NROL-4

Trumpet signal intelligence satellites were launched by Titan-4(01)A Centaur-T into highly elliptic Molniya-type orbits. In their elliptical orbits they would move very slowly over the northern hemisphere for most of their orbital period, allowing interception of microwave line-of-sight communications beams. Since they would move slowly through the beams during their orbit, a constellation of such satellites was required to monitor Soviet communications throughout the day.