What is this all about?
Since 1972, civilian scientific and later, commercial remote sensing satellites have been launched, to orbit Earth and collect, electronically, imagery over almost the entire area of our globe.
The first such satellite was Landsat 1, designed, built, launched and operated by NASA.
The capabilities were limited with a ground resolution of some 80m', but, for the first time, people had the opportunity to observe and study any area of planet Earth, free of any kind of restriction. Furthermore, the imagery was received on board the satellite in 4 different wavelengths (RGB and Near IR), a feature named 'multispectral' or MS for short, enabling extraction of information beyond that possible from a black and white (called 'Panchromatic' or 'Pan' for short) or RGB color image. Landsat 2 and 3 were similar however, in 1982 and 1983, Landsat 4 & 5 were launced with much improved capabilities - 30m in 6 spectral bands (RGB, Near IR and Mid IR) and 120m in the first-time thermal wavelength band (which measures surface temperatures).
Untill 1985, the Landsat satellites were scientific and were managed by NASA.
In 1985, Landsat 4 &5 were privatized by the EOSAT company and became the first operating commercial satellites.
The next major step was made by the French Space Agency and the SPOT Image company, with the SPOT 1 satellite, launched in February 1986, which had 10m resolution in the Pan band, had stereoscopic capability and also had a multispectral sensor with 4 bands (RGB and NIR).
From here, the world of 'open to all' remote sensing developed rapidly, overcoming regulatory restrictions (mainly regarding spatial resolution) and today there are several tens of satellites, with spatial resolutions down to 30 cm!! and spectral resolutions of up to tens or even hundreds of narrow bands (called 'hyperspectral') .
Remote sensing satellites usually have a polar orbit (orbiting Earth from pole to pole ) and thus , since Earth is rotating round the pole-to-pole axis, the satellites will cover a different, near-longitudinal, strip of the earth with each successive orbit. Thus, each such satellite is able to collect imagery over almost the entire globe, repetitively.
In principle, the imagery is collected on an electronic device which converts the radiation, reflected (which may be visible light, IR or radar or other wavelenghts) or emmited by the surface of the Earth (e.g heat) into proportional electrical currents, which may then be transmitted to a ground station on Earth where the imagery is received and is converted to digital data sets, readable and interperable by imagery processing software.The imagery is transmitted together with ancillary data regarding the optical system, the precise location and attitude of the satellite in space during the image collection, thus enabling various mathematical operations/manipulations, converting the imagery to fit any map projection.
The digital character of the imagery enables processing for a multitude of applications, by use of image processing software - computer programs which enable operations on single bands, or multiple bands.
Geographic Information Systems (GIS) enable encorpoprating digital imagery as one of many information layers which may all be converted to the same cartographic projection and scale.