Low cost GNSS receivers: navigation and monitoring activities

In this PhD research different conditions of using of low cost GNSS receivers were explored in navigation and monitoring activities. These receivers are cheap and widespread, but they are single-frequency receivers and then less accurate than the geodetic dual frequency receivers. In navigation applications the single-frequency receivers are largely used and in most of application the accuracy of tens meters usually reached in point positioning is enough, but there are some particular cases where it is needed to increase it: this is the case of the MEP (Map for Easy Path) project where the goal is to reach an accuracy of meter order in point positioning. The aim of the MEP project is to improve the urban mobility of the physically impaired people, mapping obstacles, with common mass market devices, to improve mobility. Therefore the final accuracy should be improved and two different procedures were implemented and tested with the aim to correct positions estimated with these low cost receivers: correction using data from Continuously Operating Reference Stations; correction using local cartography as reference. In monitoring activities the GNSS geodetic receivers are already used, but there are situations where these receivers are not suitable, mainly due to the great economic effort that they require. The adoption of low cost GNSS receiver can expand the field of application of GNSS techniques, but their accuracy in positioning must be increased. Navigation: correction using data from CORS Let's consider the CORS optimally estimated position and the CORS positions estimated epoch by epoch: the first one is the actual position when the others are affected by the environmental effects presents at the epoch in which the survey was done. Therefore the difference between actual position and position at a generic epoch i it can be assumed due to specific errors present at this particular epoch. For correlated in space errors, if the rover is near the CORS, you can assume that these errors will be more or less the same that afflict estimations done with the GNSS low cost receiver. It is possible to compute the difference between actual and estimated CORS position and therefore to try to improve accuracy of rover positioning applying to the rover trajectory the corrections from CORS data, computed epoch by epoch. In the correction procedure takes into account also of the different satellites visibility between CORS and rover: rover usually is in a worse position than the CORS, so it can view only a subset of satellites viewed by CORS. Two different kind of test have been done: in ideal conditions, with a static occupation and in normal working conditions, with data collected during several repetition of two test paths, in a urban scenario. In the first case, two test site have been used: both with good satellites visibility for a total of 15 days of data from static observations. Observations were processed and corrected with data from two near CORS stations: the final results were improved by the correction, with most of statistical errors indexes improved, therefore the tested correction method in static survey worked. In the second case test two test paths were defined and repeated several times collecting data using a geodetic receiver and some low cost receivers (single frequency). The data from the geodetic receiver were processed by a kinematic postprocessing and provided the benchmark epoch by epoch to evaluate the positioning accuracy of the low cost receivers tested. Errors statistic were compute before and after applying corrections from the CORS like did in the first case. Unlike what it was obtained with the data from static positioning, in this case the accuracy after the correction does not improve. This is probably the effect of local noise, due to particular properties of the area where the survey was made (like multipath), that obviously can not be removed using data from reference station, where this kind of noises are not present. Navigation: correction using local cartography as reference As reference map it was chosen OpenStreetMap (OSM), due to the several advantages offered: it is a collaborative project, the data access is free and permits to use, download and modify data freely, without particular restrictions, keeping them always up to date. Therefore it is possible get data and organize them in a local database (PostgreSQL and its extension PostGIS ) with the data collected during the survey activities and so to use the database tools to match data and then to correct the positioning. Different policies were explored to implement the corrections: path segments as reference, in this case the only acceptable position for a collected point is assumed on a OSM path segment and all the points that are not on a OSM segment must be corrected. building as exclusion: in this method the point position will be classified as wrong only if the point is inside a building; buffer as reference: this method uses the path segments like the first correction method viewed, but in this case the point position will be considered wrong only if the distance from a segment will be greater than a certain threshold. The different correction procedures were compared checking how many points, after correction procedure, are near the actual path done, where the actual path is know and specifically defined for these tests. All the methods implemented improve the accuracy of points collected, but there is not a method that is always better to another one: the final results depend significantly on the conditions in which the survey was done, the site characteristic and from the cartography used as reference. Local monitoring The purpose of local monitoring is to model displacements, evolution and deformations of civil engineering structures and local phenomena such as landslides. This monitoring activity is very important to avoid damages and possible victims. Different techniques are possible to do local monitoring, between these, in the last fifty years, the use of GNSS receivers has had a great development that permits to reach accuracy less 1 cm, in case of dual frequency receivers. Nevertheless, the high cost of these receivers can represent a problem then it could be interesting verify if it is possible do local monitoring with GNSS techniques, but using low cost receivers. The accuracy obtained with low cost receivers has not yet completely tested, so in this thesis it was verified: how reliable are the results obtained or rather, how many false or missed alarms are expected per unit of time (significance); the ability to identify at least the order of magnitude of a displacement (congruence). The experiment were planned in order to assess the precision and the accuracy of low-cost GNSS receivers in monitoring dynamic displacements, with the aim of evaluating which is the precision level reachable, comparing scheduled displacements and detected displacements obtained using these kind of GNSS receivers. The tests were done using tools to control the imposed displacements along horizontal and vertical direction. The obtained results shown how the general displacement trend is recognizable: along the horizontal direction the results are satisfactory, with the estimated points close to the imposed positions. Along the vertical direction the results are worse: the general trend is still recognizable, but the individual movements are almost impossible to identify, probably due to the high noise of the cheap antenna used. Therefore a low cost receiver can be used, under specific constraints, in survey activities. The vertical component is not well estimated, so the the main displacement in the monitored area should be in the horizontal component to use efficiently the low-cost receiver.

In questo dottorato di ricerca sono state studiatei diverse condizioni di utilizzo di ricevitori GNSS a basso costo nell'ambito della navigazione e di attività di monitoraggio. Questi ricevitori sono molto diffusi ed economici, ma sono ricevitori a frequenza singola e quindi meno accurati dei ricevitori geodetici a doppia frequenza. Nelle applicazioni di navigazione i ricevitori a singola frequenza sono ampiamente utilizzati e genericamente nella maggior parte delle applicazioni l'accuratezza dell'ordine delle decine di metri è sufficiente. Ci possono essere però dei casi particolari in cui è necessario aumentare questa accuratezza: questo è il caso del progetto europeo MEP (Map forr Easy Path), progetto in cui l'obiettivo è quello di raggiungere una precisione di ordine metro nel posizionamento puntuale. Lo scopo del progetto MEP è quello di migliorare la mobilità urbana delle persone con disabilità fisica, mappando gli ostacoli con i dispositivi comuni, pertanto la precisione finale dovrebbe essere migliorata. Due diverse procedure sono state attuate e testato con l'obiettivo di correggere la posizione stimata con questi ricevitori a basso costo: - correzione utilizzando i dati da stazione permanenti;; - correzione utilizzando la cartografia locale come riferimento. Per quanto riguarda le attività di monitoraggio in questo caso i ricevitori GNSS geodetiche sono già utilizzati, ma esistono situazioni in cui questi ricevitori non sono adatti, principalmente per il notevole sforzo economico si richiederebbe in alcune situazioni. L'adozione di ricevitore GNSS basso costo può espandere il campo di applicazione di tecniche GNSS, a condizione di riuscire a migliorarne la loro accuratezza.