Achilleas Tripolitsiotis

First Calibration Results for the SARAL/AltiKa Altimetric Mission Using the Gavdos Permanent Facilities

This work presents the first calibration results for the SARAL/AltiKa altimetric mission using the Gavdos permanent calibration facilities. The results cover one year of altimetric observations from April 2013 to March 2014 and include 11 calibration values for the altimeter bias. The reference ascending orbit No. 571 of SARAL/AltiKa has been used for this altimeter assessment. This satellite pass is coming from south and nears Gavdos, where it finally passes through its west coastal tip, only 6 km off the main calibration location. The selected calibration regions in the south sea of Gavdos range from about 8 km to 20 km south off the point of closest approach. Several reference surfaces have been chosen for this altimeter evaluation based on gravimetric, but detailed regional geoid, as well as combination of it with other altimetric models. Based on these observations and the gravimetric geoid model, the altimeter bias for the SARAL/AltiKa is determined as mean value of −46mm ±10mm, and a median of −42 mm ±10 mm, using GDR-T data at 40 Hz rate. A preliminary cross-over analysis of the sea surface heights at a location south of Gavdos showed that SARAL/AltiKa measure less than Jason-2 by 4.6 cm. These bias values are consistent with those provided by Corsica, Harvest, and Karavatti Cal/Val sites. The wet troposphere and the ionosphere delay values of satellite altimetric measurements are also compared against in-situ observations (−5 mm difference in wet troposphere and almost the same for the ionosphere) determined by a local array of permanent GNSS receivers, and meteorological sensors.

Dronesourcing: a modular, expandable multi-sensor UAV platform for combined, real-time environmental monitoring

ABSTRACT The systematic environmental monitoring of the land, atmosphere, oceans and their coupling zones, is assisted by the use of unmanned aerial vehicles (UAVs) that can operate over rural and/or urban areas to provide enhanced spatial and temporal measurement resolutions compared against corresponding satellite products. The international UAV market includes a vast number of solutions that carry sensors for environmental monitoring varying in type, flight time, carrying weight, communication, and autonomous flight. The majority of these commercial UAVs (especially the low-cost ones) have been designed for specific applications and their main disadvantage is that they can only integrate the payload they have been initially designed to carry, thus presenting minimal modularity. This work presents a modular and affordable platform where the user can easily adapt almost any type of environmental monitoring sensor, which can transmit its measurements to the UAV flight controller without the need for any additional modification. A novel communication protocol has been developed that is also capable to incorporate proximity sensors for collision avoidance. In addition, a wireless mobile telecommunications module incorporation through the use of mobile devices on the UAV provides real-time animated map generation along with cooperative capabilities for fleet missions.

Detection of small-scale rockfall incidents using their seismic signature

Several algorithms have been effectively used to identify the seismic signature of rockfall incidents, which constitute a significant threat for human lives and infrastructure especially when occurring along transportation networks. These algorithms have been mostly evaluated using data from large scale rockfall events that release a large amount of energy. However, low-energy rockfall events (< 100 Joules) triggered by small-sized individual rocks falling from small heights can be severely destructive. In this study, a three-parameter algorithm has been developed to identify low-energy rockfall events. An experimental setup was implemented to 1) validate the results obtained by this algorithm against visual inspection of seismic signals records, 2) define the optimal algorithm parameterization to minimize false alarms, and 3) investigate whether tri-axial vibration monitoring can be replaced by a uniaxial device in order to reduce the installation cost of a real-time rockfall monitoring system. It was found that the success rate of the proposed algorithm exceeds 80%independently of the parameters used, while event identification at a maximum distance with minimal false alarms was achieved when using mean± 3σ as the threshold criterion and 6 ms and 4 ms as the trigger and event window parameters respectively. Finally, it was found that for the specific experimental setup, a uniaxial device could be used for rockfall event identification.

Rockfall detection along road networks using close range photogrammetry

Early warning for rockfall incidents occurring on slopes along highways and roads is a vital safety practice for both human lives and property loss. Monitoring systems mainly rely on laser scanners, distributed sensors and precision geodetic measurements. In this study, close range terrestrial photogrammetry is evaluated towards rockfall incident detection. Photogrammetry has been extensively used for 3D mapping and reconstruction of terrain and infrastructure mainly due to its relatively low implementation cost, compared to the above mentioned approaches, especially when using non-metric digital cameras. Yet, the underlying problems in terrestrial photogrammetry include processing time, precision, night capturing limitations, and various errors, including occlusion, camera geometry, lighting, etc. In the present paper, an experimental setup along with preliminary results of the terrain photogrammetry integration into an operational slope monitoring scheme are presented. More specifically, assessment of the detection limitations are provided in order to evaluate whether photogrammetry can constitute a cost-effective alternative for rapid surface and change detection processes. In addition, since false alarm incidents cause extensive resource draining, their magnitude and occurrence are also reported.

