I would assume that it keeps the last known offset between the radar altimeter and the pressure altimeter. It may have also pulled up based on the pressure altimeter, as obviously the plane won't be flying below sea level regardless. But the latter is less clear. It appears to pull up before the "sea level" altitude gets too low, but the minimal reading may look less severe because the pilot increased the pullout to 9G.
It might also mix in data from GPS and topo maps. It's also possible they have some internal dead reckoning system to perform holdover while the craft is doing aggressive maneuvers. There are a lot of ways to skin that cat.
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I would assume that it keeps the last known offset between the radar altimeter and the pressure altimeter. It may have also pulled up based on the pressure altimeter, as obviously the plane won't be flying below sea level regardless. But the latter is less clear. It appears to pull up before the "sea level" altitude gets too low, but the minimal reading may look less severe because the pilot increased the pullout to 9G.
It might also mix in data from GPS and topo maps. It's also possible they have some internal dead reckoning system to perform holdover while the craft is doing aggressive maneuvers. There are a lot of ways to skin that cat.
I guess having a topographical map helps too.