Earlier in the month, China put three more Yaogan Weixing (Remote Sensing) satellites into LEO (low, under 600 kilometers, earth orbit). The three new satellites were called Yaogan 35 models and are apparently part of a large array of such satellites that China has been building since 2010. China describes its remote sensing satellites as civilian models controlled by the Ministry of Agriculture. That is a subterfuge dating back to the Cold War when Russia described all its Kosmos satellites are serving civilian, not military needs. The U.S. simply calls its military satellites, classified mission satellites and leaves it at that.
China is also putting up new Yaogan birds with better MSI (multispectral imaging) sensors as well as new generations of radar/optical sensors. There are a growing number of sensors available for satellites, including AESA SAR (synthetic aperture radar) as well as electro-optical and infrared (heat) sensors.
These sensors can provide real-time video as well as photos and images in several different forms. That’s because multi-spectral sensors can detect different types of vegetation or the composition of the surfaces below. The SAR provides photo-like 3-D images of small objects in any weather and at night.
Many of the new Yaogan type satellites have been sent up to monitor the Western Pacific and South China Sea, areas where the Ministry of Agriculture has little to monitor. China has also been building a multi-sensor system for obtaining constant data on the precise location of surface and submarine vessels in the South China Sea. Surface surveillance is carried out by a constellation of up to ten remote sensing satellites so that the South China Sea is under constant surveillance. Satellites at an altitude of 600 kilometers or less are equipped with SAR (synthetic aperture radar) and digital cameras. A typical SAR can produce photo quality images at different resolutions. At medium resolution (detecting items as small as three meters long) the radar covers an area 40x40 kilometers. Low resolution (20 meters) covers 100x100 kilometers. This takes care of surface ships, including diesel-electric subs when surfaced. Since 2010 China has been experimenting with such an array, initially using three satellites moving in formation over the western Pacific. China admitted that the first South China Sea satellite was launched in 2019 and all will be in orbit by 2025. The Chinese Navy already has more ships than the American fleet. Those satellites and the rest of the 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.
To make this surveillance system work China is using or adapting a lot of existing technologies. For underwater surveillance China is installing a network of underwater sensors similar to the American Cold War era American SOSUS (SOund Surveillance System). The original SOSUS consisted of several different networks. On the continental shelf areas bordering the North Atlantic was the CAESAR network. In the North Pacific there was COLOSSUS plus a few sensors in the Indian Ocean and a few other places that no one would talk about. The underwater passive sonars listened to everything and sent their data via cable to land stations. From there it was sent back to a central processing facility, often via satellite link. SOSUS was accurate enough to locate a submarine within a circle no wider than 100 kilometers. That's a large area, but depending on the quality of the contact, the circle might be reduced up to ten kilometers. The major drawback of the system was that it did not cover deep water areas more than 500 kilometers from the edge of the continental shelf. This is not a problem for the Chinese system, as most of the South China Sea is shallow and like coastal waters. South Korea began building a coastal SOSUS system after 2010 to deal with North Korea (and, by implication, Chinese) submarines off their coasts.
China also uses AUVs (Autonomous Undersea Vehicles) set loose to collect technical data on the water all the way from the surface to the sea bottom. This data collection is very useful in submarine operations. The AUVs are also called UUVs (unmanned underwater vehicles) and they have been getting cheaper, more capable, and more proliferating. These AUVs are silent, very small, and able to operate on their own for up to a year. The first models were two meters (six feet) long, weighed 59 kg (130 pounds) and operated completely on their own for up to a year collecting valuable information about underwater “weather”. What this AUV does is automatically move slowly (30-70 kilometers a day) underwater, collecting data on salinity and temperature and transmitting back via a satellite link every hour or so as the AUV briefly reaches the surface. This data improves the effectiveness of sonars used by friendly forces, making it easier to detect and track enemy submarines. That’s because the speed of sound travelling through water varies according to the temperature and salinity of the water. Having more precise data on salinity and temperature in a large body of water makes your underwater sensors (sonar, which detects sound to determine what is out there) more accurate. This data can also assist submarines in better avoiding detection. The first of these navy AUVs could dive as far down as 200 meters (620 feet) and later models were able to go down to 1,000 meters or more. Autonomous AUVs use a unique form of propulsion. They have wings, and a small pump that fills and empties a chamber. This changes its buoyancy, causing it to glide down, then back up. This maneuver moves the AUV forward. Equipped with GPS and a navigation and communications computer, the AUV is programmed (or instructed via the sat link) to monitor a particular area. The small pump uses less electricity than a propeller (to move it at the same speed).
The U.S. Navy currently has over 2,000 of these AUVs in service or on order and plans to keep increasing this robotic ASW fleet as long as they keep demonstrating they can do the job. UAVs replace many of the ocean survey ships long used for this kind of work. The survey ships take temperature and salinity readings from instruments deployed from the ship as well as a global network of several thousand research buoys. Unlike the survey ships the AUVs could be deployed in areas where hostile subs are believed to be operating, and be kept at it if needed. If successful in regular use, larger versions are planned, equipped with more sensors and longer duration.
China already has survey ships in service as well as a growing network of buoys. China will be deploying its autonomous AUVs throughout the Western Pacific and Indian Ocean, along with buoys. The AUVs and buoys will be serviced by the expanding Chinese fleet that is seen more frequently throughout the Pacific and Indian Oceans. China has already captured (or stolen, if classified) a lot of data similar to what American systems collected over the decades and is using that to enhance its own databases.
For the moment this new network concentrates on the South China Sea but will also include land-based data processing and fusion (combining data from different sensors) in one of the Paracel Islands (in the South China Sea), another in South China (Guangdong province) and third somewhere in South Asia where it will share data with other nations. The data fusion centers will also include material collected by warships and commercial ships and aircraft. China also plans to make some of this data available to everyone as a contribution to safe operation of maritime commerce and fishing.
Putting the Yaogan 35 satellites into orbit broke a record as it was the 43rd SLV (Satellite Launch Vehicle) used this year, which was the largest in any previous year and there are still two months left in 2021. China plans to introduce an SLV like the American reusable SpaceX SLVs and once that becomes a regular practice China hopes to dominate the world market for SLV use.