Functional model modification of precise point positioning considering the time-varying code biases of a receiver

Precise Point Positioning(PPP),initially developed for the analysis of the Global Positing System(GPS)data from a large geodetic network,gradually becomes an effective tool for positioning,timing,remote sensing of atmospheric water vapor,and monitoring of Earth’s ionospheric Total Electron Content(TEC).The previous studies implicitly assumed that the receiver code biases stay constant over time in formulating the functional model of PPP.In this contribution,it is shown this assumption is not always valid and can lead to the degradation of PPP performance,especially for Slant TEC(STEC)retrieval and timing.For this reason,the PPP functional model is modified by taking into account the time-varying receiver code biases of the two frequencies.It is different from the Modified Carrier-to-Code Leveling(MCCL)method which can only obtain the variations of Receiver Differential Code Biases(RDCBs),i.e.,the difference between the two frequencies’code biases.In the Modified PPP(MPPP)model,the temporal variations of the receiver code biases become estimable and their adverse impacts on PPP parameters,such as ambiguity parameters,receiver clock offsets,and ionospheric delays,are mitigated.This is confirmed by undertaking numerical tests based on the real dual-frequency GPS data from a set of global continuously operating reference stations.The results imply that the variations of receiver code biases exhibit a correlation with the ambient temperature.With the modified functional model,an improvement by 42%to 96%is achieved in the Differences of STEC(DSTEC)compared to the original PPP model with regard to the reference values of those derived from the Geometry-Free(GF)carrier phase observations.The medium and long term(1×10^(4) to 1.5×10^(4) s)frequency stability of receiver clocks are also signifi-cantly improved.

A novel satellite-equipped receiver for autonomous monitoring of GNSS navigation signal quality

Global navigation satellite system(GNSS) comes with potential unavoidable application risks such as the sudden distortion or failure of navigation signals because its satellites are generally operated until failure. In order to solve the problems associated with these risks, receiver autonomous integrity monitoring(RAIM) and ground-based signal quality monitoring stations are widely used. Although these technologies can protect the user from the risks, they are expensive and have limited region coverage. Autonomous monitoring of satellite signal quality is an effective method to eliminate these shortcomings of the RAIM and ground-based signal quality monitoring stations; thus, a new navigation signal quality monitoring receiver which can be equipped on the satellite platform of GNSS is proposed in this paper. Because this satellite-equipped receiver is tightly coupled with navigation payload, the system architecture and its preliminary design procedure are first introduced. In theory, code-tracking loop is able to provide accurate time delay estimation of received signals. However, because of the nonlinear characteristics of the navigation payload, the traditional code-tracking loop introduces errors. To eliminate these errors, the dummy massive parallel correlators(DMPC) technique is proposed. This technique can reconstruct the cross correlation function of a navigation signal with a high code phase resolution. Combining the DMPC and direct radio frequency(RF) sampling technology, the satellite-equipped receiver can calibrate the differential code bias(DCB) accurately. In the meantime, the abnormities and failures of navigation signal can also be monitored. Finally, the accuracy of DCB calibration and the performance of fault monitoring have been verified by practical test data and numerical simulation data, respectively. The results show that the accuracy of DCB calibration is less than 0.1 ns and the novel satellite-equipped receiver can monitor the signal quality effectively.