Visible light only forms a small slice of the electromagnetic spectrum centred at yellow – we have evolved to see things the way we do due to the solar peak radiation being somewhere near yellow.
Radar technology came to the forefront during World War II as an early warning for aircraft which were beyond visible range or hidden in clouds. Since then we have moved to using radars in a similar way we use flash camera, we can perform photography at microwaves.
Imaging radars typically operate in a range of frequency range of about 300MHz to 30GHz. The wavelength of operation determines the size of the structures the waves resonate with; L-band or P-band radar does not resonate with leaf scale structures in a forest and lets us see the trees through the forest, and X-band or K-band radars will see surface-detail.
Current Spaceborne Radar Constellations
The recent trend has been to launch satellites in constellations , two commercial X-band constellations are currently in operation : Cosmo-Skymed and TerraSAR/Tandem.
The Cosmo-Skymed constellation features 4 satellites, 3 of which are deployed with 120 degree orbital offset to provide high temporal density and the 4th is placed in tandem with the first to allow low temporal decorrelation interferometry.
The TerraSAR-X and Tandem-X constellation features one of the most ambitious orbits with the satellites flying in very close formation which reminds us of the DNA double helix. With this mission DLR and Infoterra aim to produce the next logical step to the global SRTM mission. There are a lot of future radar constellations proposals on the table, including the RadarSAT constellation, TerraSAR-L and Australia’s own attempt – Garada.
Radars are vital in our knowledge of the Earth’s terrain and biosphere, it also lets us do a survey of urbanisation and proliferation of man made structures. There is so much left unexplored on land, radars do the cloud bursting and let us see through the moisture that clogs up the troposphere to visible light photography.