According to Raytheon's pamphlet on the Excalibur, the "CPS-guided projectile provides accurate, first round fire-for-effect capability to all current and future 155-mm howitzers."
The shell uses software to direct it within 10-metres of its intended target, even when fired from up to 40 kilometres away, apparently minimizing unintended casualties. Standard shells are usually accurate to within 50 metres.