The Earth's atmosphere is one of the major error sources degrading the potential of kinematic positioning. In this study, we focus on the abnormal ionospheric activity during the Halloween (super-) storm of 29-31 October 2003 As a first step we discuss of the effect of such ionospheric activity on kinematic positioning.

As a second step we produced Total Electron Content (TEC) maps to characterize the ionospheric activity during this period from dual frequency continuous operating GPS stations of the EUREF permanent network (EPN) using the Bernese v5.0 software. Those maps, computed each hour with a one degree step grid, present greater spatial and temporal resolutions than CODE or IGS TEC maps, and allow us to detect more rapid and isolate abnormal activity above the Europe. In addition, we will investigate the potential benefit of complementing these TEC maps with the residuals from the geometry-free combination to better evidence small scale ionospheric effects.

In the equatorial regions, strong irregularities in the ionospheric density may affect the radio-propagation causing phase and amplitude fluctuations. Some applications like precise positioning based on Differential GPS or the use of corrections transmitted via a geostationary satellite may be interrupted during night hours. In the case of off-shore exploration, they may result in significant business loss. Monitoring and even more forecasting equatorial scintillations are of great interest for real time high precision geodesy. Taking advantage of a permanent ionospheric scintillation monitor (ISM) at Kourou - that samples the GPS signals at 50 Hz and provides the classical (S4, sigma-phi) scintillation parameters - the opportunity to use some collocated IGS data have been analysed. Even if the GPS receivers of the IGS worldwide network have much lower repetition rate (1 s or 30 s) some useful information may be extracted for operational monitoring. We have developed a tool to analyse the perturbations that affect a station, its behaviour in space and time and to separate possible other effects than ionosphere (multi-paths). We have also initiated a statistic analysis about the correlation between the scintillation occurrence observed in the two first hours of the night and the one in the following hours at Kourou. This approach may be used for a short term forecast indicator knowing that only climatological forecast models are available. In order to prepare to the coming solar cycle maximum and to the increase of scintillation effects, the implementation of an equatorial permanent ISM network and the verification of higher robustness of new GNSS signals are major points to consider.

Under perturbed external condition the high latitude ionosphere may be affected by small-scale and intermediate-scale (from centimetres to hundreds of meters) structures or irregularities imbedded in the large-scale (tens of kilometers) ambient ionosphere. Intermediate-scale irregularities produce short term phase and amplitude fluctuations in the carrier of the radio waves which pass through them. These effects are called Amplitude and Phase Ionospheric Scintillations. The gradient drift instability is often regarded as the principal mechanism producing amplitude and phase scintillations (see, e.g., Tsunoda, 1988, Wernik et al., 2003 and ref. therein).This same instability is also indicated as the main origin of the back scattering echoes which are observed by the SuperDARN coherent HF radars. On this basis, we investigate spatial and temporal correlations between GPS scintillations and SuperDARN echo characteristics in the ionosphere using data frm the GPS receiver run by INGV at Ny Alesund and from the Cutlass SuperDARN radars.

The stability of time and frequency transfer with GPS is limited by the fact that the GPS signal travels through ionosphere.

In high precision geodetic time transfer, performed using the Precise Point Positioning (PPP) approach, when dual-frequency GPS data are available, the so-called ionosphere-free combinations P3 (code) and L3 (carrier-phase) are used to remove the first order ionosphere effect.

In this paper, we investigate the impact of ionosphere perturbations (second order and higher) on time transfer when using the P3 and L3 combinations. We further correct the "ionosphere-free" time transfer solutions for second and third order ionosphere effects.

All time transfer computations have been done using the software Atomium, developed at the Royal Observatory of Belgium, based on a least-square analysis of dual-frequency carrier phase and code measurements, which is able to provide clock solutions in PPP mode. The implementation of ionosphere higher order terms in the existing Atomium P3L3-analysis procedure is described, and an illustration of the impact of unmodeled second and higher order ionosphere effects on the time transfer solutions is provided. Second order ionosphere effects can reach about 1mm on a quiet day and lead to corresponding contributions at the level of 10^-12 s in station clock synchronization error estimates, which is of the level of targeted precision of future BIPM (Bureau International des Poids et Measures) timing products.

Nowadays, Global Navigation Satellite Systems or GNSS allow to measure positions in real time with an accuracy ranging from a few meters to a few centimeters mainly depending on the type of observable (code or phase measurements) and on the positioning mode used (absolute or differential). The best precisions can be reached in differential mode using phase measurements. In differential mode, mobile users improve their positioning precision thanks to so-called "differential corrections" provided by a fixed reference station. For example, the Real Time Kinematic technique (RTK) allows to measure positions in real time with precision usually better than a decimeter. In practice, the ionospheric effects on GNSS radio signals remain the main factor which limits the precision and the reliability of real time differential positioning. As differential applications are based on the assumption that the measurements made by the reference station and by the mobile user are affected in the same way by ionospheric effects, these applications are influenced by gradients in TEC between the reference station and the user. For this reason, local variability in the ionospheric plasma can be the origin of strong degradations in positioning precision.

