VADASE: Variometric Approach for Displacement Analysis Stand-alone Engine

Global Navigation Satellite Systems (GNSS), like Global Positioning System (GPS), are nowadays very well known. Besides the mass market, GPS plays an important role in several technical and scientific activities. Ever since the early stages of development (mid 1980s), given the high level of accuracy achieved in determining the coordinates of the receiver, it became clear that the extensive deployment of GPS stations all over the world would have improved many tasks in geodesy and geodynamics.
\nIn recent years, several studies have demonstrated the effective use of GPS in estimating coseismic displacement waveforms induced by an earthquake with accuracies ranging from a few millimeters to a few centimeters (so called GPS seismology). This contribution is particularly relevant as it supports the estimation of important seismic parameters (e.g. seismic moment and magnitude Mw) without the problems of saturation that commonly affect seismometers and accelerometers close to large earthquakes. These studies were developed mainly off-line, analyzing observations acquired during strong earthquakes. Then, well-known processing strategies (single Precise Point Positioning (PPP), and differential positioning) have been developed to reduce as far as possible the latency between earthquake occurrence and coseismic displacement waveforms estimation. 
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\nThis work describes a new approach that was originally designed to detect the 3D displacements of a single GPS receiver in real-time and that was eventually appointed as an effective strategy to contribute to GPS seismology. The main goals that guided through the development aimed to obtain the coseismic displacement waveforms in real-time and using a single receiver. In particular, it was pursued to overcome the drawbacks of the two most common strategies used so far in GPS seismology: PPP uses a single receiver, but it requires ancillary products (e.g. information regarding satellites orbits, clocks; parameters describing the Earth orientation with respect to an external inertial system) currently unavailable in real-time. On the other hand, differential kinematic positioning is based on a complex infrastructure (GPS permanent network), which has to be managed by specialized research centers to obtain high accuracies in real-time.
\nThe new approach proposed in this work, named Variometric Approach for Displacement Analysis Stand-alone Engine (VADASE), is based upon a so called variometric algorithm, which is able to use a single receiver to obtain real-time results, with accuracies ranging from few centimeters up to a couple of decimeters. 
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\nIn order to prove the feasibility of the variometric algorithm, a tuned software, which was appointed with the same name (i.e. VADASE), was implemented. Exploiting this software to process a large amount of both simulated and real data, the main advantages of VADASE, as well as its limitations, were investigated in details.
\nAt a first stage, VADASE effectiveness was proven on a simulated example. First, a known displacement (1 cm East and North and 2 cm Up) was synthetically introduced into carrier phase observations collected at 1 Hz rate by the M0SE permanent GPS station (Rome, Italy). Then, these data were processed using the variometric algorithm: the displacements were estimated with an accuracy of 1-2 mm in the horizontal and the vertical directions. The solutions obtained using the GPS broadcast products available in real-time and the best quality products supplied by IGS a posteriori displayed a global agreement of 1 mm for the horizontal components and 2 mm for the height. 
\nGiven this fundamental proof of success, and provided that the variometric algorithm was originally 
\nconceived to contribute in the field of GPS seismology and tsunami warning systems, VADASE was applied to retrieve the coseismic displacements and the waveforms generated by real earthquakes. Here, it is worth underlining that all VADASE results were obtained using exclusively broadcast products that are available in real-time. The most significant outcomes were obtained by considering data collected at 1 Hz rate from the IGS station of BREW during the Denali Fault, Alaska earthquake (Mw 7.9, November 3, 2002), at 10 Hz rate from CADO station during the L'Aquila earthquake (Mw 6.3, April 6, 2009), and at 5 Hz rate from some stations included in the UNAVCO - PBO network during the Baja, California earthquake (Mw 7.2, April 4, 2010). In all cases, the agreement between VADASE and other solutions achieved by different research groups employing different methodologies was between few centimeters and a couple of decimeters.
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\nThe real-time potentialities of the variometric approach were internationally recognized during the recent tremendous earthquake in Japan (Mw = 9.0, March 11, 2011), when the GNSS research team of University of Rome "La Sapienza" was able to provide the first waveforms results among the scientific community. The results obtained for IGS stations of MIZU, USUD and from EV-network station JA01 were compared with those stemming from the PPP approach implemented in the software developed at Natural Resources Canada (NRCan). This work was made possible thanks to the precious support of Dr. Henton Joe and Dr. Dragert Herb, NRCan, who produced the PPP solutions for the aforementioned stations. The agreement between VADASE and PPP was evaluated in terms of the RMSE of the differences.
\nIn addition, it was investigated the agreement reliance with the earthquake duration. The Root Mean Square Error (RMSE) of the differences between the two solutions ranged from 0.01-0.02 m in East and North and 0.06 m in Up, after one minute, to approximately 0.05 m in East and North and (with much larger variability) 0.20 m in Up after four minutes. Further, the agreement between the two approaches evaluated in terms of peak to peak displacements appeared to be independent from the earthquake duration. In details, the differences reached approximately 0.01 m in East and North and 0.03 m in Up components. Finally, the correlation coefficient between the two solutions proved to be higher than 99% for the planimetric components (but for the East component of USUD and the North component of JA01 which showed 97% and 96% correlations, respectively). The vertical component showed slightly lower (and with larger variability) correlations: 65%, 84% and 90% for stations MIZU, USUD and JA01, respectively.
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\nAt the moment of this writing, VADASE is subject of a pending patent of University of Rome “La Sapienza” ever since June 2010. In October 2010 VADASE was recognized as a simple and effective approach towards real-time coseismic displacement waveform estimation and it was awarded the German Aerospace Agency (DLR) Special Topic Prize and the First Audience Award in the European Satellite Navigation Competition 2010.

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