Chameleonic Noise in GPS Position Time Series

International audienceIt has been a standard practice for about 2 decades to compute global positioning system (GPS) based station velocity uncertainties using the apparent noise statistics of the non linear position residuals rather than assume white noise (WN) behavior. The latter choice would yield unrealistic velocity uncertainties. The most common noise types used are power law, usually close to flicker noise (FN), over most frequencies mixed with WN at the shortest periods. The complicating impact of offsets in the position time series, mostly caused by equipment changes or tectonic events, has not been fully appreciated. These are far less benign than recently suggested. In addition to contributing a pseudo random walk noise (RW) component to the velocity errors, estimating offset parameters changes the apparent noise color toward whiter. Spectral power is effectively drained by offsets at periods longer than roughly the mean span between them. This consequently favors a Gauss Markov process as the apparently preferred noise model and, importantly, obscures the presence of RW and long period Earth deformation in the series. Both effects can lead to potentially under estimated velocity uncertainties. The full value of decadal long GPS time series for geodynamical applications is thereby greatly eroded by recurring offsets, especially when they occur quasi regularly. In addition, contrary to common assumption, the noise color is generally not fixed with time, but clearly becomes whiter in more recent data. The origin of the colored noise and its whitening over time remain elusive