'Satellite Imaging' is the process of taking photographs of Earth and other planets using man-made satellites. These satellite photos are collectively referred to as 'satellite imagery'.

The era of satellite imaging began on August 14, 1959 when Earth was first photographed by Explorer 6, a satellite owned by the US.  On the other hand, it is believed that the first satellite photos of the moon were taken on October 6, 1959 by Luna 3, a Soviet satellite. Countless other satellite photographs, some of which had created a sensation when they captured media attention, have been produced by satellites all over the world in the ensuing years. All satellite data owned by NASA are available for the public to see for free through the Earth Observatory.

Satellite images are described in terms of their resolution, of which there are two kinds: radiometric and geometric resolution.  Radiometric resolution describes the number of grayscale levels that a satellite sensor can produce, usually expressed in the number of bits used for the grayscale, i.e., 8-bit resolution means 256 grayscale levels (0-255), 11-bit means 2048 levels (0-2047), 12-bit means 4096 levels (0-4095), and 16-bit means 65,536 levels (0-65,535). The higher the bit count of the radiometric resolution, the better the image of the satellite will be.

On the other hand, geometric resolution describes the ability of a satellite sensor to capture image details on the ground. It is usually expressed in 'Ground Sample Distance' (GSD), which is defined as the smallest square area (whose side's length is the GSD number) on the ground that maps to a single pixel.  For example, a GSD of 1 meter means that a pixel is equivalent to an area on the ground that can be as small as 1 meter by 1 meter.  The lower the GSD, the more detailed the image of the satellite will be.

    

Figure 1.  'Google Earth' images such as this one are products of satellite imaging.

   

Satellite imaging has many applications, including the following:  mapping, border control, military and national security intelligence, warfare, infrastructure planning, monitoring of agricultural lands and the environment, biodiversity conservation, disaster prevention and response, training and education. 

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