Ciro Gioia

Performance assessment of GPS/GLONASS single point positioning in an urban environment

In signal-degraded environments such as urban canyons and mountainous area, many GNSS signals are either blocked or strongly degraded by natural and artificial obstacles. In such scenarios standalone GPS is often unable to guarantee a continuous and accurate positioning due to lack (or the poor quality) of signals. The combination of different GNSSs could be a suitable approach to fill this gap, because the multi-constellation system guarantees an improved satellite availability compared to standalone GPS, thus providing enhanced accuracy, continuity and integrity of the positioning. The present GNSSs are GPS, GLONASS, Galileo and Beidou, but the latter two are still in the development phase. In this work GPS/GLONASS systems are combined for single point positioning and their performance are assessed for different configurations. Using GPS/GLONASS multi-constellation implies the addition of an additional unknown, i.e. the intersystem time scale offset, which requires a sacrifice of one measurement. Since the intersystem offset is quasi-constant over a short period, a pseudo-measurement can be introduced to compensate the sacrifice.The benefit after adding a pseudo-measurement has been demonstrated in a vehicular test.

A Galileo IOV assessment: measurement and position domain

Abstract The European GNSS, Galileo, is currently in its in-orbit validation (IOV) phase where four satellites are finally available for computing the user position. In this phase, the analysis of the measurements and position velocity and time (PVT) obtained from the IOV satellites can provide insight into the potentialities of the Galileo system. A methodology is suggested for the analysis of the Galileo IOV pseudorange and pseudorange rates collected from the E1 and E5 frequencies. Several days of data were collected and processed to determine figures of merit such as root mean square and maximum errors of the Galileo observables. From the analysis, it emerges that Galileo is able to achieve better accuracy than GPS. A thorough analysis of the PVT performance is also carried out using broadcast ephemerides. Galileo and GPS PVTs are compared under similar geometry conditions showing the potential of the Galileo system.

Benefit of the NeQuick Galileo Version in GNSS Single-Point Positioning

The GNSS measurements are strongly affected by ionospheric effects, due to the signal propagation through ionosphere; these effects could severely degrade the position; hence, a model to limit or remove the ionospheric error is necessary. The use of several techniques (DGPS, SBAS, and GBAS) reduces the ionospheric effect, but implies the use of expensive devices and/or complex architectures necessary to meet strong requirements in terms of accuracy and reliability for safety critical application. The cheapest and most widespread GNSS devices are single frequency stand-alone receivers able to partially correct this kind of error using suitable models. These algorithms compute the ionospheric delay starting from ionospheric model, which uses parameters broadcast within the navigation messages. NeQuick is a three-dimensional and time-dependent ionospheric model adopted by Galileo, the European GNSS, and developed by International Centre for Theoretical Physics (ICTP) together with Institute for Geophysics, Astrophysics, and Meteorology of the University of Graz. The aim of this paper is the performance assessment in single point positioning of the NeQuick Galileo version provided by ESA and the comparison with respect to the Klobuchar model used for GPS; the analysis is performed in position domain and the errors are examined in terms of RMS and maximum error for the horizontal and vertical components. A deep analysis is also provided for the application of the exanimated model in the first possible Galileo only position fix.

GNSS Reliability Testing in Signal-Degraded Scenario

Multiconstellation satellite navigation is critical in signal-degraded environments where signals are strongly corrupted. In this case, the use of a single GNSS system does not guarantee an accurate and continuous positioning. A possible approach to solve this problem is the use of multiconstellation receivers that provide additional measurements and allows robust reliability testing; in this work, a GPS/GLONASS combination is considered. In urban scenario, a modification of the classical RAIM technique is necessary taking into account frequent multiple blunders. The FDE schemes analysed are the “Observation Subset Testing,” “Forward-Backward Method,” and “Danish Method”; they are obtained by combining different basic statistical tests. The considered FDE methods are modified to optimize their behaviour in urban scenario. Specifically a preliminary check is implemented to screen out bad geometries. Moreover, a large blunder could cause multiple test failures; hence, a separability index is implemented to avoid the incorrect exclusion of blunder-free measurements. Testing the RAIM algorithms of GPS/GLONASS combination to verify the benefits relative to GPS only case is a main target of this work too. The performance of these methods is compared in terms of RMS and maximum error for the horizontal and vertical components of position and velocity.

