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.
NASAs 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 Earths 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.