Complementing geotechnical slope stability and land movement analysis using satellite DInSAR

This paper explores the potential of using satellite radar inteferometry to monitor time-varying land movement prior to any visible tension crack signs. The idea was developed during dedicated geotechnical studies at a large open-pit lignite mine, where large slope movements (10–20 mm/day) were monitored and large fissures were observed in the immediate area outside the current pit limits. In this work, differential interferometry (DInSAR), using Synthetic Aperture Radar (SAR) ALOS images, was applied to monitor the progression of land movement that could potentially thwart mine operations. Early signs of land movements were captured by this technique well before their visual observation. Moreover, a qualitative comparison of DInSAR and ground geodetic measurements indicates that the technique can be used for the identification of high risk areas and, subsequently, for the optimization of the spatial distribution of the available ground monitoring equipment. Finally, quantitative land movement results from DInSAR are shown to be in accordance with simultaneous measurements obtained by ground means.

Multi-temporal change image inference towards false alarms reduction for an operational photogrammetric rockfall detection system

Rockfall incidents affect civil security and hamper the sustainable growth of hard to access mountainous areas due to casualties, injuries and infrastructure loss. Rockfall occurrences cannot be easily prevented, whereas previous studies for rockfall multiple sensor early detection systems have focused on large scale incidents. However, even a single rock may cause the loss of a human life along transportation routes thus, it is highly important to establish methods for the early detection of small-scale rockfall incidents. Terrestrial photogrammetric techniques are prone to a series of errors leading to false alarm incidents, including vegetation, wind, and non relevant change in the scene under consideration. In this study, photogrammetric monitoring of rockfall prone slopes is established and the resulting multi-temporal change imagery is processed in order to minimize false alarm incidents. Integration of remote sensing imagery analysis techniques is hereby applied to enhance early detection of a rockfall. Experimental data demonstrated that an operational system able to identify a 10-cm rock movement within a 10% false alarm rate is technically feasible.

Seismic Monitoring for Automatic Rockfall Detection along Transportation corridor

This geophysical research presents the setup and the preliminary results of several experiments conducted to evaluate the performance of seismic monitoring to recognize low energy ground vibrations caused by small rock free fall or rock rolling along a slope. New ideas for automatic rockfall detection based on Short Time Average over Long Time average (STA/LTA) and fractal analysis of the seismic records are introduced. Different events of vibrations were tested, such as random noise, passing cars or people, rolling or throwing rocks or combination of passing car with rockfall. Based on the experimental results it was determined that more than 95% of car stimulated records are discarded while, more than 90 % of rockfalls or simultaneous car passing and rockfalls are successfully recognized.

Land movement monitoring at the Mavropigi lignite mine using spaceborne D-InSAR

This paper examines the capability of remote sensing radar interferometry to monitor land movements, as it varies with time, in areas close to open pit lignite mines. The study area is the “Mavropigi” lignite mine in Ptolemais, Northern Greece; whose continuous operation is of vital importance to the electric power supply of Greece. The mine is presently 100-120m deep while horizontal and vertical movements have been measured in the vicinity of the pit. Within the mine, ground geodetic monitoring has revealed an average rate of movement amounting to 10-20mm/day at the southeast slopes. In this work, differential interferometry (DInSAR), using 19 Synthetic Aperture Radar (SAR) images of ALOS satellite, has been applied to monitor progression of land movement caused my mining within the greater area of “Mavropigi” region. The results of this work show that DInSAR can be used effectively to capture ground movement information, well before signs of movements can be observed visually in the form of imminent fissures and tension cracks. The advantage of remote sensing interferometry is that it can be applied even in inaccessible areas where monitoring with ground equipment is either impossible or of high-cost (large areas).

Mapping of traditional settlements by unmanned airborne vehicles towards architectural restoration

Conservation and restoration of traditional settlements are amongst the actions that international directives proclaim in order to protect our cultural heritage. Towards this end, a mandatory base step in all archaeological and historical practices includes the surveying and mapping of the study area. Often, new, unexplored or abandoned settlements are considered, where dense vegetation, damaged structures and ruins, incorporation of newer structures and renovation characteristics make the precise surveying procedure a labor intensive and time consuming procedure. Unmanned airborne vehicles (UAVs) have been effectively incorporated into several cultural heritage projects mainly for mapping archeological sites. However, the majority of relevant publications lack of quantitative evaluation of their results and when such a validation is provided it is rather a procedural error estimation readily available from the software used, without independent ground truth verification. In this study, a low-cost custom-built hexacopter prototype was employed to deliver accurate mapping of the traditional settlement of Kamariotis in east Crete, Greece. The case of Kamariotis settlement included highly dense urban structures with continuous building forms, curved walls and missing terraces, while wild vegetation made classic geodetic surveying unfeasible. The resulting maps were qualitatively compared against the ones derived using Google Earth and the Greek Cadastral Orthophoto Viewing platforms to evaluate their applicability for architectural mapping. Moreover, the overall precision of the photogrammetric procedure was compared against geodetic surveying.