In this paper, we characterize local variability in the ionosphere which can pose a threat to high precision real time differential positioning.

GNSS carrier phase measurements can be used to monitor local TEC variability: small-scale ionospheric structures can be detected by monitoring TEC high frequency changes at a single station; as ionospheric disturbances are moving, we can expect that such structures will induce TEC temporal variability which can be detected at a single station. We applied this method (called the "one-station" method) to the GPS data collected at the permanent (mid-latitude) station of Brussels from 1994 to 2006. Two main types of structures have been observed: Travelling Ionospheric Disturbances (TID's) and "noise-like" structures. We have performed a climatological study of these phenomena on a period which covers one solar cycle. TID's have strong seasonal and solar cycle dependence when noise-like structures are "ionospheric variability" which is usually observed during geomagnetic storms. The largest Rate of TEC (RoTEC) detected at Brussels during the period considered in our study were observed during severe geomagnetic storms. For example, the storm of 30th October 2003 was responsible for the RoTEC (9.839 TECU/min) observed from 1994 to 2006. The amplitude of Rate of TEC due to TID's is by far smaller than the amplitude of RoTEC due to geomagnetic storms: the analysis of a lot of TID cases shows that the maximum RoTEC value observed during the occurrence of a TID was about 1.5 TECU/min. Moreover, we found that strong irregularities occur even during solar minimum, for example, during summer 2006, where gradients up to 1.2 TECU/min were reached. This means that, even during periods where the probability of occurrence of ionospheric irregularities is very low, large RoTEC can occur.

The one-station method allows to measure variability in time but GNSS differential applications are affected by variability in space between the user and the reference station. Therefore, in a second step, we measured TEC differential variability (using double differences of phase measurements) during few typical ionospheric conditions: quiet ionospheric activity, medium and large amplitude TID's and noise-like variability due to a severe geomagnetic storm. As a last step, we developed software which reproduces positioning conditions experienced by RTK users on the field. We used this software to assess positioning errors due to the different ionospheric conditions considered in the previous step. Again, the largest effects were observed during the occurrence of geomagnetic storms where the positioning error due to ionosphere reached 80 cm. The maximal positioning errors observed for TID's were 15 cm and 22 cm, respectively for the medium-amplitude TID and the large-amplitude TID.

The Istituto Nazionale di Geofisica e Vulcanologia (INGV) and the Institute of Engineering Surveying and Space Geodesy (IESSG) of the University of Nottingham manage the same kind of GPS receivers to monitor the Total Electron Content (TEC) and the ionospheric scintillations over the European middle and high latitude regions. The activity started on 2001 and it is still in progress. As the high sampling frequency (50 Hz) of the Novatel systems deployed, integrated with a firmware able to compute and provide in near real time the ionospheric parameters, these stations contribute concretely to the understanding of the Sun-magnetosphere-ionosphere interactions throughout the observation of the scintillation effects on the GNSS systems. Such effects can abruptly corrupt the performance of the positioning systems affecting, in turn, the awareness and safety of the modern devices.

In this paper we present some first results obtained analysing the scintillation data acquired by three stations: Ny Alesund (78.9 N, 11.9 E; mlat: 76 N), Hammerfest (70.7 N, 23.7 E; mlat: 67.2 N) and Bronnoysund (65.5 N, 12.2 E; mlat: 62.6 N). The first site is located under the cusp for the majority of the time, the other two are auroral stations. The behaviour of the scintillation occurrence as function of the magnetic local time and of the corrected magnetic latitude is investigated to characterize the climatology of the scintillation conditions. The aim of this type of analysis is, once developed over a robust data set, to provide information useful for the development of forecasting algorithms.

Transionospheric radio signals may experience fluctuations in their amplitude and phase due to irregularity in the spatial electron density distribution, referred to as scintillation. Ionospheric scintillation is responsible for transionospheric signal degradation that can affect the performance of satellite based navigation systems.

Usually, the scintillation activity is measured by means of indices such as the normalised standard deviation of the received intensity and the standard deviation of the received phase.

Here, a statistical analysis on the use of some additional parameter is carried out based on 50 Hz GPS measurements recorded at Dirigibile Italia Station (Ny-Alesund, Svalbard). The usefulness of such a parameter in the understanding of the signal dynamics due to ionospheric electron density irregularities is discussed with regard to both scientific aspects and receiver performance.

The modelling of the Total Electron Content (TEC) plays an important role in global satellite navigation systems (GNSS) accuracy, especially for single-frequency receivers, the most common ones constituting the mass market. For the latter and in the framework of Galileo, the NeQuick model has been chosen for correcting the ionospheric error contribution. It has been designed to calculate the electron density at a given point of the ionosphere according to the time conditions and the solar activity. This electron density can be integrated along the path from the receiver to the considered satellite to provide the TEC. For Galileo, a parameter Az ("effective ionization level") will be provided to the model as solar activity information and will be daily updated by the ground stations.

