It is commonly assumed that the GPS ranging degradation called Selective Availability contains only low frequency components, with a correlation time on the order of several minutes. It will be shown here that, on top of this well known behavior, SA contains a small high frequency component centered around 1/3 Hz (period 3 s), with an amplitude on the order of the millimeter. This unexpected component will be studied and characterized. It will be shown that it causes the noise on the phase measurement to be non-white, and that it is responsible for errors when decimating GPS data. ¿ 1999 American Geophysical Union