Experimental Assessment of the Accuracy of GPS and RTS for the Determination of the Parameters of Oscillation of Major Structures

Based on experimental evidence, the authors explore the possibility of using GPS and robotic total stations (RTS) for measurements of oscillations of relatively rigid structures (modal frequencies up to 3–4 Hz). The strategy was to compare uni-axial oscillations of known characteristics with simultaneous recordings of both GPS and RTS, and analyze obtained time series to determine amplitude and frequency of oscillations. The conclusion of this study is that GPS can record oscillations up to 4 Hz with a minimum amplitude of 5–10 mm with an accuracy of a few millimeters, and that RTS can record peak oscillations with submillimeters to a few millimeters accuracy, but at high frequencies some cycles were lost. Based on recordings of both instruments, frequencies of oscillations were also accurately determined, though noise seems to increase with increasing frequency. Spectral analysis was based on least-square-based software which permits one to analyze discontinuous, short, and non-equispaced time series. The latter derive either from GPS signal outages and hardware/software imperfections, or from a non-constant rate of sampling for RTS. The overall conclusion is that the adopted computational approaches permit some main disadvantages of these instruments to be overcome and also allow conventional GPS and RTS instrumentation to be used for a wide range of cases of structural monitoring, especially if displacements relative to an independent coordinate system are required.

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