by the near-far effect has been set up and the effect of the far-near effect on the performance of the carrier tracking and pseudo-code tracking is analyzed.The expression of the tracking precision is simplified according to the equivalent carrier-to-noise ratio,which can be easily used in the engineering.The validity of the derived results is verified by simulation.Then,aiming at the high precision positioning direct ranging pseudolite application requirements,the performance of the double transmission technique(DTT)is analyzed,which is a technique of mitigating the near-far effect in the signal emitting layer.Firstly,the signal model of DTT whose carrier is continuous and the working principle are given.The Doppler frequency difference will seriously affect the performance and the receiver cannot acquire the GNSS signal at several certain value of the Doppler frequency difference,so it is hardly used in the place where there is a Doppler difference between the satellites and the pseudolites.Finally,the performance of the pulse signal which is used widely to mitigate the near-far effect is analyzed;especially the carrier tracking performance under the non-ideal condition.Firstly,the pulsed DS-CDMA signal model and the non-ideal equivalent channel model are established,and the expression of the pulse signal through the non-ideal equivalence model is obtained.The carrier tracking accuracy and tracking error of the two methods are deduced in different carrier discriminator.And the theoretical deduction is validated by simulation.The simulation results are compared with the theoretical results and suggestions for the receiver designing are given.The results show that the carrier tracking precision is not sensitive to the bandwidth of the receiver and does not need to redesign the bandwidth in order to receive the pulse signal.The carrier tracking accuracy is affected by the pulse duty ratiod_f and the non-ideal channel response.When the pulse signal carrier to noise ratio is fixed,the carrier tracking accuracy of the common receiver and the suppressing noise receiver is affected by1/d_f~2 and 1/d_f separately.The duty cycle and pulse signal transmission power should be selected seriously,and to avoid a specific filter of group delay fluctuation amplitude.