RAIM algorithms for aided GNSS in urban scenario

Urban canyon is a critical scenario for satellite navigation, because many GNSS signals are blocked by artificial obstacles or severely degraded; in standalone mode GPS, currently the main GNSS, cannot guarantee an accurate and continuous positioning. A possible approach to overcome these limitations is the use of multiple GNSS systems. GLONASS, the Russian navigation satellite system, is currently fully operational and is the main candidate to support this thesis. Urban scenario is mainly affected by multipath phenomenon, yielding several blunders into the measurements and unacceptable errors in the navigation solution. The integrity concept was introduced for safety-of-life application as aviation to provide timely warnings to users when a system should not be used for navigation, and then it was expanded to not safety-of-life service as urban navigation. RAIM (Receiver Autonomous Integrity Monitoring) techniques are user-level integrity methods based on consistency check of redundant measurements. This check is crucial because only at user-level certain local errors, such as multipath and local interferences, can be detected. Multi-constellation GNSS improves navigation solution in terms of accuracy and continuity; a further enhancement is achievable even in terms of integrity owing to the gained redundancy. The multi-constellation use implies a further unknown related to the intersystem time scale offset, requiring the “sacrifice” of one measurement. This parameter is observed to be quasi-constant in the short term, so an aiding can be introduced to account for its behavior. A similar approach can be adopted for altitude considering its slow variations in urban scenario. In this work GPS/GLONASS systems are combined and the benefits of the aforesaid aids are assessed, with main focus being the improvements in terms of integrity; single point GNSS and snapshot RAIM algorithms are herein considered. PVT and RAIM algorithms are developed in MatLab® environment and belong to a tool implemented by PANG (PArthenope Navigation Group).

Real-time jamming detection using the sum-of-squares paradigm

A new class of jamming detectors is derived using the hypothesis that jamming causes correlated changes in all the measured Carrier-to-Noise density power ratio (C/No) values. The decision statistics associated to the new detectors have the form of a sum of squared C/No variations and thus, the algorithms developed are named Sum-of-Squares (SoS) detectors. Real-time platforms implementing SoS detection have been developed using a GPS mass-market receiver and an Android device. The tests performed show that jamming detection can be performed using low-cost components and the SoS paradigm.

Testing the test satellites: the Galileo IOV measurement accuracy

The European GNSS, Galileo, is currently in its In-Orbit Validation (IOV) phase where four satellites are finally available for computing the user position. In this phase, the analysis of the measurements obtained from the IOV satellites can provide insight on the performance and potentialities of the Galileo system. In this paper, a methodology based on the use of precise orbits and ionospheric corrections is suggested for the analysis of the Galileo IOV pseudorange and pseudorange rate errors. Several hours of data were collected using a Septentrio PolarRxS receiver and used to determine figures of merits such as RMS and maximum errors of the Galileo observables. From the analysis it emerges that Galileo measurements have accuracies comparable with those of GPS. The benefits of combined GPS-Galileo positioning are also highlighted and results relative to the computation of a Galileo-only navigation solution based on broadcast ephemerides are provided.

A statistical characterization of the Galileo-to-GPS inter-system bias

Global navigation satellite system operates using independent time scales and thus inter-system time offsets have to be determined to enable multi-constellation navigation solutions. GPS/Galileo inter-system bias and drift are evaluated here using different types of receivers: two mass market and two professional receivers. Moreover, three different approaches are considered for the inter-system bias determination: in the first one, the broadcast Galileo to GPS time offset is used to align GPS and Galileo time scales. In the second, the inter-system bias is included in the multi-constellation navigation solution and is estimated using the measurements available. Finally, an enhanced algorithm using constraints on the inter-system bias time evolution is proposed. The inter-system bias estimates obtained with the different approaches are analysed and their stability is experimentally evaluated using the Allan deviation. The impact of the inter-system bias on the position velocity time solution is also considered and the performance of the approaches analysed is evaluated in terms of standard deviation and mean errors for both horizontal and vertical components. From the experiments, it emerges that the inter-system bias is very stable and that the use of constraints, modelling the GPS/Galileo inter-system bias behaviour, significantly improves the performance of multi-constellation navigation.

A sum-of-squares approach to GNSS spoofing detection

This paper analyses the sum-of-squares (SoS) detector that is designed to reveal the presence of a spoofing attack. The SoS decision statistic is computed using carrier phase measurements from two spatially separated Global Navigation Satellite System receivers and assumes a simple form that makes it suitable for real-time applications. The detector is theoretically characterized, and its effectiveness is shown using simulations and experiments involving real Global Positioning System data.

Stand-Alone and Hybrid Positioning Using Asynchronous Pseudolites

global navigation satellite system (GNSS) receivers are usually unable to achieve satisfactory performance in difficult environments, such as open-pit mines, urban canyons and indoors. Pseudolites have the potential to extend GNSS usage and significantly improve receiver performance in such environments by providing additional navigation signals. This also applies to asynchronous pseudolite systems, where different pseudolites operate in an independent way. Asynchronous pseudolite systems require, however, dedicated strategies in order to properly integrate GNSS and pseudolite measurements. In this paper, several asynchronous pseudolite/GNSS integration strategies are considered: loosely- and tightly-coupled approaches are developed and combined with pseudolite proximity and receiver signal strength (RSS)-based positioning. The performance of the approaches proposed has been tested in different scenarios, including static and kinematic conditions. The tests performed demonstrate that the methods developed are effective techniques for integrating heterogeneous measurements from different sources, such as asynchronous pseudolites and GNSS.