Assessment of the diurnal and semidiurnal signals induced by monument thermal effect with time series of very short GPS baselines

The monument thermal effect (MTE) displacements could result in periodical signals with several millimeters magnitudes in the vertical and horizontal GPS position time series. However, the interaction of various origins of periodic signals in GPS observations makes it difficult to isolate the millimeter-level MTE displacement from other signals and noises. In this study, to assess the diurnal and semidiurnal signals induced by MTE, we processed 12 very short GPS baselines (VSGB) with length <150 m. The monument pairs for each baseline differ in their heights, horizontal structure, or base foundations. Meanwhile, two zero-baselines were also processed as the control group. Results showed that the seasonal signals observed in VSGB time series in the horizontal and vertical directions, were mainly induced by seasonal MTE. Time-varying diurnal and semidiurnal signals with amplitude up to 4 mm were observed in the vertical direction for baselines with monument height difference (MHD) larger than 10 m. Horizontal diurnal signal with an amplitude of about 2 mm was also detected for baselines with non-axisymmetric monument structure. The orientation of the detected horizontal displacement was coherent with the direction of daily temperature variation (DTV) driven by direct solar radiation, which indicates that the diurnal and semidiurnal signals are likely induced by MTE. The observed high-frequency MTE displacements, if not well modeled and removed, may propagate into spurious long-term signals and bias the velocity estimation in the daily GPS time series.