Salvatore Troisi

Low-Cost and open-source solutions for automated image orientation --- a critical overview

The recent developments in automated image processing for 3D reconstruction purposes have led to the diffusion of low-cost and open-source solutions which can be nowadays used by everyone to produce 3D models. The level of automation is so high that many solutions are black-boxes with poor repeatability and low reliability. The article presents an investigation of automated image orientation packages in order to clarify potentialities and performances when dealing with large and complex datasets.

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.

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.

A photogrammetric approach to survey floating and semi-submerged objects

The article presents an innovative methodology for the 3D surveying and modeling of floating and semi-submerged objects. Photogrammetry is used for surveying both the underwater and emerged parts of the object and the two surveys are combined together by means of special rigid orientation devices. The proposed methodology is firstly applied to a small pleasure boats (approximately 6 meters long) - hence a free floating case - and then to a large shipwreck (almost 300 meters long) interested by a 52 m long leak at the waterline. The article covers the entire workflow, starting from the camera calibration and data acquisition down to the assessment of the achieved accuracy, the realization of the digital 3D model by means of dense image matching procedures as well as deformation analyses and comparison with the craft original plane.

Low-cost human motion capture system for postural analysis onboard ships

The study of human equilibrium, also known as postural stability, concerns different research sectors (medicine, kinesiology, biomechanics, robotics, sport) and is usually performed employing motion analysis techniques for recording human movements and posture. A wide range of techniques and methodologies has been developed, but the choice of instrumentations and sensors depends on the requirement of the specific application. Postural stability is a topic of great interest for the maritime community, since ship motions can make demanding and difficult the maintenance of the upright stance with hazardous consequences for the safety of people onboard. The need of capturing the motion of an individual standing on a ship during its daily service does not permit to employ optical systems commonly used for human motion analysis. These sensors are not designed for operating in disadvantageous environmental conditions (water, wetness, saltiness) and with not optimal lighting. The solution proposed in this study consists in a motion acquisition system that could be easily usable onboard ships. It makes use of two different methodologies: (I) motion capture with videogrammetry and (II) motion measurement with Inertial Measurement Unit (IMU). The developed image-based motion capture system, made up of three low-cost, light and compact video cameras, was validated against a commercial optical system and then used for testing the reliability of the inertial sensors. In this paper, the whole process of planning, designing, calibrating, and assessing the accuracy of the motion capture system is reported and discussed. Results from the laboratory tests and preliminary campaigns in the field are presented.

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...

P-RANSAC: An Integrity Monitoring Approach for GNSS Signal Degraded Scenario

Satellite navigation is critical in signal-degraded environments where signals are corrupted and GNSS systems do not guarantee an accurate and continuous positioning. In particular measurements in urban scenario are strongly affected by gross errors, degrading navigation solution; hence a quality check on the measurements, defined as RAIM, is important. Classical RAIM techniques work properly in case of single outlier but have to be modified to take into account the simultaneous presence of multiple outliers. This work is focused on the implementation of random sample consensus (RANSAC) algorithm, developed for computer vision tasks, in the GNSS context. This method is capable of detecting multiple satellite failures; it calculates position solutions based on subsets of four satellites and compares them with the pseudoranges of all the satellites not contributing to the solution. In this work, a modification to the original RANSAC method is proposed and an analysis of its performance is conducted, processing data collected in a static test.

Robust estimation methods applied to GPS in harsh environments

Satellite navigation is very widespread in civil society; many devices and services exploit this technology and several systems are in use or in development phase. GNSS receiver, embedded in devices used in daily life (smartphones, cars and so on), works in several conditions and operational scenarios. Ensuring good positioning accuracy is challenging, especially in environment where receiver measurements are affected by gross errors, such as urban canyons. In this paper, the benefit of robust estimators in case of multiple simultaneous blunders is investigated; several robust estimators were implemented and their performances are compared with classical techniques used in GNSS context (Weighted Least Square, Receiver Autonomous Integrity Monitoring) using real data. Effectiveness of these methods raised from tests conducted in static and kinematic mode. Experimental results show significant enhancements for the proposed robust estimators, up to 45% for the horizontal RMS error and up to 52% for the vertical one.

The first Galileo FOC satellites: From useless to essential

In August 2014, the European Global Navigation Satellite System Galileo successfully completes its In-Orbit Validation phase and the first two Full Operational Capability satellites were launched. Currently (February 2015), Galileo consists of four IOV satellites and two FOC ones, but only three IOVs and one FOC are working correctly. The two FOCs were injected in a not nominal orbit; at the time of this study only one of them (specifically FOC-FM1) was shifted into a more circular orbit. However, several studies and analysis were carried out by ESA to monitor the impact on the health status of the FOC satellites in wrong elliptical orbits. This paper investigates the potential improvements on constellation geometry provided by the inclusion of the FOCs and analyzes Galileo single-point positioning performance. Different methods for satellite position are considered due to the lack of navigation messages.

Validity period of NeQuick (Galileo version) corrections: Trade-off between accuracy and computational load

PVT accuracy, in GNSS-based navigation, depends on various factors such as the quality of the measurements and the broadcast navigation data. GNSS broadcast signals (in L-band) are strongly influenced by the atmospheric layers. In particular ionospheric effects are the most important in open-sky. The ionosphere effect can be reduced using linear combination of dual frequencies measurements or differential methods. In single point positioning, the receivers have to apply a correction model to limit ionospheric effect. NeQuick-G is the emerging model developed for Galileo, it over-performs Klobuchar model, in the both measurement and position domains. These improvements are compensated by a higher computational load which is fundamental, for mass market devices. The main goal of this work is to identify a trade-off between accuracy and computational load; to this purpose NeQuick corrections are not computed every epoch, but only periodically. The effects of this approximation are assessed for several validity periods in position domain using GPS measurements. The coefficients necessary to use NeQuick-G model are extracted by Galileo navigation message. From the analysis emerges that an optimal threshold for the updating rate is about 15 minutes, this trade-off reduces the effort of the receiver without degrading PVT accuracy.