Scene Context: After the heat shield is jettisoned, the module "falls blind" for a few seconds until the radar and lidar begin providing actionable data through the dust and turbulent haze.
Essence of the Phenomenon: As long as the heat shield is attached, the downward-facing sensors cannot function nominally relative to the surface. After it is dropped, the lander shifts from an aerodynamic survival mode to a local navigation mode, where its own altimeters, radar, and lidar become mission-critical.
Scientific Basis: In Martian lander concepts, radar and lidar antennas are indeed integrated into the lower section of the capsule or descent stage and are activated after the transition from the hypersonic phase to controlled descent. It is at this stage that the vehicle no longer relies on the plasma-noised external channel as its primary source of local geometry.