Using spectral analysis of Cabril dam GNSS monitoring system time series for detecting periodic displacements

Abstract

A measurement system based on GNSS (Global Navigation Satellite System) was installed at Cabril dam in order to monitor displacements at the top of the central section. The GNSS system consists of two receivers (stations), one located on the crest of the dam, in central section, and the second located on the left bank, on the top of a medium voltage pole of a line deactivated and cut to be about 6 m high, to work as reference station. Unfortunately, this GNSS monument is exposed to solar radiation and wind effects. Consequently, it is expected that these environmental factors could cause sub-daily noise in the estimation of the dam displacements due to the thermal tilts of the monument. This paper is concerned with the assessment of the impact of this GNSS monument on the noise in the continuous GNSS time series. In GNSS time series analysis, frequency-domain signature is obtained by converting time-domain data into its unique frequency components using a Fast Fourier Transform (FFT). Through the analysis of frequency-domain signal signatures, the dam displacement seasonal can be detected and isolated from the noise eventually caused by the GNSS monument instability.