Which of the following is a common source of radar clutter on ATC displays?

• A. Aircraft transponder interference
• B. Solar flares
• C. Ground clutter
• D. Cosmic rays

Answer: (C)

Ground clutter is radar echoes that come from reflections off the ground, terrain, buildings, and other near-surface objects. On ATC displays these returns appear as persistent, low-elevation echoes that aren’t aircraft, creating a background “blotchiness” that can obscure real targets. Because the ground and man-made surfaces don’t move, these echoes tend to stay in roughly the same place scan after scan, making clutter a common, ongoing issue. Controllers and radar systems mitigate this with clutter suppression and processing, such as moving-target indication and Doppler filtering, to distinguish actual aircraft from steady ground returns.

Aircraft transponder interference would show as secondary SSR blips from replies, not continuous ground-like clutter. Solar activity can affect radio propagation or equipment, but it isn’t a typical source of the steady, surface-based clutter seen on primary radar displays. Cosmic rays produce random, isolated disturbances rather than consistent ground echoes.