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   GPS receiver model. Models the GPS constellation.

   Failure mode is zero output.

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   Form:
   y = GPSReceiver( action, d )
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   Inputs
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   action      (1,:)     'initialize', 'update', 'output', 'set failure' 
   d           (1,1)     Depends on the action

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   Outputs
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   y           (1,1)     Depends on the action

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