Global Positioning System (GPS) Calculated Altitudes

Most GPS receivers will deliver to you a calculated altitude.  Not all, but most do.  The accuracy of this altitude indication in a Wide Area Augmentation System (WAAS) GPS receiver today is plus or minus one meter, never worse than two meters. In the event you lose your primary altimeter, the calculated altitude may be used to good advantage. For a non WAAS GPS receiver, the altitude accuracy will be plus or minus 12 meters. Prior to the shutting down of the Selective Availability system, which happened on May 1, 2000 the calculated altitude accuracy was much worse - plus or minus 300 meters.