Northrop Grumman Corp. won a six-year U.S. Air Force contract valued at up to $1.2 billion to upgrade ground surveillance radar known as Jstars. The $1.2 billion isn't guaranteed. The contract allows the Air Force to award work to Northrop for planned upgrades over the next six years without having to draft a separate contract each time the Air Force gets funding approved. Jstars, short for Joint Surveillance Target Attack Radar System, can track the movement of armored and wheeled vehicles and helicopters and transmit three-dimensional terrain images to ground and naval units for attack. Two Jstars-equipped planes are being used by the NATO mission in Kosovo. Los Angeles-based Northrop said the first firm project under the contract, valued at $40 million, is to develop an integrated satellite communications system for Jstars. Northrop shares fell 81 cents to close at $58 on the NYSE.