Salvatore Gaglione

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.

Cigarette Butt Decomposition and Associated Chemical Changes Assessed by 13C CPMAS NMR

Cigarette butts (CBs) are the most common type of litter on earth, with an estimated 4.5 trillion discarded annually. Apart from being unsightly, CBs pose a serious threat to living organisms and ecosystem health when discarded in the environment because they are toxic to microbes, insects, fish and mammals. In spite of the CB toxic hazard, no studies have addressed the effects of environmental conditions on CB decomposition rate. In this study we investigate the interactive effects of substrate fertility and N transfer dynamics on CB decomposition rate and carbon quality changes. We carried out an experiment using smoked CBs and wood sticks, used as a slow decomposing standard organic substrate, incubated in both laboratory and field conditions for two years. CB carbon quality changes during decomposition was assessed by 13C CPMAS NMR. Our experiment confirmed the low degradation rate of CBs which, on average, lost only 37.8% of their initial mass after two years of decomposition. Although a net N transfer occurred from soil to CBs, contrary to our hypothesis, mass loss in the medium-term (two years) was unaffected by N availability in the surrounding substrate. The opposite held for wood sticks, in agreement with the model that N-rich substrates promote the decomposition of other N-poor natural organic materials with a high C/N ratio. As regards CB chemical quality, after two years of decomposition 13C NMR spectroscopy highlighted very small changes in C quality that are likely to reflect a limited microbial attack.

Time-differenced carrier phases technique for precise GNSS velocity estimation

Abstract Classically, a stand-alone GNSS receiver estimates its velocity by forming the approximate derivative of consecutive user positions or more often by using the Doppler observable. The first method is very inaccurate, while the second one allows estimation of the order of some cm/s. The time-differenced carrier phase (TDCP) technique, which consists in differencing successive carrier phases, enables accuracies at the mm/s level. A study on the existing TDCP velocity estimation algorithms has revealed that the use of different broadcast ephemeris sets to calculate the satellite positions and clock offsets produces a discontinuity in the TDCP measurements that affects the velocity estimation. We propose a method to overcome this limitation based on the use of the same set of ephemeris to calculate the satellite positions and clock offsets at consecutive epochs. We describe in detail the TDCP algorithm used, and the complete implementation in MATLAB is included.

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).

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.

Digital Surface Models for GNSS Mission Planning in Critical Environments

AbstractGlobal Navigation Satellite System (GNSS) surveys performed in critical environments (e.g., urban canyons, mountainous areas, or areas of dense vegetation) usually suffer from a lack of satellite coverage as a result of obstacles such as buildings and vegetation. GNSS mission-planning software provides an estimate of satellite visibility and dilution-of-precision (DOP) values along a planned trajectory to establish the best time frame over which to perform the survey. However, such an estimate is not reliable in a complex scenario because the surrounding environmental morphology is not considered. This paper introduces a new method to improve the prediction of GNSS satellite visibility. This method involves computing GNSS satellites position by means of the orbital parameters, as well as using three-dimensional digital surface models (DSMs) to develop a more reliable mission plan. The time evolution of key parameters describing the GNSS constellation is computed by means of a visibility georeferen...

Soil fertility promotes decomposition rate of nutrient poor, but not nutrient rich litter through nitrogen transfer

Background and aimsLitter decomposition is a critical process in terrestrial ecosystems and understanding the effects of soil fertility on the litter decay rate is of great ecological relevance. Here we test the hypothesis that N transfer from soil to litter will promote the decay rate of N poor but not N rich litter types.MethodsTen organic substrates, encompassing a wide range of biochemical quality in terms of C/N and lignin/N ratios, were decomposed in microcosms over three soil types with different N content, but inoculated with the same microbiome. Organic substrates were characterized for mass loss, C and N content to assess N transfer from soil to litter.ResultsThe decay rate response to soil fertility was related to their initial N content: positive for substrates with little initial N content and not significant for N rich plant residues. A significant N transfer, generally larger from N rich soil to N poor substrates, was found. Litter C/N and lignin/N ratios showed variable relationships with the litter decay according with the soil fertility gradient, with positive and negative correlations in N rich and N poor soils, respectively.ConclusionsOur study demonstrated that the decomposition of N rich litter proceeded irrespective of soil fertility while the decay rate of N poor substrates, either lignin poor or rich, was controlled by soil fertility likely as a result of N transfer. Litter C/N and lignin/N ratios were reliable indicators of litter quality to predict their decay rate in N poor soil, but not in N rich soils.