Volume 8, Issue 2

The presented work aims to give an overview of different calibration methods for magnetic field sensors, which are used for attitude determination. These methods are applicable in the field without any additional equipment. However, sometimes they require simplification assumptions. The paper addresses the validity of these assumptions, the accuracy and efficiency of the methods and the influence of the calibration error on the orientation estimation. Both simulations and measurements are used for evaluation. The measurements are performed using a GNSS multi-antenna system, providing an orientation reference (roll, pitch, yaw) without unknown external magnetic disturbances and with a sufficient accuracy (about 0.5 degrees). It is shown in simulations, that a full calibration of the sensor (including soft and hard iron disturbances by nearby materials) is possible without any additional equipment. However, experiments show, that some parts of the full calibration procedure are sensitive to an accurate execution of the necessary movements, which may lead to calibration errors in the same order of magnitude as a simplified method, which ignores the presence of soft iron disturbances.

Triangulation-based range sensors, e.g. laser line scanners, are used for high-precision geometrical acquisition of free-form surfaces, for reverse engineering tasks or quality management. In contrast to classical tactile measuring devices, these scanners generate a great amount of 3D-points in a short period of time and enable the inspection of soft materials. However, for accurate measurements, a number of aspects have to be considered to minimize measurement uncertainties. This study outlines possible sources of uncertainties during the measurement process regarding the scanner warm-up, the impact of laser power and exposure time as well as scanner’s reaction to areas of discontinuity, e.g. edges. All experiments were performed using a fixed scanner position to avoid effects resulting from imaging geometry. The results show a significant dependence of measurement accuracy on the correct adaption of exposure time as a function of surface reflectivity and laser power. Additionally, it is illustrated that surface structure as well as edges can cause significant systematic uncertainties.

Monitoring small size area deformations calls for increasingly precise data with greater space and time resolution. To this aim, a GPS data processing method designed for wireless networks of mono-frequency GPS receivers is proposed. The different steps of the method are explained: the GPS processing is first detailed with the design of parametric and stochastic models, their inversion by an extended Kalman filter and the mitigation of the main errors: multipath. A parsimonious radio data transfer protocol is then proposed. It allows a real-time positioning thanks to a wireless transfer of GPS carrier phases data from receivers to a processing computer. Finally the method is tested on a network of mono-frequency receivers developed by the French National Mapping Agency (IGN). Tests prove its ability for real-time positioning with a fifteen receivers network, and a precision under the centimeter level is reached.

The Canary Archipelago is an active volcanic region located in the African plate, at 100 Km of the northwest coast of Africa. The Complutense University of Madrid, Institute of Astronomy and Geodesy (CSIC-UCM) and University of Jaén, established a GPS Network in this region and carried out six observations campaigns from 2002 to 2009. The focus of thiswork is processing and analyzing these observations to study the geodynamic behavior of the stations of this network. The data have been computed with the Bernese GPS Software Version 5.0 obtaining individual solutions of coordinates for each session and campaign, the coordinate velocities of the stations and the time series of baselines formed between them. Results show consistency of station velocities compared to behaviour of areas permanent stations and baselines stability. No deformations could be detected.

The old city center of L’Aquila is rich in historical buildings of considerable merit. On April 6 th 2009 a devastating earthquake caused significant structural damages, affecting especially historical and monumental masonry buildings. The results of a study carried out on a monumental building, former headquarters of the University of L’Aquila (The Camponeschi building, XVI century) are presented in this paper. The building is situated in the heart of the old city center and was seriously damaged by the earthquake. Preliminary visual damage analysis carried out immediately after the quake, clearly evidenced the building’s complexity, raising the need for direct and indirect investigation on the structure. Several non-destructive test methods were then performed in situ to better characterize the masonry typology and the damage distribution, as well. Subsequently, a number of representative control points were identified on the building’s facades to represent, by their motion over time, the evolution of the structural displacements and deformations. In particular, a surveying network consisting of 27 different points was established. A robotic total station mounted on top of a concrete pillar was used for periodically monitoring the surveying control network. Stability of the pillar was checked through a GNSS static survey repeated before any set of measurements. The present study evidences the interesting possibilities of combining geomatics with structural investigation during post-earthquake monitoring of ancient monumental buildings.

This study presents validation of BeiDou measurements in un-differenced standalone mode and experimental results of its application for real data. A reparameterized form of the unknowns in a geometry-free observation model was used. Observations from each satellite are independently screened using a local modeling approach. Main advantages include that there is no need for computation of inter-system biases and no satellite navigation information are needed. Validation of the triple-frequency BeiDou data was performed in static and kinematic modes, the former at two continuously operating reference stations in Australia using data that span two consecutive days and the later in a walking mode for three hours. The use of the validation method parameters for numerical and graphical diagnostics of the multi-frequency BeiDou observations are discussed. The precision of the system’s observations was estimated using an empirical method that utilizes the characteristics of the validation statistics. The capability of the proposed method is demonstrated in detection and identification of artificial errors inserted in the static BeiDou data and when implemented in a single point positioning processing of the kinematic test.