This is a guidance system that uses fins as control surfaces. It is similar to the guidance kit that Cabatlı posted above. Although this type o kits offer good accuracy, they are expensive and can't be used in mass. The kit that I posted uses an air brake mechanism and a simpler IMU rather than moving fins and GPS/INS to guide the shells. It is very cheap to produce. It can be produced from the existing production lines without much investment, you can produce tens of thousands of such kits without bankrupting yourself.Weird, Denel claimed 10 meter accuracy for their kit but this one is much higher.
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In a battle, ammo is used like tap water by the artillery batteries. You need to use guided shells for maximum effectiveness but you also need to use huge amounts of them. This is a good sweet spot of cost-effectiveness. Of course, ideally, you should have GPS/INS and laser-guided shells for high-value stationery (for GPS/INS) and moving targets(for SAL) that should be used in small numbers to complement simple cheaper guidance kits when the battlefield and mission demands.