When the supplemental sensors detect an item of interest, the system control computer begins an active search for more information from other on-board and ground-based sensors.

The ground-based sensors work like sensitive ears to detect very low-frequency sound waves, which radiate outward around the globe over hundreds and thousands of miles.

In addition, SDI envisioned many space-based systems in fixed orbits, ground-based sensors, command, control and communications facilities, etc.

Not enough to be picked up by a radiation-detecting aircraft or satellite, but plenty to be picked up by ground-based sensors.

Certainly any ground-based sensors wouldn't notice him tucking a few more pieces' amid the clutter that spun in disarray round the world.

Its ground-based sensors are even more sensitive than orbiting satellites to the repercussions of meteor blasts.

Data gathered from the Hubble Space Telescope and ground-based sensors were combined to form a 3D volume visualization of the nebula.

And when the satellite technology has been augmented by ground-based sensors, it has helped measure millimeter-sized movements along earthquake fault lines.

Currently, most lightning data provided in real-time is from terrestrial sources, specifically, networks of ground-based sensors, although airborne sensors are also in operation.

The data received at NORAD from ground-based sensors are too sketchy for a detailed accounting of human activity in the heavens.