How will the computer know where the ground is if the metal box is blocking the radar?
The same way they claim to have landed every other lander using retrorockets.
Hold on, let me count those...
Mars 3 - Through aerodynamic braking, parachutes, and retrorockets, the lander achieved a soft landing at 45°S 158°W and began operations. However, after 20 seconds the instruments stopped working for unknown reasons.
Mars 6 - The descent module entered the atmosphere at 09:05:53 UT at a speed of 5.6 km/s. The parachute opened at 09:08:32 UT after the module had slowed its speed to 600 m/s by aerobraking. During this time the craft was collecting data and transmitting it directly to the bus for immediate relay to Earth. Contact with the descent module was lost at 09:11:05 UT in "direct proximity to the surface", probably either when the retrorockets fired or when it hit the surface at an estimated 61 m/s. Mars 6 landed at 23.90°S 19.42°W in the Margaritifer Terra region of Mars. The landed mass was 635 kg. The descent module transmitted 224 seconds of data before transmissions ceased
That's two that used retrorockets to land.
However, none of them have ever landed softly. Their weights were also less.
That's the best argument you have; NASA is to incompetent to properly place a radar altimeter?
Yeah... the only place you could put it that wouldn't have it ripped apart by the 200mph wind force is the bottom of the box. And tell me, what happens when you land a box with a radar panel on the bottom onto a surface with a lot of rocks?
As for the Dust, it's not about the lenses so much (though I'd love to see you design dust resistant covers that can retract mechanically) but also the amount of dust that could get into all the various doors and crevices that require a sterile environment, such as the whole internal lab. High velocity dust is bad. And when you blast dust with a rocket, the dust particles fly pretty fast.
But here's a lot of other reasons:
http://en.wikipedia.org/wiki/Mars_Science_LaboratoryFor several reasons, a different landing system was chosen for MSL compared to previous Mars landers and rovers. Curiosity was considered too heavy to use the airbag landing system as used on the Mars Pathfinder and Mars Exploration. A legged lander approach would have caused several design problems.[64] It would have needed to have engines high enough above the ground when landing not to form a dust cloud that could damage the rover's instruments. This would have required long landing legs that would need to have significant width to keep the center of gravity low. A legged lander would have also required ramps so the rover could drive down to the surface, which would have incurred extra risk to the mission on the chance rocks or tilt would prevent Curiosity from being able to drive off the lander successfully. Faced with these challenges, the MSL engineers came up with a novel alternative solution: the sky crane.[64] The sky crane system lowered the rover with a 7.6 m (25 ft)[64] tether to a soft landing—wheels down—on the surface of Mars.[57][71][72] This system consists of three bridles lowering the rover and an umbilical cable carrying electrical signals between the descent stage and rover. As the support and data cables unreeled, the rover's six motorized wheels snapped into position. At roughly 7.5 m (25 ft) below the descent stage the sky crane system slowed to a halt and the rover touched down. After the rover touched down, it waited 2 seconds to confirm that it was on solid ground by detecting the weight on the wheels and fired several pyros (small explosive devices) activating cable cutters on the bridle and umbilical cords to free itself from the descent stage. The descent stage flew away to a crash landing 650 m (2,100 ft) away.[73] The sky crane powered descent landing system had never been used in missions before.[74]
http://spaceflightnow.com/mars/msl/120731skycrane/Basically:
Rocks cause problems with ramps.