I dont know what the authothrottles use except that they add 5 knots at all times. The unit flys +5 based on either gear or flap extension, I can't remember which. It uses radio altitiude for flare mode and unlike other autothrottle systems goes to idle at 50ft rad alt.
Most 80's don't have IRS or IRU systems to provide ground speed, the +5 as I understand it is just built into the system.