RADAR SEES THE EARTH AND ANGELS
When some of the outgoing radar energy strikes the earth, part of it is reflected back and one sees echoes of the ground. This result is reasonable and easy to understand. On the other hand, radarscopes sometimes show echoes from regions in the sky which are absolutely clear. In the early days of radar these echoes were given such interesting names as "ghost echoes," "phantom echoes," and "angel echoes." The last one has caught on, and it is common to hear echoes which cannot be easily explained called angels. We now have some good ideas about the source of these mysterious echoes. Let us first examine the ground echoes and come back to the angels.
Almost all PPI pictures from ground-based radar sets show some echoes from the ground at very short ranges. They are often called "ground clutter" because they almost never serve a useful purpose. In