Alvaro Santamaría-Gómez

GET, Université de Toulouse, CNES, CNRS, IRD, UPS, Toulouse, France, GET, Université de Toulouse, CNES, CNRS, IRD, UPS, Toulouse, France
Corresponding Author:alvaro.santamaria@get.omp.eu
Author ProfileAbstract
It has been standard practice for about two decades to compute 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 towards whiter, to the point that FN can no
longer be considered the model that objectively best describes the
observed GPS position spectra. Spectral power is effectively drained by
offsets at periods longer than roughly the mean span between them. This
consequently promotes a Gauss-Markov process as the preferred noise
model and, importantly, obscures the presence of RW and long-period
Earth deformation in the series. Both effects can lead to potentially
underestimated 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.