In the last few years China and several Western firms put a new generation of satellites equipped with SAR (synthetic aperture radar) into orbit. These SAR systems are powerful and inexpensive enough for a wide variety of commercial customers to use. Typically, a commercial firm wants SAR images over a specific area for a specific time. Whenever you see a monochrome photo in the media showing what appears to be a 3D view of the ground, it is usually from an SAR satellite. Being a radar, SAR sees through clouds and other atmospheric obstructions that plague photo-satellites. SAR can also penetrate trees and vegetation if the user wants that.
Since the 1980s SAR radars used by aircraft and UAVS have been developed a lot more capabilities, including greater resolution, smaller size and weight and lower cost. All SAR radars are now digital and the smallest ones are often found in the warheads of missiles where they identify and home in on targets they are programmed to find and destroy. SAR radars have also been reducing their electricity needs, which is a major item for SARs used in space satellites where sustained electric power is from solar panels.
Since 1978 satellites equipped with SAR (synthetic aperture radar) have been in orbit. Many of the early ones were for ocean surveillance and for mapping remote land areas. These were analog and limited in capabilities and resolution. Since the 1990s a new generation of less expensive digital image SAR space satellites have emerged. A typical SAR can produce photo quality images at different resolutions. At medium resolution (3 meters) the radar covers an area 40x40 kilometers. Low resolution (20 meters) covers 100x100 kilometers. SAR satellites operate in a LEO (Low Earth Orbit of 600-800 kilometers)
SAR satellites are often launched in groups (or arrays) of three and they travel together to quickly take 3D photos of what is below them. With enough SAR arrays available you can put an area under persistent coverage and use the sequence of SAR images to detect movement of surface ships, mainly commercial vessels tracked for safety purposes. Over the Western Pacific several nations use their SAR satellites to detect and track warships, including diesel-electric subs when surfaced. Since 2010 China has been experimenting with such an array, using three satellites moving in formation over the western Pacific. China put the their first South China Sea satellite up in 2019 and all will be in orbit by 2025. By 2030 the Chinese Navy will have 30 percent more ships than the American fleet. Those satellites, including non-SAR reconnaissance satellites with other sensors will add to that numerical superiority and make the South China Sea a very dangerous place for anyone the Chinese do not want there.
Meanwhile the growing number of commercial SAR satellites means the public will also be able to see what the Chinese and other naval powers (U.S., South Korea and Japan) in the Pacific can see with their military grade SAR satellites.