You can get pretty close......but remember, a mortar is an area fire weapon. When I was a mortar platoon leader the aim was to get with 25 meters of the target because it was assumed you would hit the target with shrapnel. You also have factor in air temperature and density, comparative elevation of gun, target and observer and that you are firing a fin stabilized round from a smooth bore weapon. I had only one time round on a hard target (and I mean fireball flash from hitting right on top of an old APC. But then, I am not expert......but I did stay at a Holiday Inn Express last weekend!

Mortars have a beaten zone like an MG. Our army employs mortars in sections of two with three or four sections making the battalion mortar platoon, we have no company mortars. A section has an area of fire (25m x 50m IIRC) with a half platoon area of fire a little bigger. Then finally the platoon area of fire is reasonably large.

GPS is not 'extremely' accurate, I think the current mil version has a CEP of about 4 metres. That means the combined CEP is just under 6 metres. Not having a mortar firing table I can't give you inherent PEs for a mortar, However, mortars are short range so the numbers are small. As an indicator, a 155mm gun with modern ammo(something neither US nor Aust have) the PErange at 5km is about 5 metres (PEline will be a fraction of that). Suffice to say they vary with range and charge. The last issue is corrections for non standard conditions (temp, wind, etc). Traditionally mortars haven't worried too much about these, however, mortars are increasingly being integrated into the arty system and met data automatically distributed to them (and in some armies mortars are using the same ballistic computers as arty) so this source of error (which is usually fairly small for mortars, although larger than for guns of similar calibre at the same range) will be removed. To answer your real question, if you know exactly where the mortar muzzle is (almost the same as baseplate centre) and exactly where the target is (I'm talking mm here) and the ballistic computations are precise and compensating for non standard conditions, then 50% your bombs will fall within a few metres of the target point and be skewed to it(ie not evenly distributed in the area). There's a very good chance you'll get a direct hit with a few bombs. However, there is one small snag, the mortar sights. These are graduated in mils and at 5km 1 mil subtends 5 metres laterally and perhaps averages about 2 metres in range. So if the relationship between muzzle and target point doesn't compute at a whole mil you have an inherent problem.