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.
The importance of satellite altimetry in monitoring the complex ocean-atmosphere system led to the approval of the
Sentinel-3 ocean topography mission. This mission is scheduled to be launched in 2013. It will incorporate new
instruments and measuring modes that are expected to provide high-accuracy measurements for the determination of sea
level as well as ocean and land color, sea and land surface temperature with improved spatial and temporal coverage.
Nevertheless, satellite altimeter measurements, as in also the case for Sentinel-3, of homogenous quality and reliability
have to be maintained over longer periods of time. Hence, the Sentinel-3 altimetry observations, such as sea-surface
heights and sea-level anomaly fields, need to be continuously and independently connected in a common, reliable but
also long-term manner. This can be achieved by satellite calibration using dedicated research infrastructures. A
permanent calibration facility for satellite altimeters has been operating in Gavdos island, Greece as of 2004. This
facility has already been successfully and continuously determined the OSTM/Jason-2 altimeter bias. This work presents
the plans and actions to be performed to calibrate the altimeter of Sentinel-3, using the existing Gavdos Cal/Val facility,
as well as the newly developed microwave transponder.
The Gavdos calibration facility for satellite radar altimeters has been operational as of 2004. The island is located along
repeating ground tracks of Jason-1 and Jason-2 satellites (crossover point for passes No.109 ascending and No.018
descending and adjacent to Envisat), and because of its small size, both altimeter and radiometer measurements are not
significantly contaminated by land. This makes Gavdos an ideal place for the calibration of satellite altimeters. In this
work, three different techniques have been applied for calibrating the Jason altimeter measurements at Gavdos Cal/Val
facility. These are: (i) The conventional: In-situ observations made by tide gauges, GNSS receivers, meteorological and
other sensors in conjunction with precise geoid models are applied for determining the altimeter bias; (ii) The MSS:
instead of the geoid, the mean sea level, provided by the CLS10_MSS model, is used as a reference surface for
estimating the bias; and (iii) Microwave transponder measurements are implemented and examined over the cross over
point on land to produce the altimeter bias as well. This paper presents the results regarding these calibration techniques.