State modelling of the land mobilepropagation channel for dual-satellite systems

The quality of service of mobile satellite reception can be improved by using multi-satellite diversity (angle diversity). The recently finalised MiLADY project targeted therefore on the evaluation and modelling of the multi-satellite propagation channel for land mobile users with focus on broadcasting applications. The narrowband model combines the parameters from two measurement campaigns: In the U.S. the power levels of the Satellite Digital Audio Radio Services were recorded with a high sample rate to analyse fast and slow fading effects in great detail. In a complementary campaign signals of Global Navigation Satellite Systems (GNSS) were analysed to obtain information about the slow fading correlation for almost any satellite constellation. The new channel model can be used to generate time series for various satellite constellations in different environments. This article focuses on realistic state sequence modelling for angle diversity, confining on two satellites. For this purpose, different state modelling methods providing a joint generation of the states ‘good good’, ‘good bad’, ‘bad good’ and ‘bad bad’ are compared. Measurements and re-simulated data are analysed for various elevation combinations and azimuth separations in terms of the state probabilities, state duration statistics, and correlation coefficients. The finally proposed state model is based on semi-Markov chains assuming a log-normal state duration distribution

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

Statistical characterization of strong and mid solar flares and sun EUV rate monitoring with GNSS

The global network of permanent Global Navigation Satellite Systems (GNSS) receivers has become an useful and affordable way of monitoring the Solar EUV flux rate, especially -for the time being- in the context of Major and Mid geoeffective intensity Solar Flares (M. Hernandez-Pajares et al., SpaceWeather, doi:10.1029/2012SW000826, 2012). In fact the maturity of this technique (GNSS Solar FLAre Indicator, GSFLAI) has allowed to incorporate it in operational real-time (RT) conditions, thanks to the availability of global GNSS datastreams from the RT International GNSS Network (M. Caissy et al, GPS World, June 1, 2012), and performed in the context of the MONITOR and MONITOR2 ESA-funded projects (Y. Beniguel et al., NAVITEC Proc., 978-1-4673-2011-5 IEEE, 2012). The main goal of this presentation is to summarize a detailed recent study of the statistical properties of Solar Flares (E. Monte and M. Hernandez-Pajares, J. Geophys. Res., doi:10.1002/2014JA020206, 2014) by considering the GNSS proxy of EUV rate (GSFLAI parameter) computed independently each 30 seconds during the whole last solar cycle. An statistical model has been characterized that explains the empirical results such as (a) the persistence and presence of bursts of solar flares and (b) their long tail peak values of the solar flux variation, which can be characterized by: (1) A fractional Brownian model for the long-term dependence, and (2), a power law distribution for the time series extreme values. Finally, an update of the Solar Flares’ occurrence during the recent months of Solar Activity, gathered in RT within MONITOR2 project, will close the paper.Postprint (published version

MONITOR Ionospheric Monitoring System: GNSS performance estimation

MONITOR is a project from the European Space Agency’s GNSS Evolutions Programme started in 2010, dedicated to the collection of data and products during active periods of solar activity for later understanding of the impact of ionospheric effects on EGNOS and Galileo system performance. In the frame of this project several tasks have been achieved, in particular the deployment of a network of scintillation receivers (Novatel + Septentrio + GISMO) mainly at low and high latitudes, the development of a real time Central Archiving and Processing Facility (CAPF) and the development of dedicated processors to generate user oriented outputs for TEC, scintillation, and space weather issues. This project, in its new phase started in 2014, is moving forward with an improved and updated scope, addressing in addition to general ionospheric monitoring, the generation of dedicated products and reports to EGNOS system evolution, international collaboration in related ionospheric topics including feasibility studies in Africa. The main new features are: an upgraded data archiving system providing improved accessibility, the integration of data from SAGAIE network from French Space Agency, CNES and the exploitation of its data for new products, new station deployment in regions of interest (mainly in West and Central Africa and in high latitudes in Europe), and the upgrade and development of new products allowing better analysis of geophysical conditions during periods of compromised system performance and service. As an example, the Along Arc TEC Rate (AATR index) is computed routinely, as it has proven to be a clear indicator of ionospheric activity that degrades SBAS system performance. In addition, Monitor already produces VTEC maps (obtained using various techniques and algorithms), several space weather indicators including solar flare detection, ROTI maps, indices related to the quality of measurements and scintillation analysis tools. This paper focuses on the relationship of an SBAS system (EGNOS, WAAS) to the ionosphere’s variability and will analyse in detail the ionospheric parameters leading to a decrease or compromise of system performance. Several case studies will highlight significant EGNOS events for this purpose. The paper will demonstrate how AATR is able to discriminate availability degradation due to ionospheric events from other effects. The ionosphere scintillation aspects and the last developments of the GISM model will also be addressed for this issue.Peer ReviewedPostprint (author's final draft

SNACS – The Satellite Navigation Radio Channel Signal Simulator

A versatile GNSS signal simulation tool for time-domain simulations at sample-level with a focus on wave propagation effects, such as multipath or atmospheric disturbances, is introduced. The Satellite Navigation Radio Channel Signal Simulator (SNACS) uses a modular, object-oriented approach implemented in C++. The application is thoroughly designed in a multi-threaded way to provide highly accurate simulation results in a reasonable processing time. SNACS not only fills the gap of a software tool using models of the GNSS channel from a GNSS signal generator as input for a GNSS software receiver, it also provides an implementation that spans the whole simulation chain from the transmitter to the receiver loops. SNACS can be adopted and enhanced easily since it is published under an open source license

GNSS Software Simulation System for Realistic High-Multipath Environments

Compared to the various effects which degrade GNSS performance in general, nowadays multipath propagation accounts for the most dominant error in satellite navigation. Other error sources like satellite clock deviation and atmospheric effects for example can be compensated to a certain degree by the use of high-stability timing equipment (as for example the hydrogen maser in GIOVE-B represents), SBAS corrections, multi-frequency Galileo ranging and pilot signals, and the future availability of civil signals with a higher bandwidth than the currently available C/A code signal. Especially in high-multipath environments like urban and suburban areas, the performance of GNSS receivers is severely affected by multipath propagation. Extensive measurement campaigns were undertaken in 2005 by the German Aerospace Center (DLR) for different scenarios such as the vehicular urban, sub-urban, and rural environments, and the pedestrian use case to record and model effects caused by multipath signal reception. These measurement campaigns lead to the availability of sophisticated channel models consisting of combined stochastic and deterministic parts that allow for the investigation of multipath effects on GNSS receiver performance. These realistic channel models provide series of channel impulse responses (CIR) as outputs where the plethora of distinct echoes in urban scenarios is represented by Dirac impulses at quasi time-continuous instants. However, the application of such time-continuous CIRs to an accurate GNSS signal propagation simulation proves to be a demanding task due to the high sampling rates which are necessary to cover the channel's complexity. Before the time continuous CIRs can be applied in a simulation they have to be adjusted to fit to the time discrete sampling instants. This process is done using low-pass interpolation with a sinc function. The presented work introduces a modular C++ framework that is able of reproducing the harsh conditions of urban environments in a very precise manner. This new efficient and flexible software tool implements the whole GNSS simulation chain consisting of “signal generation – channel model – receiver” in time-domain as a sample-true simulation. The software’s main features are given after a description of the DLR GNSS urban channel model. Additionally the interpolation process of transforming the time-continuous channel impulse responses to FIR coefficients is outlined. Eventually, a demonstration of simulation runs using the urban channel model, a BOC(1,1), and a CBOC signal is given