A refined carrier-to-code levelling method for retrieving ionospheric measurements from dual-frequency GPS data

Abstract

The retrieval of ionosphere measurements (IMs) constitutes the first key step in the process of ionospheric monitoring with the global positioning system (GPS). Carrier-to-code levelling (CCL) is one of the most widely used retrieval methods, mainly because of its simplicity and effectiveness. However, CCL-derived IMs have been proven to be susceptible to so-called levelling errors, which is mainly due to the presence of code multipath effects and the occurrence of short-term variability in receiver code biases (RCBs). To address these levelling errors, we propose a refined CCL (rCCL) method, in which the code multipath and RCB variations are jointly estimated as time-varying parameters and thus removed. Compared to the customary CCL method, this new method has two advantages. First, rCCL is able to exclude the effect of code multipath and time-varying RCBs from IMs, thereby greatly reducing the levelling errors of IM estimates. Second, the RCB variations are estimated for each individual code observation using a single receiver, allowing easy and effective calibration of the short-term variability in RCBs and the linear combination of, for instance, receiver differential code biases (rDCBs).