Simultaneous multiplicative column normalized method (SMART) for the 3D ionosphere tomography in comparison with other algebraic methods

The accuracy and availability of satellite-based applications like GNSS positioning and remote sensing crucially depends on the knowledge of the ionospheric electron density distribution. The tomography of the ionosphere is one of the major tools to provide link specific ionospheric corrections as well as to study and monitor physical processes in the ionosphere. In this paper, we introduce a simultaneous multiplicative column-normalized method (SMART) for electron density reconstruction. Further, SMART+ is developed by combining SMART with a successive correction method. In this way, a balancing between the measurements of intersected and not intersected voxels is realised. The methods are compared with the well-known algebraic reconstruction techniques ART and SART. All the four methods are applied to reconstruct the 3-D electron density distribution by ingestion of ground-based GNSS TEC data into the NeQuick model. The comparative case study is implemented over Europe during two periods of the year 2011 covering quiet to disturbed ionospheric conditions. In particular, the performance of the methods is compared in terms of the convergence behaviour and the capability to reproduce sTEC and electron density profiles. For this purpose, independent sTEC data of four IGS stations and electron density profiles of four ionosonde stations are taken as reference. The results indicate that SMART significantly reduces the number of iterations necessary to achieve a predefined accuracy level. Further, SMART+ decreases the median of the absolute sTEC error up to 15, 22, 46 and 67% compared to SMART, SART, ART and NeQuick respectively

High-Resolution Reconstruction of the Ionosphere for SAR Applications

Caused by ionosphere’s strong impact on radio signal propagation, high resolution and highly accurate reconstructions of the ionosphere’s electron density distribution are demanded for a large number of applications, e.g. to contribute to the mitigation of ionospheric effects on Synthetic Aperture Radar (SAR) measurements. As a new generation of remote sensing satellites the TanDEM-L radar mission is planned to improve the understanding and modelling ability of global environmental processes and ecosystem change. TanDEM-L will operate in L-band with a wavelength of approximately 24 cm enabling a stronger penetration capability compared to X-band (3 cm) or C-band (5 cm). But accompanied by the lower frequency of the TanDEM-L signals the influence of the ionosphere will increase. In particular small scale irregularities of the ionosphere might lead to electron density variations within the synthetic aperture length of the TanDEM-L satellite and in turn might result into blurring and azimuth pixel shifts. Hence the quality of the radar image worsens if the ionospheric effects are not mitigated. The Helmholtz Alliance project “Remote Sensing and Earth System Dynamics” (EDA) aims in the preparation of the HGF centres and the science community for the utilisation and integration of the TanDEM-L products into the study of the Earth’s system. One significant point thereby is to cope with the mentioned ionospheric effects. Therefore different strategies towards achieving this objective are pursued: the mitigation of the ionospheric effects based on the radar data itself, the mitigation based on external information like global Total Electron Content (TEC) maps or reconstructions of the ionosphere and the combination of external information and radar data. In this presentation we describe the geostatistical approach chosen to analyse the behaviour of the ionosphere and to provide a high resolution 3D electron density reconstruction. As first step the horizontal structure of the ionosphere is studied in space and time on the base of ground-based TEC measurements in the European region. In order to determine the correlation of measurements at different locations or points of time the TEC measurements are subtracted by a base model to define a stationary random field. We outline the application of the NeQuick model and the final IGS TEC maps as background and show first results regarding the distribution and the stationarity of the resulting residuals. Moreover, the occurred problems and questions are discussed and finally an outlook towards the next modelling steps is presented

Mathematical Approaches in GNSS Positioning and Integrity Monitoring

The development of Global Navigation Satellite System (GNSS) augmentation systems plays an important role for the warranty of high accuracy and integrity in satellite based positioning. Since 2006 the DLR has developed an experimental “Maritime Ground Based Augmentation System (MGBAS)” for safety critical maritime applications (e.g. port and docking manoeuvres) in the research port Rostock. On the one hand, this system allows the users to mitigate typical GNSS errors, like atmospheric delay, by the provision of Real-Time Kinematic (RTK) services. On the other hand, it supports the detection of faulty satellite signals and the assessment of the possible positioning accuracy aboard. At the beginning, this talk will outline the least squares based principle of satellite positioning. Afterwards the currently implemented RTK algorithm based on Kalman-Filter and Integer Least Squares (ILS) will be described. Concluding, a brief introduction to issues of integrity monitoring will be presented

