---------------------------------------------------------------------------
   GPS receiver model with navigation message model.
   Output is compatible with GPSOrbit. 

   Failure mode is zero output.
---------------------------------------------------------------------------
   Form:
   y = GPSReceiverNav( action, d )
---------------------------------------------------------------------------

   -------
   Inputs
   -------
   action      (1,:)     'initialize' 'update' 'output' 'set failure' 
   d           (1,1)     Depends on the action

   -------
   Outputs
   -------
   y           (1,1)     Depends on the action

---------------------------------------------------------------------------