Space Communications

This paper presents briefly the interaction between the ionosphere and the navigation signals, the ionosphere activity with its different effects and when possible a guideline on the different models that encompass the relevant effect.

A regular EGNOS Test Bed (ESTB) data collecting, 24 h per week, and analysis campaign has been developed by Eurocontrol along 2002 and 2003. During this period a geomagnetic storm was experienced on October 29th–31st 2003, degrading the ESTB performance in Europe.

In this work we present a study of the effects of this storm in several European sites on October 30th. The ESTB performance is monitored from a network of GPS receivers widely distributed over Europe, including the ESTB reference stations (RIMS), and the geographical degradation of the accuracy is analyzed. Also, the correlation between the geomagnetic activity index with the number of events with protection level failures is shown. In the most severe stormy periods, the errors in the ESTB ionospheric corrections and its integrity bounds are analyzed to explain the peaks in the navigation system error, which produces Misleading Information events.

This study is devoted to analyze the performance of the ESTB system, which is a simple prototype of the “true” final EGNOS system with several design limitations.

The IESSG, at the University of Nottingham, first initiated studies on the effects of ionospheric scintillation and Total Electron Content (TEC) gradients on GNSS users late in 2000. A network of four state-of-the-art GPS Ionospheric Scintillation Monitor receivers was set up in June 2001 to collect GPS phase and amplitude scintillation parameters, as well as TEC data, forming a Northern European monitoring network. Investigations were then carried out involving in particular the analysis of standalone GPS, DGPS, EGNOS aided DGPS and carrier phase user errors, which have been correlated with observed scintillation levels and with geomagnetic indices. A comprehensive statistical analysis was carried out, aiming to characterise ionospheric scintillation over Northern Europe. Amongst many results from our study, which mostly covered our 2002 data archive, analyses of occurrence of high levels of scintillation disturbing simultaneously a number of satellites showed that, on a day of enhanced geomagnetic activity, for up to nearly 2% of the time, two satellites may be concurrently affected. If it can be shown that scintillation levels over a certain threshold will lead to receiver loss of lock on satellites, then in addition to constellation geometry degradation, this may prove crucial during periods when only 4 or 5 satellites are in view. This scenario, of simultaneous failure of a number of satellites, is the consequential situation most likely to impact on the user community in Northern Europe, especially those involved in safety-critical applications. Also, SBAS (Satellite Based Augmentation Systems) reference stations may be adversely affected during these periods of time as they rely on both GPS carriers (L1 and L2) to compute ionospheric delay corrections for dissemination to users. This paper focuses on our latest results and includes further analyses involving isolated geomagnetically active periods as well as statistical analyses covering a larger part of our data archive (2002–2003), which have recently been carried out aiming to better establish the implications of ionospheric scintillation for GNSS users during the high of the solar cycle.

This paper deals with the problem of ionosphere scintillations and their impact on the GPS positioning errors.

To develop and test real-time Fade Mitigation Techniques control algorithms, propagation time series are needed. An alternative to using real data collected from propagation experiments is to generate typical fading time-series making use of climatological characteristics as well as geometrical and radio-electrical parameters of the link. The objective of this paper is to present the improvement of the rain attenuation dynamic model based on the Maseng–Bakken model. Some of the theoretical hypotheses have been verified, a methodology to assess input parameters has been tested, and the sensitivity of the model with respect to internal parameters has been studied. Then, a new validation methodology that aims at assessing the physical soundness of synthesized events has been developed. Finally it has been applied to the ONERA-CNES channel model.

The simulation of a fine space-time structure of rain has great potential applications in many fields of research such as radar-meteorology, hydrology, and telecommunications.

A rainfall simulator has been developed based on simple assumptions on the release of raindrops from cloud bases, and on a model of raindrop interarrival time and size distributions at ground level, derived from a statistical analysis of actual measurements using CETP's disdrometer over more than a year. Among the simple assumptions used, the probability of precipitating drops follows an exponential law related to the drop size.

This Monte-Carlo simulation can be performed in two dimensions giving the spatial distribution of rain drops in a vertical plane. From this simulation, time series of rain rates, but also of microwave attenuations at a given frequency and elevation angle can be derived.

The simulator is designed so that the main characteristics of rain observed on the ground are preserved: size distribution, interarrival time distribution and autocovariance of drop sizes. Results show that quantities derived from the series of raindrops generated by the simulator, such as time series of rain rate or attenuation, are in good agreement with observations.

This paper describes two methods for generating synthetic rain rate time-series capable of being used to simulate the performance of radio communication systems operating above 10 GHz. Rain rates are modelled because they are widely available and because an approximate link to signal attenuation can be established owing to the significant correlation between them. The proposed models are based on hierarchic Markov chains. Rain and no rain events are simulated by the outer chain which simulates the rain event duration on the basis of the experimental statistics. The inner chain of both models deals with the rain intensity generation. The models therefore produce simulated rain samples whose statistics very closely match those of the experimental data without using any stored rain time series.

This paper is related to the modelling of rain fields and attenuation fields at small (size of a rain cell), mid (∼150 km²) and large scale (∼1000 km²). The methodology lies on a cellular description of rain fields by the HYCELL model. The latter allows not only to describe the rain cell horizontal structure but also to generate two dimensional rain rate fields at mid scale (∼150 km², terrestrial network, satellite telecommunication beam) which account for the local Cumulative Distribution Function given as an input parameter. Nevertheless, the mid-scale fields generated that way are not spatially correlated. To overcome this limitation, using radar observations at mid-scale, the rain field internal organisation is analysed as a function of the rain intensity (intercellular distances, nearest neighbour distance, inter aggregate distance at mid-scale). This observational study allows to model the rain cell spatial location within a mid-scale area by a doubly aggregative isotropic random walk. Coupled with the HYCELL modelling of rain field at mid-scale, the random walk allows to generate rain fields spatially correlated at mid-scale, while accounting for the local climatology characteristic to the simulation area.

The next step of the paper is related to the modelling of rain field at large scale (∼1000 km², French national territory for example). In this context, low-pressure systems are modelled in the Fourier plane, assuming an anisotropic covariance function. The large scale field obtained that way is then split into mid-scale areas over which we proceed to the generation of (mid-scale) rain fields according to the HYCELL methodology presented above. The rain field is then spatially correlated at mid and large scale. It accounts for the climatological characteristics over each of the mid-scale areas which compose it. Assuming a telecommunication link with OLYMPUS (19°W), the rain fields simulated at large scale over the French national territory are then turned into attenuation fields. The statistical properties of the resulting rain fields and attenuation fields are then compared to radar observations and ITU-R recommendations. It is concluded that this large scale model is a new tool which deserves to be considered by system designers to compute propagation parameters. In this context, typical two-dimensional rain rate fields and attenuation fields over an area corresponding to the size of a country, a satellite beam, a Local Multipoint Distribution Services (LMDS) network or a radar coverage can be simulated to evaluate diversity gain, terrestrial or slant path attenuation for different azimuth directions while accounting for the local meteorological characteristics.