The U.S. Air Force on Oct. 20 issued a request for proposals (RFP) for the next development phase in a competition to build a $3.5 billion network of S-band ground radars to track satellites and debris in low Earth orbit.

Three companies have been working under low-level study contracts to conceptualize the Air Force’s next-generation Space Fence since 2006. Lockheed Martin Corp., Raytheon Co. and Northrop Grumman Corp. in June 2009 were awarded $30 million risk-reduction contracts, and in February the service chose Lockheed Martin and Raytheon to continue design work, according to an Oct. 27 Air Force press release.

Under the upcoming phase, the Air Force will issue up to two 18-month contracts worth as much as $214 million combined to develop preliminary system designs, radar performance analysis and prototypes, and carry out other technical activities, the release said. After that work concludes in 2012, the Air Force will choose a single contractor to deploy the system.