Comparison of SAGE and classical multi-antenna algorithms for multipath mitigation in real-world environment

The performance of the Space Alternating Generalized Expectation Maximisation (SAGE) algorithm for multipath mitigation is assessed in this paper. Numerical simulations have already proven the potential of SAGE in navigation context, but practical aspects of the implementation of such a technique in a GNSS receiver are the topic for further investigation. In this paper, we will present the first results of SAGE implementation in a real world environmen

Contributions to high accuracy snapshot GNSS positioning

(English) Snapshot positioning is the technique to determine the position of a Global Navigation Satellite System (GNSS) receiver using only a very brief interval of the received satellite signal. In recent years, this technique has received a great amount of attention thanks to its unique advantages in power efficiency, Time To First Fix (TTFF) and economic costs for deployment. However, the state of the art algorithms regarding snapshot positioning were based on code measurements only, which unavoidably limited the positioning accuracy to meter level. The present PhD research aims at achieving high-accuracy (centimetre level) snapshot positioning by properly utilizing carrier phase measurements. Two technical challenges should be tackled before such level of accuracy can be achieved, namely, satellite transmission time inaccuracy and the so-called Data Bit Ambiguity (DBA) issue. The first challenge is essentially originated from the lack of absolute timing accuracy in the receiver, as only the coarse time information is available from an external assistance module and its error can be up to a few seconds. Applying a conventional Coarse Time Filter (CTF) can increase this timing accuracy to millisecond level. However, this is still not enough for carrier-phase based positioning since the satellite position errors introduced by such timing errors range up to one meter, which certainly impedes the carrier phase Integer Ambiguity Resolution (IAR). A method is proposed to set a global time tag and correspondingly construct the pseudoranges with full period corrections. The second challenge is caused by the fact that snapshot measurements are generated based on the results of the correlation between the received signal and the local replicas. Multiple replicas are typically produced in snapshot positioning following the Multi Hypothesis (MH) acquisition architecture. It may happen that more than one local replica (i.e. hypothesis) result in the maximum correlation energy. Hence, we need to identify the actual secondary codes or data bit symbols encoded in the received signal, i.e. to resolve the DBA. Particularly, when the local replica is generated with exactly opposite symbols to the actual ones, the resulting carrier phase measurement contains a Half Cycle Error (HCE) and impedes also the IAR step. A method has been proposed in this PhD to resolve the DBA issue for pilot signals with encoded secondary codes. This method attempts to form a consensus among all satellites regarding their secondary codes under the assistance of their flight time differences. A different approach has been developed for data signals. It amends the carrier phase HCEs one after another by an iterative satellite inclusion procedure. This approach uses the Real Time Kinematics (RTK) LAMBDA Ratio Factor (LRF) as an indicator to evaluate the potential existence of the HCEs. The present PhD focuses on implementing the so-called Snapshot RTK (SRTK) technique. As in the classic RTK technique, SRTK cancels most of the measurement errors through the Double-Differenced (DD) process. The workflow details of SRTK are explained incorporating the aforementioned new algorithms. Several experiments were performed based on real world signal recordings and the results confirm the feasibility of obtaining SRTK fix solutions. The performance of SRTK is numerically demonstrated under different parameters of signal bandwidth, integration time and baseline distance. The SRTK fix rates can reach more than 90% in most of the scenarios, with centimetre-level positioning errors observed in the fixed solutions. It can be concluded that upon the implementation of the global time tag method, high accuracy snapshot positioning becomes feasible with the SRTK technique and its performance varies depending on the SRTK configuration. The algorithms developed for the DBA issue and carrier phase HCEs also prove to effectively improve the performance of SRTK.