Complex sentence: Lockheed had intended to use a derivative of the F-35's APG-81 radar but is now believed to be using an Elta system; both it and the Thales system will begin flight trials in the summer of 2014 with Main Gate in 2016.
Split into simpler sentences: Lockheed had intended to use a derivative of the F-35's APG-81 radar but is now believed to be using an Elta system. Both systems will begin flight trials in the summer of 2014 ahead of Main Gate in 2016.