Noise in GPS coordinate time series

We assess the noise characteristics in time series of daily position estima tes for 23 globally distributed Global Positioning System (GPS) stations wi th 3 years of data, using spectral analysis and Maximum Likelihood Estimati on. A combination of white noise and flicker noise appears to be the best m odel for the noise characteristics of all three position components. Both w hite and flicker noise amplitudes are smallest in the north component and l argest in the vertical component. The white noise part of the vertical comp onent is higher for tropical stations (+/-23 degrees latitude) compared to midlatitude stations. Velocity error in a GPS coordinate time series may be underestimated by factors of 5-11 if a pure white noise model is assumed.