Integrity concepts for future maritime Ground Based Augmentation Systems

Global Navigation Satellite Systems (GNSS) require augmentation to achieve integrity and accuracy performance for high-precise safety of life applications. The current standard maritime GNSS augmentation system is a differential GPS (DGPS) beacon system, which provides correction data and integrity information according to the IALA-standard [IALA-R-121]. They are broadcasted in the 300 kHz radio-navigation band in accordance with ITU-R Recommendations [DIN EN 61108-4]. Even if such systems, also called Ground Based Augmentation Systems (GBAS), increase the accuracy and integrity of GNSS substantially, the performance reached by these systems is not sufficient to meet all International Maritime Organization (IMO) requirements, especially those for critical traffic areas like ports and for e.g. automatic docking manoeuvres [IMO A.915(22)]. In order to support the applicability of satellite navigation in such areas, the German Aerospace Centre (DLR) has started to develop a maritime GBAS that meets all IMO requirements. While the current IALA (International Association of Marine Aids to Navigation and Lighthouse Authorities) GBAS is a Code-based Differential GNSS (C-DGNSS), what means it broadcasts information concerning code corrections, our developments aim for multi-frequency Phase-based Differential GNSS (P-DGNSS). For this purpose DLR has installed an experimental maritime GBAS in the port of Rostock (Germany) enabling algorithm development in the ground and user subsystem as well as their validation. The ground subsystem consists of two independent stations. The first station is operating as reference station and the second one as integrity monitoring station. This is similar to the hardware architectural design of the current IALA Beacon DGNSS architecture [IALA-R-121], whereby the GBAS uses high-rate receivers to enable a fast signal assessment in real time. Moreover, the proposed software architecture consists of real time processor chains that enable a hierarchical assessment from single data types via satellite signals up to the used GNSS with respect to the supported P-DGNSS service. Each of the implemented processors provides quality parameters like code and phase noise, Signal to Noise Ratio (SNR), Horizontal Positioning Error (HPE). These are considered as suitable input data for the GBAS integrity monitoring and the conditional provision of augmentation data and integrity flags. Thus Performance Key Identifiers (PKI) must be specified for each quality parameter which allows distinguishing between the nominal and the disturbed behaviour of GNSS and GBAS according to different positioning performances. The GBAS is complemented by a statistical analysis, which is deriving statistical performance parameters with respect to real time quality parameter collected during the previous 24 hours. The statistical performance parameters are used in the first instance to gradually improve the measuring models by an auto-adaptive system and to specify PKIs described by valid value ranges and thresholds. Then they are employed to detect outliers in real time and to estimate protection levels. The proposed quality parameters and related PKIs have been derived from 20 Hz GPS raw data of four GBAS stations in Germany (Research Port Rostock, DLR in Neustrelitz, Braunschweig) and France (Toulouse). Based on examples it will be shown that the nominal signal behaviour at the reference station can be employed to detect signal disturbances during GBAS operation in real time. In addition to the investigation of the single performance key identifiers, special attention is paid to the description of dependencies between the various performance key identifiers

Midlatitude Ionospheric density depletion and its impacts on GNSS

Ionospheric disturbances are the source of accuracy degradation of Global Navigation Satellite System (GNSS) observables and they can cause harm to GNSS positioning techniques, especially for standalone users. The disturbance effects can be rapid and most enhanced by large geomagnetic storm that is still challenging to predict and model. Higher resolution measurement is one of the keys to better understand the storm time impacts on GNSS. During the geomagnetic storm on 17.03.2015, known as St. Patrick’s Day storm, large perturbation of geomagnetic fields was observed even in middle latitudes in European region. In northeast Germany, the largest magnetic disturbances and accompanying plasma density changes were observed from afternoon to evening hours. Multiple GNSS satellites measured unusually large drops of Total Electron Content (TEC) in negative phases of the storm. We demonstrate the performance of GNSS positioning in Single Point Positioning (SPP) and Precise Point Positioning (PPP) techniques for the different phases of ionospheric disturbances. The large errors seems to be occurred when the electron density drops sharply compared to quiet time conditions

Investigations of Decorrelation Effects on the Performance of DGNSS Systems in the Baltic Sea

Differential Global Navigation Satellite Systems (DGNSS) are a commonly applied technique for safety critical (Safety-of-Life) navigational operations. Since the nineties an augmentation system following the IALA Beacon DGNSS standard has been employed in the maritime sector. As main components the system comprises a reference station and an integrity monitoring station. With the help of the reference station code based corrections are calculated. Simultaneously the reference station and integrity monitoring station run tests regarding the performance of the system to inform the user within a specified time when the system should not be used for navigation. The gained corrections and integrity information are transmitted in the RTCM format via a medium frequency antenna and can be received by users in the surroundings of almost 300 kilometres. The provided corrections represent one of the two key functions of the DGNSS and allow the user to mitigate errors falsifying the own received pseudoranges. The calculated corrections are generated at the reference station site at a certain time. Due to this fact the longer the distance between the reference station and the user is and the more delayed the corrections are the less they are valid. The IALA has specified the accuracy degradation with 0.4 to 1m for each 100nm. Based on measurement activities in the Baltic Sea the paper discusses the performance of the current maritime DGNSS regarding the spatial and temporal decorrelation effects

Comparing different assimilation techniques for the ionospheric F2 layer reconstruction

From the applications perspective the electron density is the major determining parameter of the ionosphere due to its strong impact on the radio signal propagation. As the most ionized ionospheric region, the F2 layer has the most pronounced effect on transionospheric radio wave propagation. The maximum electron density of the F2 layer, NmF2, and its height, hmF2, are of particular interest for radio communication applications as well as for characterizing the ionosphere. Since these ionospheric key parameters decisively shape the vertical electron density profiles, the precise calculation of them is of crucial importance for an accurate 3-D electron density reconstruction. The vertical sounding by ionosondes provides the most reliable source of F2 peak measurements. Within this paper, we compare the following data assimilation methods incorporating ionosonde measurements into a background model: Optimal Interpolation (OI), OI with time forecast (OI FC), the Successive Correction Method (SCM), and a modified SCM (MSCM) working with a daytime-dependent measurement error variance. These approaches are validated with the measurements of nine ionosonde stations for two periods covering quiet and disturbed ionospheric conditions. In particular, for the quiet period, we show that MSCM outperforms the other assimilation methods and allows an accuracy gain up to 75% for NmF2 and 37% for hmF2 compared to the background model. For the disturbed period, OI FC reveals the most promising results with improvements up to 79% for NmF2 and 50% for hmF2 compared to the background and up to 42% for NmF2 and 16% for hmF2 compared to OI