2012-04-23 at 19PM GMT+1, FLEXIPILOT (my own closed-source development) with special Terrain Follower firmware, controlling EasyUAV, has made its first long duration loiter at 1.5-3m AGL above sloped terrain. Loiter diameter was around 110m.
10m Flight took place in steady weather, while norhtern part of the field was 4-5m higher than southern part of the loop.
The autopilot exploited 3.5m of this range, maintaining reliably airplane altitude above uneven field borders. The barometric pressure, exhibiting usual long-term drift during 8min loiter, has confirmed aggressive altitude changes spanning 4m once autopilot flew into ground proximity zone.
Sensors used is a mixture of specific optical and ultrasound sensor, modified by me.
The effect of brutal pitch changes has destabilised flight speed, slightly 'smearing' the flight track.