GSA - GPS DOP and active satellites

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 $--GSA,a,a,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x.x,x.x,x.x*hh<CR><LF>

 Field Number: 
  1) Selection mode
  2) Mode
  3) ID of 1st satellite used for fix
  4) ID of 2nd satellite used for fix
  ...
  14) ID of 12th satellite used for fix
  15) PDOP in meters
  16) HDOP in meters
  17) VDOP in meters
  18) checksum