U.S. District Judge Reggie Walton has been appointed presiding judge of a secretive special court that oversees warrants for government surveillance in spy and terrorism cases.

Chief Justice John Roberts appointed Walton to head the Foreign Intelligence Surveillance Court effective Feb. 22, replacing Judge John D. Bates, whose term is expiring.

The court meets in secret at the federal courthouse in Washington to hear classified evidence from government attorneys. No defense attorneys are present. At least one of its 11 judges is on call to issue warrants any time of the day or night.

Walton has served on the surveillance court since 2007. He also has presided over a number of high-profile federal court cases, including last year's Roger Clemens perjury trial.