Proceeding of the Joint International Symposium on Deformatuon Monitoring. 2-4 November, 2011. Hong Kong. China. Report. Measurements of bridge oscillations by collocated GPS receivers, at the limit of the instrument range of operation, revealed important differences in the instrument output, and this result led us to investigate this problem on the basis of systematic experiments. 10Hz measurements from various types of collocated stationary GPS receivers were analyzed. The analysis was focused on short-duration measurements (10 to 104sec) usually corresponding to signals of earthquakes and of oscillations of various structures. The output of these experiments was to confirm the existence of differences between identical, collocated instruments, even an absence of correlation between their recordings, which was not, however, due to phase shifts. Spectral analysis revealed that differences between short duration GPS records are mainly due to their low frequency (below ~0.2Hz) components which are dominated by colored noise, while their high-frequency components (above ~2.5Hz) contain only white noise. The limit between colored and white noise seems to be a function of the duration of observations, and for this reason, long-term observations are practically contaminated by white noise only and hence permit mm-level accuracy. This result puts some constraints in the use of GPS for short-duration recordings, for instance efficiency in identification of dynamic movements, but not of small amplitude, semi-static movements.