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Description
Dual-frequency GNSS ground observations are commonly used to compute precise ionospheric total electron content (TEC) maps also known as Global Ionospheric Maps (GIMs). Precise ionospheric maps are useful in mitigating atmospheric effects in GNSS navigation, positioning and timing (PNT) applications. The estimation of ionospheric TECs, and the satellite and receiver Differential Code Biases (DCBs) are correlated. When estimating the DCBs, one of the significant error sources is the mapping (slant TEC to vertical or vice versa) error, which affects the accuracy of the vertical TEC (VTEC) parameterization, while the VTEC parameterization is also crucial in DCB estimations.
Low Earth Orbiting (LEO) satellites are flying mostly above the ionospheric peak density height and therefore the same ionosphere mapping function (MF) as the ground data cannot be used. Again, since different LEO missions are flying at significantly different heights (e.g., 400 – 1300 km altitude) the same MF cannot be used straightforward for all missions. Also, the ionospheric and plasmaspheric density profile varies with geographic location, local time, seasons and solar cycle variations which has direct impact on the selection of MF algorithm (or parameters defining MF). Therefore, the choice of mapping function influences the ionospheric determination as well as the determination of the LEO receiver Biases using space data.
A universal multi-layer MF approach applicable to LEO and ground data has been implemented in which the ionospheric and plasmaspheric electron densities are modeled as a function of height using Chapman layer function and exponential decay function, respectively.
We executed various test cases covering different LEO missions and different ionospheric conditions. The execution of the test cases provided ionospheric data products such as GNSS satellite and receiver DCBs, and absolute/calibrated topside TECs. We found that the multi-layer MF outperforms the single-layer MF when computing GNSS receiver DCBs especially at low latitude and equatorial regions where ionosphere is highly dynamic and difficult to model. When compared with the International GNSS Services (IGS) products, we found that the mean receiver DCB estimation is improved by about 0.14 – 0.25 ns and 0.32 – 0.78 ns during days in 2019 and 2023, respectively. We found that the receiver DCB estimation improves for about 68-87% receivers. This is also reflected in GIMs showing better performance for the multi-layer MF when comparing with IGS GIMs.
Acknowledgement:
The work is funded by the LMAP (LEO Ionospheric Mapping Assessment and Derivation For Precise PVT Applications) project under the ESA Contract No. 4000142821/23/NL/MGu/my.
