TacSat-3 "Hyperspectral" Spy Satellite
The idea of hyperspectral sensing is not, however, merely to “see” in the usual sense of optical telescopes, infrared nightscopes and/or thermal imagers. This kind of detection is used on spy satellites and other surveillance systems, but it suffers from the so-called “drinking straw effect” — that is, you can only view a small area in enough detail to pick out information of interest. It’s impossible to cover an entire nation or region in any length of time by such means; you have to know where to look in advance.
Hyperspectral imaging works differently. It’s based on the same principle as the spectrometry used in astronomy and other scientific fields – that some classes of objects and substances will emit a unique set of wavelengths when stimulated by energy. In this case, everything on the surface below the satellite is being stimulated by sunlight to emit its unique spectral fingerprint.
By scanning across a wide spectrum all at once across a wide area, it’s then possible to use a powerful computer to crunch through all wavelengths coming from all points on the surface below (the so-called “hyperspectral cube”, made up of the full spectrum coming from all points on a two-dimensional surface).
If the sensor is good enough and the computer crunching powerful and discriminating enough, the satellite can then identify a set of points on the surface where substances or objects of interest are to be found, and supply map coordinates for these. This is a tiny amount of data compared to the original “hyperspectral cube” generated by ARTEMIS and crunched by the satellite’s onboard processors, and as such it can be downloaded to a portable ground terminal (rather than a one with a big high-bandwidth dish). Within ten minutes of the TacSat passing overhead, laptop-sized ROVER ground terminals can be marking points of interest on a map for combat troops nearby.