An Air Force program to build unmanned surveillance aircraft, made by Los Angeles-based Northrop Grumman Corp. and called Global Hawks, has increased in cost by $900 million since 2001, the Washington Post reported, citing a report by the Government Accountability Office.

The aircraft can see through clouds or sandstorms and transmit data in real-time to command centers, the newspaper said. The Defense Department thinks the aircraft may “transform military operations,” the report said, according to the Post.

The cost of the overall Global Hawk program increased to $6.3 billion from $5.4 billion in 2001, while the number of aircraft the Air Force plans to buy dropped to 51 from 63, the report said, according to the Post.

Most of the increase in cost is related to development of a second, heavier version of the aircraft, and the GAO recommended delaying buying more aircraft, the newspaper reported. An unidentified Northrop spokesman declined to comment and the Pentagon’s response, included in the report, said a delay might cost another $400 million, the Post reported.