Errors Analysis and Improvement on Estimating Low Latitude Ionospheric Delay Gradient Based on GPS/BDS Observations

摘要

Researchers proposed various methods for estimating ionospheric delay and gradients. The developed methods usually involve utilizing simultaneous dual frequency carrier phase and code delay observations from a number of short- and medium- baseline GNSS station networks. The ionospheric delays and satellite and receiver inherent differential code biases (DCBs) at each station are estimated. At each epoch, the slant ionospheric delay gradients along satellite-receiver line-of-sight, between selected pairs of receivers viewing at the same satellite are determined by dividing the differential slant ionospheric delays by the baseline distance of the two receivers. Presently, applying the previously developed methods to obtain ionospheric gradient based on GPS/BDS observations in low latitude region would face greater challenges. At first, recent investigations showed that receiver DCBr exhibited significant variations over intervals of hours. Some of these variations could be attributed to changing temperature conditions at the receiver antenna, along the cable, or in the internal receiver hardware. Secondly, systematic bias errors were found from BDS MEO/IGSO/GEO code-delay multipath and would lead to greater leveling slant ionospheric delay errors. Lastly, ionospheric variability which is much more pronounced in low latitude would influence the accuracy of estimated satellites and receivers DCBs. The purpose of the paper is analysis of error sources in slant ionospheric observations and method of improving accuracy of low latitude ionospheric gradient determination based on GPS/BDS observations. The main works consist of: A method of analyzing error sources in ionospheric observations based on zero-, short- and medium-baseline between-receiver single difference observations is proposed. The method considers such factors as ionospheric variability, code DCB temporal variability, and code and phase multipath and noise. Single differences of hourly averaged difference of code-derived minus phase-derived ionospheric observations are utilized to study error source in leveling processes and the differential code bias (DCBr) variations. In addition, Single differences of ionospheric observations are used to assess the phase multipath and noise variation level. In order to assess performance of the proposed method, the data collected by zero-short, short- and medium-baseline stations in middle and low latitude are processed and analyzed. Error analysis of single difference of ionospheric observations is presented for zero-, short-, and medium-baseline in middle and low latitude. It is presented that low latitude ionospheric delay relative variation can be accurately detected using single difference observations of GEO satellites, which is free of effects of arc-related BR-DCB and code multipath effects. This method is very suited for accurately detecting and analyzing the low latitude ionospheric gradients and ionospheric scintillations due to relatively fixed orbital position and continuously long time observation from BDS GEOs.