Ambrogio Manzino

GPS & GLONASS Mass-Market Receivers: Positioning Performances and Peculiarities

Over the last twenty years, positioning with low cost Global Navigation Satellite System (GNSS) sensors have rapidly developed around the world at both a commercial and academic research level. For many years these instruments have only acquired the GPS constellation but are now able to track the Global’naja Navigacionnaja Sputnikovaja Sistema (GLONASS) constellation. This characteristic is very interesting, especially if used in hard-urban environments or in hard conditions where satellite visibility is low. The goal of this research is to investigate the contribution of the GLONASS constellation for mass-market receivers in order to analyse the performance in real time (Network Real Time Kinematic—NRTK positioning) with post-processing approaches. Under these conditions, it is possible to confirm that mass-market sensors could be a valid alternative to a more expensive receiver for a large number of surveying applications, but with low cost hardware the contribution of the GLONASS constellation for fixing ambiguities is useless, if not dangerous.

Performances comparison of different MEMS-based IMUs

MEMS inertial sensors are widely used for navigation applications where size, weight, power and cost are key sides, such as autonomous vehicular control and pedestrian navigation. Otherwise, if there is no doubt that MEMS technologies represents an interesting turning point for low cost inertial-based sensors and applications, nevertheless it is absolutely true that, in order to obtain good positioning accuracies, it is necessary to investigate very well the behaviour of these MEMS sensors and realize special test calibrations, both in static and kinematic conditions. In order to evaluate the potentialities (and the limits) of these sensors, comparative tests have been realized considering MEMS inertial sensors with different characteristics and different performances, First of all, a static calibration of the sensors has been made, in order to compare the bias values and their stability with respect to the time. In particular, an Allan-variance analysis and a modified six position static test were carried out for each sensor, preserving carefully the same environment conditions for all the tests. After the lab tests, the performances of all the sensors were compared in a field kinematic test, integrating their data with a GPS solution.

Reference frames for GNSS positioning services: Some problems and proposed solutions

Abstract Positioning services based on GNSS Permanent Networks (PNs) disseminate the reference frame to users; hence the networks must be adjusted by constraining the ITRF coordinates of some permanent stations (PS), with the satellite orbits and clocks constrained to the values published by the International GNSS Service (IGS). Furthermore, the positioning service provider must use analysis methods, conventions and practices fully consistent with IGS standards. In this framework some problems arise. A first one is that long term solutions for the IGS PS coordinates are not always fully consistent with weekly IGS products, requiring an appropriate use of IGS PS weekly solutions. Moreover, the consistence of the reference frame implied by different positioning services must be checked, to avoid discontinuities between neighbouring networks. Clearly, in order to monitor the PS coordinates, the adjustment of a network needs to be performed on a quasi real time continuous (daily or weekly) basis. However, from a practical point of view, a continuous update of the PS coordinates is neither appropriate nor possible and alternative proposals will be presented. Finally, most users of real-time positioning services need coordinates in another reference frame, perhaps a local one or the national one. In this case the real-time service provider must distribute the transformation between the broadcasted frame and the user required reference frame. In this work some possible solutions of the aforementioned problems are presented, based on experiences obtained during data processing and adjustments of the Piemonte and Lombardia (Northern Italy) networks.

Real-time monitoring for fast deformations using GNSS low-cost receivers

Landslides are one of the major geo-hazards which have constantly affected Italy especially over the last few years. In fact 82% of the Italian territory is affected by this phenomenon which destroys the environment and often causes deaths: therefore it is necessary to monitor these effects in order to detect and prevent these risks. Nowadays, most of this type of monitoring is carried out by using traditional topographic instruments (e.g. total stations) or satellite techniques such as global navigation satellite system (GNSS) receivers. The level of accuracy obtainable with these instruments is sub-centimetrical in post-processing and centimetrical in real-time; however, the costs are very high (many thousands of euros). The rapid diffusion of GNSS networks has led to an increase of using mass-market receivers for real-time positioning. In this paper, the performances of GNSS mass-market receiver are reported with the aim of verifying if this type of sensor can be used for real-time landslide monitoring: for this purpose a special slide was used for simulating a landslide, since it enabled us to give manual displacements thanks to a micrometre screw. These experiments were also carried out by considering a specific statistical test (a modified Chow test) which enabled us to understand if there were any displacements from a statistical point of view in real time. The tests, the algorithm and results are reported in this paper.

A GNSS/INS-based architecture for rescue team monitoring

The present work shows the results that have been obtained using a low-cost pedestrian system made up of a GPS receiver and an inertial platform. This positioning system could be used by rescue teams to locate hazard zones and escape routes. Our study shows how multiple sensors can be used in a non-traditional way: the inertial platform is used as an odometer (step counter) in which the magnetometers allow the estimation of gyroscope drifts. The GPS receiver has instead been used to correct the bias when the GPS observations are sufficiently reliable. We have in particular focused on the estimation of sensor errors and on the reliability of the system, which is the main problem of this equipment.

