After 22:00 UTC September 12, GPS began transitioning satellite SVN25 (PRN25) into the broadcast almanac for all satellites, according to a Navstar advisory. Meanwhile, satellite SVN24 (PRN24) is being moved out of the almanac.
Below is the text of the advisory:
NOTICE ADVISORY TO NAVSTAR USERS (NANU) 2009065 NANU TYPE: GENERAL
*** GENERAL MESSAGE TO ALL GPS USERS ***
On 12SEP2009, after 2200Z, GPS will transition SVN 25 (PRN 25) into the
broadcast almanac for all satellites. At the same time SVN 24 (PRN 24)
will be transitioned out of the broadcast almanac for all satellites.
The almanac transition, one satellite at a time, will require 24 hours
to complete. During the transition SVN 25 (PRN 25) will be USABLE.
Source: http://www.gpsworld.com/