(Español) El posicionamiento instantáneo es la técnica para determinar la posición de un receptor del Sistema Global de Navegación por Satélite (GNSS) utilizando solo un intervalo muy breve de la señal recibida. En los últimos años, esta técnica ha recibido una gran atención gracias a sus ventajas únicas en eficiencia energética, tiempo hasta la primera posición (TTFF) y reducidos costes económicos para la implementación. Sin embargo, el estado del arte de los algoritmos relacionados con el posicionamiento de señales instantáneas utilizaron solo medidas de código, lo que inevitablemente limitó la precisión del posicionamiento a al nivel del metro. La presente Tesis Doctoral tiene como objetivo lograr un posicionamiento instantáneo de alta precisión (nivel centimétrico) mediante las medidas de fase de la portadora. Para ello, deben abordarse dos desafíos técnicos antes de que se pueda alcanzar ese nivel de precisión: resolver la inexactitud del tiempo de transmisión del satélite y el llamado problema de ambigüedad de bit de datos (DBA). El primer desafío se origina esencialmente por la falta de precisión de tiempo absoluto en el receptor, ya que solo está disponible la información del tiempo aproximado desde un módulo de asistencia externo y su error puede ser de hasta unos segundos. Así, se propone un método para establecer una etiqueta de tiempo global y construir correspondientemente los pseudorangos con correcciones de período completo. El segundo desafío se debe al hecho de que las mediciones instantáneas se generan en función de los resultados de la correlación entre la señal recibida y las réplicas locales. Las múltiples réplicas generalmente se producen en el posicionamiento de instantáneas siguiendo la arquitectura de de adquisición de el Múltiples Hipótesis (MH). Por lo tanto, se necesita identificar los códigos secundarios reales o los símbolos de bits de datos codificados en la señal recibida, para resolver el DBA. En particular, cuando la réplica local se genera con símbolos exactamente opuestos a los reales, el resultado de la medición de la fase de la portadora contiene un error de medio ciclo (HCE) e impide también la resolución de ambigüedad entera (IAR). Se ha propuesto un método en esta Tesis Doctoral para resolver el problema de DBA para señales piloto con códigos secundarios. Este método intenta formar un consenso entre todos los satélites con respecto a sus códigos secundarios bajo la asistencia de sus diferencias de tiempo de vuelo. Un enfoque diferente ha sido desarrollado para señales que contienen datos del mensaje de navegación. Se modifica los HCE de la fase de portadora uno tras otro mediante un procedimiento iterativo de inclusión de satélites. Este método utiliza el factor de relación LAMBDA (LRF) utilizado en posicionamiento relativo en tiempo real (RTK) como indicador para evaluar la existencia potencial de los HCE. La presente tesis doctoral se centra en implementar la técnica denominada Snapshot RTK (SRTK). Se realizaron varios experimentos basados ?en ?señales del mundo real. Las grabaciones y los resultados confirman la viabilidad de obtener soluciones SRTK con IAR. El rendimiento de SRTK es numéricamente demostrado bajo diferentes parámetros tales como el ancho de banda de señal, tiempo de integración y distancia de línea de base. Las tasas de fijación IAR de SRTK pueden alcanzar más del 90% en la mayoría de los escenarios, observándose errores de posicionamiento centimétricos en las soluciones fijas. Se puede concluir que tras la implementación del método de etiqueta de tiempo global, que el posicionamiento de instantáneas de alta precisión se vuelve factible con la técnica SRTK y las prestaciones varían dependiendo de la configuración. Los algoritmos desarrollados para la resolución de DBA y los HCE de fase portadora también demuestran que mejoran efectivamente el rendimientoCiència i tecnologies aeroespacial

