An educated guess
I'm not familar with Beagle 2's innards. Viking and Pathfinder used radar altimeters and inertial measurement units. The Pioneer Venus Large Probe and Galileo Probes used sequence timers that were open loop to the trajectory. My guess is that Beagle 2 uses a timer because it has neither the room or energy resources for a radar. A big question that I have is: Does Beagle 2 have radioisotope heaters attached to its electronics? All American interplanetary spacecraft use radioisotope heaters to keep their electronic's warm (little slugs of Pu-238). If Beagle 2's electronics gets too cold then it won't wake up when it reaches the Martian atmosphere.
Gary