Nicolas Bergeot

Distribution and mitigation of higher‐order ionospheric effects on precise GNSS processing

Higher-order ionospheric effects (I2+) are one of the main limiting factors in very precise Global Navigation Satellite Systems (GNSS) processing, for applications where millimeter accuracy is demanded. This paper summarizes a comprehensive study of the I2+ effects in range and in GNSS precise products such as receiver position and clock, tropospheric delay, geocenter offset, and GNSS satellite position and clock. All the relevant higher-order contributions are considered: second and third orders, geometric bending, and slant total electron content (dSTEC) bending (i.e., the difference between the STEC for straight and bent paths). Using a realistic simulation with representative solar maximum conditions on GPS signals, both the effects and mitigation errors are analyzed. The usage of the combination of multifrequency L band observations has to be rejected due to its increased noise level. The results of the study show that the main two effects in range are the second-order ionospheric and dSTEC terms, with peak values up to 2 cm. Their combined impacts on the precise GNSS satellite products affects the satellite Z coordinates (up to +1 cm) and satellite clocks (more than ±20 ps). Other precise products are affected at the millimeter level. After correction the impact on all the precise GNSS products is reduced below 5 mm. We finally show that the I2+ impact on a Precise Point Positioning (PPP) user is lower than the current uncertainties of the PPP solutions, after applying consistently the precise products (satellite orbits and clocks) obtained under I2+ correction

Comparison of Regional and Global GNSS Positions, Velocities and Residual Time Series

More than 10 years (1996–2008) of weekly GPS solutions of 299 globally distributed stations have been used to quantify the impact of the reference frame definition and especially the size of the network on the estimated station positions, velocities, and residual position time series. For that purpose, weekly regional solutions (covering the European region) and global solutions have been respectively stacked to obtain regional and global station positions, velocities, and residual position time series. In both cases, the estimated long-term solutions have been tied to the ITRF2005 under minimal constraints using a selected set of reference stations. This study shows that: (1) regional position and velocity solutions can present biases with respect to each other and to global solutions, while in comparison, global solutions are much more stable; (2) the obtained residual position time series are affected by the size of the network with significantly reduced periodic signals in the regional networks, e.g. a 27% reduction of the annual signals in the height component.

Influence of ionospheric perturbations in GPS time and frequency transfer

The stability of GPS time and frequency transfer is limited by the fact that GPS signals travel through the ionosphere. In high precision geodetic time transfer (i.e. based on precise modeling of code and carrier phase GPS data), the so-called ionosphere-free combination of the code and carrier phase measurements made on the two frequencies is used to remove the first-order ionospheric effect. In this paper, we investigate the impact of residual second- and third-order ionospheric effects on geodetic time transfer solutions i.e. remote atomic clock comparisons based on GPS measurements, using the ATOMIUM software developed at the Royal Observatory of Belgium (ROB). The impact of third-order ionospheric effects was shown to be negligible, while for second-order effects, the tests performed on different time links and at different epochs show a small impact of the order of some picoseconds, on a quiet day, and up to more than 10 picoseconds in case of high ionospheric activity. The geomagnetic storm of the 30th October 2003 is used to illustrate how space weather products are relevant to understand perturbations in geodetic time and frequency transfer.

Erratum to: Influence of different GPS receiver antenna calibration models on geodetic positioning

In the original publication, the last five sentences of the third paragraph under the section Results were incorrectly published. The corrected text is given below. ‘‘Specifically the update of the type mean robot calibration of the TRM29659.00/NONE caused a position offset of -3.9 mm in the east component and 5.8 mm in the up component. Note that its type mean calibration in the igs08.atx is based on 18 individual calibrations compared to 12 in the igs05.atx, hence the importance of including RMS values in the IGS antenna calibration files.’’

Higher-order ionosphere perturbations in GPS time and frequency transfer

In high precision geodetic time transfer (i.e. based on precise modeling of code and carrier phase GPS data), the so-called ionosphere-free combination of the code (P3) and carrier phase (L3) measurements, made on the two GPS frequencies, is used to remove the first-order ionosphere effect. In this paper, we quantify the impact of residual second- and third-order ionosphere perturbations on geodetic time and frequency transfer solutions, for both continental baselines and intercontinental baselines. All time transfer computations have been done using the ATOMIUM software, developed at the Royal Observatory of Belgium. In order to avoid contamination by some imperfect modeling of the second- and third-order ionosphere effects in the satellite clock products, only single-difference (Common-View) processing is used, based on both code and carrier phase measurements. The results are shown for weak and for strong solar activity, as well as for particular epochs of ionosphere storms.