Artificial neural network for detecting incorrectly fixed phase ambiguities for L1 mass-market receivers

One of the main challenges in global navigation satellite systems (GNSS) network real-time kinematic positioning is phase ambiguity estimation. We describe methods that predict false fixing (FF) of phase ambiguities in mass-market receivers. In this work, FF is defined to occur when the differences between 3D coordinates estimated in real-time differ by more than 20xa0cm with respect to the reference coordinates. Phase ambiguity FF events occur for many reasons, such as wrong estimation of phase ambiguities by the network software, noise in the corrections, and the environment of the rover. Moreover, one of the main reasons for phase ambiguity FF is the high level of noise and the low redundancy of observation by receivers that track L1 frequencies only. We develop and analyze a specific tool utilizing an artificial neural network that, when trained, tested, and refined specifically for GNSS mass-market receivers, can predict and detect FF. This tool comprises three inputs for all epochs, the Horizontal Dilution of Precision index, the latency of the differential correction, and the number of satellites with fixed phase ambiguities seen by the rover. It provides as output an index consisting of values 0 or 1, i.e., 0 for no FF and 1 for FF. A description of the training and validating phases is provided. The results of tests show that the algorithm has a 99.7% probability of detecting phase ambiguity FF in these cases.

How reliable is a Virtual RINEX

Virtual RINEX (VR) is a common product for Global Navigation Satellite System (GNSS) post-processing applications, useful if the distance between the rover and master is greater than 30 km. Many studies have been conducted in order to analyse the content of this kind of data, but few have proved the positioning performances obtainable in particular conditions. The VR is a synthetic file created from real data by software that manages continuous operating reference system networks. It depends on algorithms of the software to compute and model the bias interpolation. This paper focuses attention on the evaluation of the quality and accuracy of positioning using VR near the border of two different continuous operating reference system networks, managed by two different network software (GNSMART provided by GEO++ and Spidernet by Leica Geosystems) in the same reference system. To assess the reliability and quality of VR provided by two different GNSS network software with inter-station distances of about 40 km (typical of Italian networks), 15 points equally distributed near the border between the two networks were chosen, using points with the same distances between real existing stations of both networks. For each point, a VR file 24 hours in length was generated with a sampling rate of 1 second. Tests were performed to assess the importance of the location of permanent stations at different altitudes to estimate biases for the VR generation in a better way. To assess the quality of the VR of the two networks, the baseline vector between the VR generated by the two network software was evaluated using relative positioning. The files have been processed for session lengths of 5 minutes, 10 minutes, 30 minutes, 1 hour, and 24 hours.

An Innovative Method to Predict and to Detect the False Fixing of the GNSS Ambiguity Phase

One of the most critical points during the GNSS NRTK (Network Real Time Kinematic) positioning is the correct fixing of the ambiguity phase. This work wants to try to focus attention on the quality control of the real-time GNSS positioning, both from the point of view of what the network provides, and from one of the network products is used by the rover receiver. The quality of the positioning is a parameter that must be monitored in real time to avoid an incorrect ambiguity fixing, also called FF (false fixing), occurring; this can be due both to internal problems of the network software and, more often, to the environment (obstructions, multipath and so on) within which where the receiver works. To achieve this control a tool was designed that, starting from the data available in real time from a user connected to an NRTK positioning service, can identify with a certain probability threshold the effective presence, or the possibility, of a false fixing. The FF estimator will be composed of a neural network, trained a priori with some datasets, and will have, as a single output, the probability that the current fixing is a false fixing of the ambiguity phase.

Effectiveness of time-series analysis for thermal plume propagation assessment in an open-loop groundwater heat pump plant

Thermal perturbation produced in the subsurface by open-loop groundwater heat pumps (GWHPs) represents a complex transport phenomenon that is affected by several factors, including intrinsic characteristics of the exploited aquifer, abstraction and reinjection well features, and the temporal dynamics of the accessed groundwater. Post-GWHP water may have become warmed or cooled before being reinjected into the aquifer, thereby creating a thermal plume, known as the thermal affected zone (TAZ), which can alter aquifer temperature. The TAZ is propagated mainly by advection, after which the plume tends to degrade via conductive heat transport and convection within moving water. Groundwater monitoring and multiparametric probes are used to check the dynamics of plume propagation and whether a system’s thermal plumes are generating unsuitable interference with wells, subsurface infrastructure, or land use. Analyses of time-series groundwater monitoring data can be used to monitor TAZ movement. In this paper, the thermal plume velocity was calculated by both an analytical solution and cross-correlation. Cross-correlation calculated between temperature measured in the reinjection well and control downstream piezometers can reveal plume dynamics and demonstrate the importance of advective transport in aquifer heat transfer.

Recovery of cadastral boundaries with GNSS equipment

The purpose of this work is to propose a new redefinition of cadastral boundaries using GNSS equipment and cadastral maps. These maps are the ‘original’ maps of the Italian Land Cadastre, the first cartographic support built directly from measures carried out by technicians during implantation of the Italian land cadastre. They are called ‘originali di impianto’ – ‘originals of implantation’ or ‘implant maps’. As such, these maps are valuable and are kept with great care. Recently, the Italian cadastre has carried out an accurate digitisation of these maps in a raster format at a high resolution. In this work, the authors propose the use of these digital maps for the recovery of cadastral boundary. The original cadastral map, one of the primary sources relied upon in defining legal boundaries, generally uses the Bessel ellipsoid localised in Genova and the Cassini-Soldner projection; the GNSS equipment, on the other hand, uses the geocentric ellipsoid with global or continental realisations. After an RTK positioning, the receivers usually provide the cartographic coordinates in a Gauss projection. However, our study deals with the problem of using different projections and reference systems within the limits of a map. In this context, the transition between systems and projections can be made through a conformal transformation with deformations slighter than graphical errors in the map. The difficulty of finding identifiable points in both reference systems is partially solved through a new way of carrying out the redefinition of boundaries by exploiting geometric information.