[Paddlewise] GPS and D-GPS

     Here's the scoop on GPS and D-GPS.
     
     The distortion (error) that the government injects into the GPS signal 
     is called Selective Availability (SA).  They do this so that enemy 
     foreign powers can't use our GPS system for their weapons against us.  
     Don't expect to have it turned off anytime soon, though.  As long as 
     we are involved in some type of conflict, they will leave it on.  The 
     rumor that it is being turned off usually comes around on an annual 
     basis, but never comes true.
     
     Regarding D-GPS, you won't find it currently integrated into 
     handhelds.  The problem is the Differential Beacon stations that 
     transmit the differential information operate on frequencies between 
     280 Khz and 325 Khz.  In most applications, a whip antenna is required 
     to receive these signals, so a handheld is out.  JRC does make a 
     sensor that utilizes an H-field antenna (flat patch) but that is part 
     of a combo sensor that does GPS and differential in one package.  This 
     sensor is connected to chartplotters and navigators for accurate 
     position fixes.
     
     You can buy Differential Beacon Receivers for some of the handhelds 
     (Garmin has one), but you have to mount (and usually ground) the 
     sensor and put a 2' to 4' whip on it.  Not really kayak friendly.
     
     There is a possibility for differential in the future, though.  The 
     Air Force is putting up a differential satellite umbrella which is 
     called WAAS (Wide Area Augmentation System).  This was designed for 
     aircraft approach positioning but will also cover many coastal waters 
     since many airports are located on the coast.  (This system will be 
     operational in the continental US only).  GPS and WAAS frequencies 
     will be very close together allowing the same antenna to be used for 
     both.  Most of the GPS engines being developed today have WAAS 
     capability built in and the on-board processors will be able to handle 
     the added calculations.
     
     Operational WAAS testing is supposed to begin next month but won't be 
     fully operational until fall of next year (in a perfect world).  You 
     should start seeing WAAS capable products sometime next year.
     
     JP
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