Neumann-Hoffman Code Evasion and Stripping Method for BeiDou Software-defined Receiver

© 2016 The Royal Institute of Navigation. The acquisition and tracking strategies of the BeiDou navigation satellite signals are affected by the modulation of Neumann-Hoffman code (NH code), which increases the complexity of receiver baseband signal processing. Based on the analysis of probability statistics of the NH code, a special sequence of incoming signals is proposed to evade the bit transitions caused by the NH code, and an NH Code Evasion and Stripping method (NCES) based on the NH-pre-modulated code is proposed. The NCES can be applied in both 20-bit NH code and 10-bit NH code. The fine acquisition eliminates the impact of NH code on the traditional tracking loop. These methods were verified with a BeiDou PC-based software-defined receiver using the actual sampled signals. Compared with other acquisition schemes which try to determine or ignore the NH code phase, the NCES needs fewer incoming signals and the actual runtime is greatly reduced without sacrificing much time to search in the secondary code dimension, and the success rate of acquisition is effectively improved. An extension of Fast Fourier Transform (FFT)-based parallel code-phase search acquisition gives the NCES an advantage in engineering applications

Multi-purpose TDM Component for GNSS

International audienceThis article proposes a Time-Division-Multiplexing (TDM) technique applied at PRN code level as a signal design solution able to cope with the provision of several functionalities in one signal component: the allocation of the signal to the different functionalities is made at PRN code level. The functionalities targeted in this article are low-complexity acquisition, fast Time-ToFirst-Fix Data (TTFFD), Security Code Authentication (SCA) and, additionally, non-coherent signal processing. The interest of using a TDM component signal design lays on the introduction of just one new component to reduce the complexity to be added to the legacy GNSS satellite payload and to the GNSS receiver. Moreover, a TDM signal design solution presents a great flexibility able to adapt the signal design to the different GNSS strategic directives. The TDM component is constituted of period blocks called short basic blocks and advanced blocks; the introduction of such blocks simplifies the TDM component processing by a GNSS receiver. The TDM component is divided first in a continuous stream of short basic blocks of 20ms, where the short basic blocks are used to provide a signal periodic structure for the acquisition functionality. Then, the short basic blocks are grouped in advanced blocks to provide the signal periodicity for fast TTFFD and SCA. The low-complexity acquisition functionality is provided by the first PRN codes of a short basic block: PRN codes are selected to have a low duration and are always at the same position inside the block. Code Shift Keying Modulation is used to provide the fast TTFFD and the SCA key delivery. An example of application on the Galileo E1 civil signals is presented with different target scenarios or type of users: lowcomplexity user, high performance – no TTFFD, high performance – TTFFD and high dynamics user

Performance Limits of GNSS Code-Based Precise Positioning: GPS, Galileo & Meta-Signals

This contribution analyzes the fundamental performance limits of traditional two-step Global Navigation Satellite System (GNSS) receiver architectures, which are directly linked to the achievable time-delay estimation performance. In turn, this is related to the GNSS baseband signal resolution, i.e., bandwidth, modulation, autocorrelation function, and the receiver sampling rate. To provide a comprehensive analysis of standard point positioning techniques, we consider the different GPS and Galileo signals available, as well as the signal combinations arising in the so-called GNSS meta-signal paradigm. The goal is to determine: (i) the ultimate achievable performance of GNSS code-based positioning systems; and (ii) whether we can obtain a GNSS code-only precise positioning solution and under which conditions. In this article, we provide clear answers to such fundamental questions, leveraging on the analysis of the Cramér–Rao bound (CRB) and the corresponding Maximum Likelihood Estimator (MLE). To determine such performance limits, we assume no external ionospheric, tropospheric, orbital, clock, or multipath-induced errors. The time-delay CRB and the corresponding MLE are obtained for the GPS L1 C/A, L1C, and L5 signals; the Galileo E1 OS, E6B, E5b-I, and E5 signals; and the Galileo E5b-E6 and E5a-E6 meta-signals. The results show that AltBOC-type signals (Galileo E5 and meta-signals) can be used for code-based precise positioning, being a promising real-time alternative to carrier phase-based techniques

Scoping Study on Pseudolites

Pseudolites or pseudo-satellites are an emerging technology with the potential of enabling satellite navigation indoors. This technology found several applications that are not limited to indoor navigation. Precise landing, emergency services in difficult environments and precise positioning and machine control are few examples where pseudolite technology can be employed. Despite the great potential of this technology, severe interference problems with existing GNSS services can arise. The problem can be particularly severe when considering non-participating receivers, i.e., legacy devices not designed for pseudolite signals. The design of pseudolite signals is thus a complex problem that has to account for market requirements (modifications of existing receivers for enabling the use of pseudolite signals, measurement accuracy, target application), regulatory aspects (frequency bands to be allocated for pseudoliteservices) and interference problems. The main aspects for the design of a pseudolite signal standard minimizing the interference problem without compromising the location capabilities of the system are considered. The focus is on the signal characteristics and topics relevant for the signal design. A literature review on the different pseudolite applications, prototypes and solutions adopted for minimizing the interference problem is first conducted. Recommendations on the aspects that should be further investigated are then provided.JRC.DG.G.6-Security technology assessmen

