| With the coming of the intelligent era,people put forward higher requirements for Positioning,Navigation and Timing(PNT)performance(such as accuracy,integrity,continuity,and robustness).Although the Global Navigation Satellite System(GNSS)can provide global and all-weather PNT services,it is impossible to be applied in physical shielding and electromagnetic interference environment because the GNSS signals suffer from blocking,jamming and spoofing when arrived the earth ground.Pseudolite systems(PLS)are emerging as powerful supplements,backups and enhancement solutions.Therefore,it is very valuable and meaningful to research PLS.This paper aims to improve the PLS independent positioning performance and its augmentation to GNSS.The main work and contributions are as follows:(1)A anechoic chamber-type absolute field calibration method is proposed.This calibration method is based on the use of a transceiver unit that can calibrate both receiver and transmitter antennas.In order to reduce the effect of variations in hardware delay on calibration,a method to monitor the variations in hardware delay is proposed.Multiple calibrations can be performed at different sites to randomize the multipath errors and to improve the accuracy of calibrations.The experiment showed that the antenna phase pattern accuracy of the proposed calibration method could reach millimeter level.It lays a solid foundation for the realization of PLS high-precision positioning and integer ambiguity resolution.(2)Thanks to the accurate calibration of the antenna phase center,it is possible to recover the integer nature of phase ambiguity.However,transmitter phase bias(TPB)is still an obstacle to integer ambiguity resolution.This paper proposed an on-the-fly TPB calibration method and analyzed its characteristics.In this study,it is shown that the TPBs are rather stable in time unless the PLS is restarted.As a result,the TPBs do not need to be broadcast to users frequently.After obtaining the TPB broadcast by PLS,the user receiver can use it to correct the carrier phase observation to achieve integer ambiguity resolution,thus accelerating convergence and improving positioning accuracy.(3)PLS can not only position independently,but can also be used to augment GNSS.A long convergence time for GNSS PPP is a critical obstacle to its wide application.Since pseudolites(PLs)are relatively close to receivers,moving receivers can produce rapid spatial geometry changes,which reduced the correlation of parameters and facilitates their convergence fast.Based on this advantage of the PLS,we investigate the PLS-augmented GNSS real-time kinematic PPP.Single-frequency PLS raw observations are combined with dual-frequency GNSS raw observations by an undifferenced and uncombined PPP model to augment the GNSS PPP.A real-world experiment demonstrated that the convergence time was dramatically shortened by 99.5%from about 900 s to nearly 4 s using PLS augmentation.In the 900-s sessions,the average success rate of GNSS ambiguity resolution can reach approximately 90%using PLS augmentation.Furthermore,PLS-augmented GNSS PPP showed a better positioning accuracy compared with GNSS-only PPP.The contributions of the PLS are still remarkable even with a short-time PLS augmentation.For example,after position parameters converged,if PLS can provide another 8-s augmentation,GNSS ambiguity parameters will also converge,then GNSS PPP can work independently and provide continuous high-precision positioning.In addition,longer augmentation times can promote higher success rates of GNSS ambiguity resolution.In summary,the positioning accuracy,convergence speed,and success rate of GNSS ambiguity resolution of GNSS PPP are significantly improved using PLS augmentation.(4)Considering the limited coverage of PLS,the augmentation time is limited.It is therefore crucial to make the GNSS PPP parameters(especially the GNSS ambiguity parameters)converge within the limited augmentation time.Therefore,the potential of PLS should be explored as much as possible,and the gains brought by PLS should be effectively utilized.In order to use the fixed solution information of PLS more safely and reliably,a dual-filter tightly coupled model is proposed,and the contributions of PLS on GNSS under different couple models(dual-filter tightly coupled,tightly coupled,and semi-tightly coupled)are analyzed.When the number of PLs is sufficient and the augmentation time is long enough,the positioning performances of these couple models are not significantly different.However,when the number of PLs is not enough to support PLS positioning alone,the semi-tightly coupled model will be invalid.In the case of short-time enhancement,the dual-filter tightly coupled model exhibits the best augmentation performance,it can significantly reduce the augmentation time required by GNSS,which means that the required coverage of PLS can be smaller and will be more flexible and convenient for applications. |