Since NeQuick was chosen for Galileo purpose, a new version of the model has been released. It involves simplifications in the representation of the bottomside as well as a unique formula for a key parameter of the topside formulation previously defined through two equations, each one used for six months of the year. Hence we decided to investigate consecutive improvements and remaining weaknesses of this new formulation.

To this extent, we take benefit of various ionosphere data from several European stations (Chilton in UK, Dourbes in Belgium, El Arenosillo and Roquetes in Spain, Pruhonice in Czech Republic) where ionosonde and GPS TEC data are available for different solar activity levels. These data allow us to study NeQuick representation of the ionosphere at mid-latitudes. We investigate the difference between GPS-derived vTEC and corresponding values from NeQuick for the latest years (between solar maximum in 2000 and minimum in 2007) in order to observe the temporal dependencies towards Universal Time, season and solar activity. We use ionosonde data to constrain the model so that we can concentrate on its formulation of the profile only. We especially highlight the improvements from the latest (second) version of NeQuick and show the critical importance of the topside formulation.

We investigated the response of TEC over Europe to summer and winter geomagnetic storms using observations of European GPS network. The spatial and temporal evolution of TEC during storm was analyzed on base of TEC maps which created with 1 hour interval in latitudinal range of 35-70 N. Comparison of storm effects was carried out for severe geomagnetic storm disturbances of July 22-28, 2004 and November 6-12, 2004. Both storms started near midnight.

July storm had three active phases and November one had two phases, maximal sum of Kp reached the value of 60 for both cases. The positive and negative disturbances were observed for summer and winter events. Strong enhancement of TEC was detected in winter on higher latitudes mainly at nighttime. In summer storm the negative disturbances were prevailed the whole days of the considered period. The strong short duration of positive perturbations were detected in daytime in winter storm. Features of the summer storm were short-term positive perturbations which in contrast winter storm were observed against daily depression of TEC several days after beginning of disturbances. It is interesting feature of the storm was occurrence of main trough in TEC distribution in summer condition, though, as known the trough usually is well pronounced in winter conditions. The trough was recognized on the TEC maps in evening and night-time ionosphere. During maximal geomagnetic activity the trough was migrated until latitudes of 50N.

In the coming years it is expected that a periodic disturbance in the Earth’s ionosphere will affect the radio signals that are transmitted by GNSS (Global Navigation Satellite System, i.e., GPS, GLONASS and future Galileo and Compass) satellites. This phenomenon is caused by a period of increased solar activity that repeats itself every 11 years, a period during which the Sun’s surface shows an increased number of dark areas, the so-called sun spots. In March 2008 we reached the end of a quiet period and it is expected that we will see the effects of an increase in ionospheric activity on GNSS signals from the second half of 2008 onwards. The highest levels of these ionospheric disturbances are expected to be reached around 2011-2012 and the next minimum around 2018. The effects of these disturbances will be most prominent in equatorial (essentially following the geomagnetic equator) and Polar Regions. Mid-latitude areas are less affected. In anticipation of the above effects, all high precision GNSS services provided by Fugro are prepared to mitigate the adverse effects that can result from this phenomenon. This is done by a variety of methods, the most important being the utilization of dual frequency GNSS receivers that provide data that can be used to virtually eliminate ionospheric effects. As the correction signals that are transmitted from reference to mobile receivers can also be affected by increased solar activity, Fugro provides an option to deliver corrections via multiple independent satellite links in order to reduce the risk of failing communication in these adverse conditions. For the areas most affected by ionospheric disturbances, Fugro also uses terrestrial radio links. Further, the DGNSS services that Fugro provide use a network of reference stations, many of which contain a combined GPS/GLONASS dual frequency receiver. The GPS/GLONASS stations are located in regions that are affected most by solar activity. The additional GLONASS data can be used to reduce the effects of ionospheric disturbances in those regions even further. In the coming year, Fugro will expand its GLONASS capabilities and should Galileo become available, then also this system will be fully incorporated to provide the highest quality services possible to support our client base. Within the framework of GPS modernization more satellites will become operational that transmit an additional, new civil signal on L2, called L2C, allowing for much improved signal tracking. The L2C signal will help to mitigate the scintillation effects that cause GPS signals to vary very rapidly and may make GPS receivers lose lock on the existing L2 frequency. Fugro’s hardware and services are fully prepared to take advantage of the new L2C signal. Currently six out of 31 GPS satellites are transmitting this additional signal, a number that will increase in the years to come. All Fugro's high end DGNSS hardware systems and services are prepared for dual frequency GPS, including L2C, GLONASS and Galileo to provide a solid foundation for further expansion of performance and services.