FUNTIMES – Future Navigation and Timing Evolved Signals

International audienceThe European Galileo system moves clear steps forward towards the completion of its space and ground segment infrastructures, after starting providing early services in 2016 and with the plan to achieve the full operational capability (FOC) in 2020. Also the user segment is rapidly expanding, with the increasing introduction of mass market chipsets fully supporting Galileo in a constantly growing number of smartphones. In this context a strong need for R&D activities in the field of navigation signal engineering has been identified by various Programme's stakeholders. Considering the long process required for introducing new signals and features in a system that is already deployed and finds itself in the exploitation phase, early R&D activities become essential to investigate potential evolutions and new concepts to improve the Galileo signals and services in the short, medium and long term. The Future Navigation and Timing Evolved Signals (FUNTIMES) project is a European GNSS mission evolution study funded by the European Commission within the Horizon 2020 Framework for Research and Development. It aims at identifying, studying and recommending mission evolution directions and at preliminary supporting the definition, design and implementation of the future generation of Galileo signals. The project is led by Airbus Defence and Space as prime contractor, supported by Ecole Nationale de l‘Aviation Civile (ENAC) and Istituto Superiore Mario Boella (ISMB) as subcontractors and was run under the supervision of the European Commission and its Joint Research Centre. The research activities were conducted according to the following high level evolution directions: - Improve the Galileo OS reliability by providing an enhanced authentication service based on both navigation message authentication and spreading code authentication, in such a way that the two solutions can take advantage of their combination. - Improve the sensitivity and/or reduce the complexity of the acquisition of the Galileo OS signals, e.g. by studying the potential introduction of a new signal component for this purpose. - Make use of new concepts and techniques for the delivery of the data messages, to improve the time-to-data performance and robustness. - Consider options for providing an effective high data rate component suitable for satellite navigation purposes, e.g. in view of a possible evolution of the signals providing the Galileo Commercial Service. The project started by defining the key elements characterizing GNSS signals, describing the current signal plans of the major global and regional satellite systems and carrying out a literature survey on the various proposals for the evolution and optimization of navigation signals. A key role in the project was then played by a specific task on the definition of signal user requirements, which, besides providing by themselves an added-value to the project outcomes, were taken into account to select and consolidate the R&D topics defined at the beginning of the study. For what concerns the core navigation signal R&D activity, various solutions belonging to the following areas were considered: new and evolved modulations and multiplexing techniques, new concepts and techniques for the data message, solutions providing services with higher reliability, solutions for improved navigation performance. In the followings, some highlights about the main project tasks are provided. *Adding New Signal Components to Galileo E1 OS* Due to backward compatibility constraints, the Galileo legacy signals defined in the current SIS-ICD do not offer much space for further modifications. The possibility to add new signal components to the Galileo E1 signal was investigated with the goals of providing a fast and reliable authentication service and better acquisition performance while keeping the complexity of the acquisition process low. Various options were investigated, considering new components centered at E1 or ones presenting a carrier offset. The options were studied in terms of ranging performance, compatibility with other signals in E1/L1, multiplexing efficiency and backward compatibility. The outcome of this task was then combined with the other solutions investigated during the project and briefly introduced in the followings. *Signal User Requirements Survey* This task aimed at identifying and understanding the current and future needs of various GNSS user groups in order to derive requirements and evolution directions for the Galileo signals. The work logic followed was based on a 3-step approach: - Definition of the user communities - Analysis of available documentation and state-of-the-art for each user communities to extract high level and, if possible, low level requirements - Consultation of representative of the various user communities by means of questionnaire on signal user requirements. The considered user communities are representative of 7 classes of users: - Traditional Safety-of-Life Applications (Navigation of Civil Aviation aircrafts, Train Control) - Automotive Location-Based Charging (LBC) and Vehicle Motion Sensing (VMS) - Mobile Location-Based Services (LBS) - Surveying - Timing and Synchronization - Search and Rescue - Remotely-Piloted Aircraft Systems (RPAS). As mentioned above, the consortium prepared a questionnaire which was distributed to companies and organizations representative of various GNSS user communities. After collecting the answers, personal interviews were conducted to deepen the outcomes of the survey and collect more details about their expectations. From the received answers, the following points were considered particularly relevant for the identification/consolidation of signal evolution directions: - The need for integrity and authentication is present also in non-safety of life applications (e.g. precise positioning) - Very wide-spread need for fast authenticated PVT (fast data and pseudo-range authentication) - Interest in fast Time-To-First-Fix (TTFF) Data, or in other words, fast provision of the Clock error corrections and satellites Ephemeris Data (CED). - Need for precise clock and orbit data, freely accessible through the navigation message transmitted through conventional signals (at L1/E1 or L5/E5) - Importance of stand-alone operation mode despite the increasing number of connected users (network connection still judged not reliable enough). - Need for multipath/NLOS resistant signals - Need for RFI resistant signals - Interest for an alert/emergency service. *Reed-Solomon Codes for the Improvement of the I/NAV Message* Despite the growing number of connected user devices, the reception of the clock and ephemeris data (CED) is still a major factor impacting the TTFF. The current approach for the dissemination of these data can be defined as "data carouseling": the data are repeatedly sent to the users with a certain repetition rate. For example the repetition rate of the CED contained in the Galileo E1 OS message is equal to 1 every 30 s. A different approach is offered by Maximum Distance Separable (MDS) codes like Reed-Solomon