GPS errors are a combination of noise, bias, blunders. 
     Noise, Bias, and Blunders 
      Noise errors are the combined effect of PRN code noise (around 1

The following is from the University of Texas at Austin, considered a reliable resource for GPS information. See http://www.utexas.edu/depts/grg/gcraft/notes/gps/gps.html GPS Error Sources GPS errors are a combination of noise, bias, blunders. Noise, Bias, and Blunders Noise errors are the combined effect of PRN code noise (around 1 meter) and noise within the receiver noise (around 1 meter). Bias errors result from Selective Availability and other factors Selective Availability (SA) SA is the intentional degradation of the SPS signals by a time varying bias. SA is controlled by the DOD to limit accuracy for non-U. S. military and government users. The potential accuracy of the C/A code of around 30 meters is reduced to 100 meters (two standard deviations). The SA bias on each satellite signal is different, and so the resulting position solution is a function of the combined SA bias from each SV used in the navigation solution. Because SA is a changing bias with low frequency terms in excess of a few hours, position solutions or individual SV pseudo-ranges cannot be effectively averaged over periods shorter than a few hours. Differential corrections must be updated at a rate less than the correlation time of SA (and other bias errors). Other Bias Error sources; SV clock errors uncorrected by Control Segment can result in one meter errors. Ephemeris data errors: 1 meter Tropospheric delays: 1 meter. The troposphere is the lower part (ground level to from 8 to 13 km) of the atmosphere that experiences the changes temperature, pressure, and humidity associated with weather changes. Complex models of tropospheric delay require estimates or measurements of these parameters. Unmodeled ionosphere delays: 10 meters. The ionosphere is the layer of the atmosphere from 50 to 500 km that consists of ionized air. The transmitted model can only remove about half of the possible 70 ns of delay leaving a ten meter un-modeled residual. Multipath: 0.5 meters. Multipath is caused by reflected signals from surfaces near the receiver that can either interfere with or be mistakenfor the signal that follows the straight line path from the satellite. Multipath is difficult to detect and sometime hard to avoid. Blunders can result in errors of hundred of kilometers. Control segment mistakes due to computer or human error can cause errors from one meter to hundreds of kilometers. User mistakes, including incorrect geodetic datum selection, can cause errors from 1 to hundreds of meters. Receiver errors from software or hardware failures can cause blunder errors of any size. Noise and bias errors combine, resulting in typical ranging errors of around fifteen meters for each satellite used in the position solution. IMHO, Loran-C is still the most reliable, cost effective, widely available (continental US) technique for providing navigational redundancy. Mel Kowal MEI LEE Swantown Marina, Olympia, WA