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.Comment: 25 pages, 10 eps figures, 1 table, accepted in Journal of Advances in Space Research, Special Issue "Recent advances in space weather monitoring, modelling and forecasting

Modelling and assessing ionospheric higher order terms for GNSS signals

High precision positioning and time transfer are required by a large number of scientific applications: seismic ground deformations, sea level monitoring or land survey applications require sub-centimeter precision in kinematic position; monitoring of stable atomic frequency standards requires an increasing sub –nanosecond precision. Differential GNSS is presently the best tool to reach such precisions, as it removes the majority of the errors affecting the GNSS signals. However, the associated need for dense GNSS observation networks is not fulfilled for many locations (e.g. Pacific, Africa). An alternative is to use Precise Point Positioning (PPP), but this technique requires correcting signal delays at the highest level of precision, including high order ionospheric effects. It is thus essential to accurately characterize the higher order ionospheric terms (I2+), i.e. I2, I3, I4, geometric bending and differential STEC bending, which is the goal of this paper. For that, we used a network of well-distributed GPS stations, and the Bernese v5.0 software. We have focused our attention in the I2+ terms, studying two approaches: A) Combining independent and simultaneous measurements of the same transmitter-receiver pair at three (or more) different frequencies, in order to remove the I2 term: it is theoretically possible to cancel out both I1 and I2 similarly as it is done typically in precise dual-frequency GNSS measurements for I1. It is shown that, as expected, due to the proximity of the corresponding frequencies in L-band, the high noise of the combinations makes this approach unpractical to either isolate or remove I2. B) Modelling the I2+ terms, in function of estimates of electron content, geomagnetic field and electron density values. Their characterization has been done in a realistic and full-control environment, by using the last version of the International Reference Ionosphere model (IRI2012) and International Geomagnetic Reference Model in its 11th version (IGRF11). Two metrics have been considered to assess the importance of the different higher order ionospheric corrections and their approximations: a) At the signal level, or range level, directly provided by the corresponding slant delays. b) At the geodetic domain level, provided by the impact of such values in the different geodetic parameters estimated consistently (i.e. simultaneously) from a global GNSS network.Peer ReviewedPostprint (author's final draft

Impact of higher order ionospheric delay on precise GNSS computation

Peer ReviewedPostprint (published version

Modèles de la convection actuelle dans le manteau terrestre

Doctorat en sciences physiques -- UCL, 199

Generation of new time scale at ROB: Combination of Cesium clocks and hydrogen masers

The tuning of the master clock frequency at ROB, is performed using a daily monitoring based on GNSS geodetic time transfer with different UTC(k)’s located in remote time laboratories. To enhance the independence of the ROB, a time scale algorithm is under process, using the ensemble of cesium clocks and H masers data. Equal clock weighting has been replaced by time-varying noise characteristics of the two different types of clocks. Correction of frequency drift is required by the masers. This paper will present the algorithm and the different tests performed in order to increase its robustness and reliability

Length of day variations due to zonal tides for an inelastic earth in non-hydrostatic equilibrium

We present a refined theoretical model for length-of-day (lod) variations induced by the zonal part of the tide-generating potential. The model is computed from a numerical integration, from the Earth's centre up to the surface, of the equation of motion, the rheological equation of state and Poisson's equation. The Earth is modelled as a three-layered body, with an inelastic inner core, an inviscid fluid core and an inelastic mantle sustaining convection, which,induces deviations from hydrostatic equilibrium. The model also incorporates ocean corrections deduced from dynamic ocean models. It is shown that the non-hydrostatic structure inside the Earth has an effect of less than 0.1 per cent on the transfer functions, while the different modellings of mantle inelasticity (different combinations of possible values for the inelastic parameters) can lead to a wide range of results. Finally, we show that the precision of the geodetic observations of UT1 and the precision of the oceanic and atmospheric corrections are not yet sufficient to obtain information about mantle inelasticity from the comparison between theoretical models and geodetic observations