The U.S. Air Force is now convinced that air burst JDAM smart bombs are often much more effective than those that detonate on contact with the ground. This was determined after closely examining the use of such bombs in Iraq and Afghanistan. The air burst is achieved using the DSU-33B/B Proximity Sensor, which is a fuze that uses radio waves to sense how high the bomb is, and detonates the bomb when it is 20 feet above the ground (an "air burst"). This maximizes the blast and fragmentation effect of a high explosive bomb. This is more effective against troops and vehicles out in the open. If a normal contact fuze is used, the bomb explodes when it hits the ground, digging a crater, but putting a lot of the blast and fragments into the ground. The air force has just bought another 7,000 of these fuzes, for $1,130 each. The U.S. Navy and Australia have also bought this fuze.