Active imaging is necessary for measuring things that do not naturally give off or reflect a specific type of light or energy. This approach to imaging involves generating and emitting an energy signal that will reflect off a surface. The reflected signals are collected and analyzed to produce images that show information about the reflection surface.

NASA’s Shuttle Radar Topography Mission (SRTM) actively emitted radar pulses to Earth and recorded the reflected signals. By measuring the time the signals took to make the round trip from the shuttle to Earth’s surface and back again, scientists could calculate the distance the signals had traveled. The shuttle mission used this active imaging technique to make detailed measurements of elevation for most of our planet.