Secular Changes in Differential Code Bias of COMPASS System

Abstract

Differential Code Bias (DCB), named Timing Group Delay (TGD) for satellite and Inter-Frequency Bias (IFB) for receiver, is a crucial parameter in COMPASS system, which is needed in satellite clock correction and ionospheric delay modeling. Though DCB parameters were measured in laboratory initially, it may change with working environment as well as with time elapsed, necessitating DCB estimation for the deployed satellites and receivers. DCB estimates can be obtained by combining COMPASS data in three frequencies and accurate ionospheric Total Electron Content (TEC) models (e.g. Global Ionosphere Model, GIM from International GNSS Service). However, TEC model errors as revealed by discrepancies with GPS observations would translate into and contaminate DCB estimates. This study concludes that the error GIM brought into TGD is under 1 ns that is equivalent to a positioning error of 0.18 m, an ionospheric time delay correction error of 0.4–0.7 m and TEC models error of 3TECU. Analysis of COMPASS data for 290 days in 2011 shows a secular change in TGDs with amplitude of about 1.5 ns for GEO satellites and 4 ns for IGSO satellites respectively and in IFBs with amplitude of 3 ns. The DCB time series could be well fitted by a second-degree polynomial with residuals of 0.4–0.6 ns for TGDs and 0.6–0.9 ns for IFBs, based on which the TGD could be predicted with accuracy of 0.5 ns for GEO satellites and 0.5–1 ns for IGSO satellites respectively.