Artillery weapons are designed to launch projectiles at distant targets, using a high-arcing trajectory to shoot over intervening obstacles. Early forms of artillery use stored energy in leather ropes or gravity to launch projectiles long distances. Artillery became a major force on the battlefield once gunpowder was invented leading to the creation of cannons and rockets substantially increasing their effectivness. Modern artillery weapons can be cannons or rocket-propelled warheads. Artillery weapons are typically deployed in substantial numbers, allowing them to saturate large areas of a battlefield with explosive ordinance. Proper use of artillery requires a forward observer of some kind to locate targets and relay information about their position and movements to the weapon operators.