codes, whose erasure correction capability allows to retrieve the entire information contained in k data blocks from any combination of k received blocks of the codeword. During the project, the performance of Reed-Solomon codes when applied to the Galileo I/NAV message as proposed in [1] were studied, in terms of Time-to-Data, with extensive simulations in the AWGN and mobile channel. The results were then compared with the legacy implementation and with the performance of the GPS L1C signal and showed a very significant improvement, with a reduction of the Galileo E1 OS TTFF by up to 50% in difficult urban environments. Also received processing scheme and complexity aspects were taken into account in the work. *Spreading Code Authentication Techniques* The increasing awareness concerning the vulnerability of GNSS signals to potential spoofing attacks suggested to dedicate an important part of the project R&D activities to investigating new concepts and ideas to improve the reliability of the provided PNT service. This need was also confirmed by the conducted user requirements survey. The investigation of possible authentication techniques has been carried out on the basis of both quantitative results and qualitative analyses, considering a set of criteria useful to weight the overall performance of different options in realistic scenarios. The methodology used to trade-off different options took into account four main criteria: - the authentication performance, aiming to assess the techniques mainly in terms of Time Between Authentications (TBA) and Time To Alarm (TTA) metrics; - the spoofing robustness, that measures the level of resilience to different specific spoofing attacks; - the implementation readiness, that assesses the level of complexity required both at the system and receiver levels and the backward compatibility; - the legacy signal valorization, with the objective to assess the level of reuse and valorization of today’s signal and messages structures, e.g. considering the current Galileo plans to provide navigation message authentication for his Open Service. When considering authentication solutions, it is important not to focus only on the benefits of future participant users, i.e., those able to exploit the features of the authenticated signals, but also to take into account the possible impact on the existing satellites, ground segment, and other receivers (i.e. non-participant users). Therefore the activities included the assessment of the impact of authentication schemes on user receivers. In detail, the analysis covered the possible degradation of the performance of non-participant users, in terms of C/N0 degradation and impact on acquisition and tracking, and the evaluation of the performance of participant users in relation with the authentication technique parameters. In addition, a novel high-level concept for spreading code authentication, based on the idea of reusing the E1-B OS NMA data, was investigated. The proposed concept, already anticipated in [2], foresees the use of two types of SCA bursts, inserted in the open Pseudo-Random Noise (PRN) code sequence at different rates: - “Slow rate” SCA bursts, which are intended for a robust a-posteriori verification with moderate latency (i.e., TBA of about 10 seconds); - “Fast rate” SCA bursts, potentially suitable to improve the authentication performance (e.g. TBA of about 2 seconds) under a wide set of spoofing attacks. The proposed solution can potentially exploit the information received from all the in-view satellites by means of a two-steps authentication procedure. *CSK Modulation and Channel Codes for a High Data Rate Component* The Code Shift Keying (CSK) modulation is an orthogonal M-ary modulation (M orthogonal symbols are used in order to transmit U =log_2?(M) bits) which was specially designed to increase the bandwidth efficiency of a DS-SS signal, i.e. the bit rate to signal bandwidth ratio, without affecting the PRN code structure. The usage of CSK for the improvement of GNSS data delivery was already investigated in the past (e.g. in [3]). Within the FUNTIMES project the main scope of this task was to prove the expected benefits of this technique by applying it to a number of signal design options, considering various data rates, power distributions between data and pilot components and demodulation strategies at the receiver. The first advantage of CSK is the possibility to increase the bit rate of a DS-SS signal without increasing the PRN code number of bits and without increasing the signal chip rate (and thus signal bandwidth). The increased data rate could be used to increase the number of services provided by the signal and/or to improve the services already available, e.g. by sending correction data. The second benefit is enhanced flexibility of the signal bit rate as the CSK modulation allows to change the number of symbols of the modulation alphabet from one codeword to another one. This allows the GNSS signal to provide more robustness to fundamental data and less robustness to less relevant or optional data since the bit rate is directly relate to the demodulation sensitivity. The third major benefit of a CSK modulation is the possibility of implementing a non-coherent demodulation process that does not require the estimation of the incoming signal carrier phase. Therefore, when in degraded environments and/or for high dynamic users, the PLL cannot be in lock for a certain time, the GNSS receiver could still be able to demodulate the data signal. The results obtained in terms of signal availability and reduced Time-to-First-Fix are very promising and bring a significant improvement when compared with the data delivery performance of today's navigation signals. For what concerns the study of channel codes that could be best suited for high data rate transmission and, especially, in combination with a CSK scheme, the investigation focused on LDPC codes with a bit interleaved coded modulation (BICM/BCIM-ID). As Galileo transmits a navigation signal intended to deliver value-added data in a significant amount (high accuracy service through the E6-B signal), it was decided to study a potential application of the studied CSK schemes to a similar use case. From the results obtained, depending on the C/N0 value considered, an increase of the information bit rate from the current 500 bps up to 5000 bps can be feasible, while still reaching a WER equal to 10-3 for a signal component C/N0 equal to 37 dB-Hz. The project allowed to study new elements in the field of GNSS signal engineering and to consolidate solutions that were already investigated in the recent literature, paving the way to the evolution of the Galileo signal plan but also offering elements and ideas that can be adopted by any other GNSS. The variety of solutions proposed presents different levels of maturity. In some cases the solutions are ready to be implemented in the currently deployed systems, while in other cases they would require a corresponding evolution of the space and ground segments. Where deemed necessary, specific recommendations for future R&D work in the areas studied in the project were provided

