Carlson Field can use an external Light Bar for determining
elevation differences and centerline offsets. Light Bars can
indicate whether your current position is in cut, fill or on-grade
when set vertically. When set horizontally, Light Bars can give
centerline left/right offsets. Currently Carlson Field supports a
light bar made by Mikrofyn named RD-4 1137551, as well as by
Apache, that has arrows for up/down, or left/right, and a row of
lights for on-grade. The Light Bar must be connected to a separate
serial port than the GPS.