[Paddlewise] GPS and Selective Availability

     Hi everyone,
     
     I work for Raytheon Marine company and thought I could offer a little 
     insight into SA getting turned off.
     
     There are two types of signals that can be received from the GPS 
     system; C/A (course acquisition) and P code (precision).  The P code 
     is used by the military and some civilian applications for precise 
     positioning (like Tomahawk missiles).
     
     The rest of us have access to only the C/A code.  With S/A on, 
     accuracy is stated as "better than 100 meters 95% of the time".  With 
     S/A off, that drops to around 15 meters so there is significant 
     improvement.  Any GPS receiver will realize this increase in accuracy.
     
     How will this affect differential?  Both the land based differential 
     and the new WAAS satellite differential systems will make GPS twice as 
     accuracte even with S/A off.  Typical differential accuracy is 1 to 5 
     meters.
     
     But this is great news for paddlers with handheld GPS's.
     
     JP Stohrer

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