For geodetic data acquired with the Global Positioning Sys- tem (GPS), a variety of time-correlated processes in addition to monument noise corrupt velocity estimates and, in fact, likely dominate the error budget at the present time. In other words, GPS velocity estimates may not yet be accurate enough to ob- serve monument noise except in extreme cases. Other sources of time-correlated noise include mismodeled satellite orbits, other reference frame effects (e.g., Earth orientation), mismodeled atmospheric effects, and mismodeled antenna phase center ef- fects, which may vary with satellite elevation, azimuth, and local environmental factors. Studies of time-correlated noise in GPS time series have been hampered by the relatively short time that high-quality time se- ries have been available. Rigorous analysis of time-correlated noise in GPS data may well require decade or longer time series, but high-precision results from continuously operating stations have been available only since about 1992 or 1993. The present study reports the noise characteristics of 23 globally distributed GPS sites that have operated more or less continuously for about 3 years.