Satellite-based sensors can see much larger areas of the Earth than sensors closer to the Earth can see. This allows large-scale simultaneous observation of the Earth's surface and atmosphere, and communication between and simultaneous broadcast to large parts of the Earth. Because the atmosphere blocks transmission of many types of electromagnetic waves, some kinds of astronomical observations can only be made from space. Moreover, space is much better suited to some types of operations than to others. Electromagnetic signals (light and radio waves) can be transmitted over large distances almost instantaneously and with very little energy cost. Space therefore favors activities that entail sending and receiving electromagnetic signals over activities that involve transporting large amounts of mass from the Earth into space or that involve significant maneuvering in space, which can require a large mass of propellant. As a result, the applications for which space basing is uniquely well suited include:

• large-scale environmental monitoring of, for example, atmospheric
  behavior, climate change, and deforestation


• large-scale weather monitoring for weather forecasting astronomy


• global communication, broadcast, and data transfer


• highly accurate navigation and position determination reconnaissance on a global or large-scale basis


• detection on a global basis of missile launches, to provide early warning of attacks and information about the missile testing programs of nations

Some of these tasks could, as discussed later, be accomplished by ground- and air-based alternatives, if urgently needed, albeit on a regional rather than global basis.

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Wright, David, Laura Grego et al. Physics of Space Security. Washington, D.C.: Union of Concerned Scientists, May 2005. [ 5 quotes ] [ page 6-7 ]