Click here for high-resolution version of the image.
DSP-F21 is a satellite of the US Air Force’s Defense Support Program (DSP) which operates the Satellite Early Warning System. The TLEs for this satellite (USA-159) are distributed by SpaceTrack. The satellite was launched in 2001 and is still used for missile launch detection. The satellite emits a constant carrier at 2237.5 MHz which sometimes becomes active with data during Early Warning and LEO SIGINT operations.