Errors analysis and improvement on estimating low latitude ionospheric delay gradient based on GPS/BDS observations

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.