However, military analysts and human rights observers say the IDF is still using unguided, indirect fire with high-explosive shells, which they argue is inappropriate for a densely populated area like Gaza – where 1.8 million people live in an area the size of the Isle of Wight. The biggest artillery weapon being used is a 155mm howitzer, mounted on tracks to make it mobile. It typically fires a fragmentation shell weighing 44kg that spreads shrapnel over a wide area. Such shells have a lethal radius of 50 to 150 metres and causes injury up to 300 metres from its point of impact. Furthermore, such indirect-fire artillery (meaning it is fired out of direct sight of the target) has a margin of error of 200 to 300 metres.
The use of indirect artillery fire in residential areas is not forbidden under international humanitarian law, but its legality depends on a balance between potential military gain against risk of harm to non-combatants. On that basis, Donatella Rovera, Amnesty International's senior crisis response adviser, said it was hard to see a military justification for its use in Gaza. "The margin of error of an artillery shell is well known. If you are shooting into a civilian area, there is a high chance of hitting civilians and a low chance of hitting your military objective, unless your military objective is to terrorise and punish a population," she said.
The IDF is reported to have a "safety zone", the closest an artillery target can be to civilians areas, of 100 metres. That is significantly less than the casualty radius of 155mm artillery shells, and Human Rights Watch found in a 2007 report that the rule appeared to be ignored in Gaza.
Where howitzers are used to strike back at missiles or mortars launched from inside Gaza, the guns are aimed with the help of the Shilem radar system and a linked, computerised fire direction centre. The coordinates for any protected building – the UN school for example – are supposed to have been fed into that computer database to block fire in that direction automatically. The UN says it had provided the coordinates of all its buildings, but it is not clear whether that data was incorporated into IDF systems.