Analysis and Detection of Outliers in GNSS Measurements by Means of Machine Learning Algorithms

L'abstract è presente nell'allegato / the abstract is in the attachmen

IF-level signal-processing of GPS and Galileo Radionavigation signals using MATLAB/Simulink®: Including Effects of Interference and Multipath

Open-source GNSS simulator models are rare and somewhat difficult to find. Therefore, Laboratory of Electronics and Communications Engineering in the former Tampere University of Technology (and now Tampere University, Hervanta Campus) has took it upon itself to develop, from time to time, a free and open-source simulator model based on MATLAB/Simulink® for signal processing of a carefully selected set of GNSS radionavigation signals, namely, Galileo E1, Galileo E5, GPS L1, and GPS L5. This M.Sc. thesis is the culmination of those years which have been spent intermittently on research and development of that simulator model. The first half of this M.Sc. thesis is a literature review of some topics which are believed to be of relevance to the thesis’s second half which is in turn more closely associated with documenting the simulator model in question. In particular, the literature review part presents the reader with a plethora of GNSS topics ranging from history of GNSS technology to characteristics of existing radionavigation signals and, last but not least, compatibility and interoperability issues among existing GNSS constellations. While referring to the GNSS theory whenever necessary, the second half is, however, mainly focused on describing the inner-workings of the simulator model from the standpoint of software implementations. Finally, the second half, and thereby the thesis, is concluded with a presentation of various statistical results concerning signal acquisition’s probabilities of detection and false-alarm, in addition to signal tracking’s RMSE