Ocean Satellites: Protecting the Blue Homeland

Ocean satellites are also, strictly speaking, earth observation satellites. However, the ocean and land have different forms, and different sensors are required for satellite remote sensing.

Ocean-1 Ocean Color Satellite

If you observe the ocean images taken by satellites, you will find that most of the time, the sea is quite monotonous, with patches of azure blue. However, a detailed analysis will reveal that the blue in different sea areas is different, and different types of information are hidden behind different blues. Generally speaking, the color of the sea is determined by three factors: chlorophyll in plankton, inorganic suspended matter, and organic yellow matter. These three factors determine the optical properties of seawater, including absorption and scattering properties. Seawater reflects sunlight, which is a beautiful sight for people on vacation, but it is a headache for marine researchers. In order to minimize the impact of reflected light, observations of water color should be made from directly above the sea surface.

Traditionally, ocean researchers throw a disc with 24 colors into the sea water, and record the color that matches the local sea water. This method sounds good, but it is an impossible task to measure the global ocean color. Using satellites to measure water color is currently the most efficient way.

The first ocean satellite launched by China, Ocean No. 1, is a water color satellite. It includes four satellites, A, B, C, and D, which have been launched into space one after another. Their main payload is a water color remote sensor, which has a very narrow operating frequency band, mainly concentrated in the visible light to near-infrared band; the spatial resolution is very low, on the order of hundreds of meters. Like ordinary remote sensing satellites, the Ocean No. 1 satellite operates in a sun-synchronous orbit. However, it chooses a different time point, around 12 noon, so as to ensure that the ocean can be observed from directly above. The detection of ocean water color can let people understand the productivity of the ocean. Simply put, it is how many fish, shrimps and sea vegetables can be bred in an ocean, and how much humans can catch or cultivate without destroying the ecological balance. This is crucial for us who love food.

3D simulation of Ocean No.1 satellite

Ocean 2 Dynamic Environmental Satellite

The sea has endless blue waves, and it looks flat when there are no winds or waves. However, this is not the truth. Due to the influence of factors such as wind, temperature, and underwater topography, the sea surface is actually constantly fluctuating. The sea water is constantly moving. There are ocean circulations on a scale of thousands of kilometers, vortices on a scale of hundreds of kilometers, and sea breezes, waves, and currents on a scale of tens of kilometers, thousands of kilometers, or even smaller. These are all classified as ocean dynamic phenomena. Systematically mastering the laws of these ocean dynamic processes can serve the fields of marine disaster prevention and mitigation, marine resource development, marine scientific research, and national defense construction. To detect ocean dynamic environmental phenomena, it is necessary to observe sea breezes, waves, sea surface heights, and sea surface temperatures, which are prerequisites for effective understanding of the ocean. Therefore, people need to develop another type of ocean satellite-an ocean dynamic environment satellite.

Ocean dynamic environment satellites are generally equipped with radar altimeters, microwave scatterometers, and microwave radiometers, which can measure altitude, observe waves, listen to wind, and measure temperature. China has not only developed its own ocean dynamic environment satellites, but also cooperated with France to develop the Sino-French ocean satellite.

China's ocean dynamic environment satellite is the Haiyang-2 series, and four have been launched so far. The first satellite has been retired, and the three satellites Haiyang-2B, C, and D are currently in orbit. Haiyang-2 is mainly used to detect sea surface wind fields, temperature fields, sea surface heights, wave fields, flow fields, etc., to obtain global ocean wind vector fields and surface wind stress data, as well as global ocean circulation, ocean geoid, gravity field and polar ice sheet data. Among them, the determination of the ocean gravity field plays an important role in the detection of seabed topography, and is also one of the major projects of the National Natural Science Foundation. Under the sea, there are also complex terrains, including mountains, plains and valleys. The detection of underwater terrain not only has important military value, but also has important reference value for the future development of marine resources.

The Sino-French Ocean Satellite (CFOSAT) was launched in 2018. It uses a microwave scatterometer developed by China and an ocean spectrometer developed by France, and for the first time achieved large-scale, high-precision synchronous observation of ocean surface wind and waves worldwide. It is particularly important to emphasize that the microwave radiometer on this satellite is currently the satellite-borne microwave scatterometer with the highest spatial resolution, the most observation angle coverage, and the highest wind field measurement accuracy. The relevant data has entered the French Meteorological Agency's operational forecast system. This is the first time that China's ocean payload data has entered international operational applications.

Demonstration image of the Sino-French ocean satellite in orbit (Photo credit: French National Center for Space Studies)

Ocean-3 surveillance satellite

Of course, high-resolution images are also needed in ocean research. At this time, what we need is not necessarily a high-resolution camera, but a radar. One advantage of the sea surface is that the signal reflection characteristics of the entire seawater in each sea area are the same. If there is any foreign object, such as a ship, a garbage dump or even a group of marine life, it will be clearly depicted in the radar echo. In addition, the wake of the ship will also be very obvious in the radar image. This is very helpful for tracking the movement of ships. Even the traces of underwater submarines will leave a series of ripples on the sea surface.

Therefore, people developed a third type of ocean satellite, which is ocean surveillance satellite. Their main payload is synthetic aperture radar with adjustable resolution.

China's ocean surveillance and monitoring series of satellites are divided into two stages of development.

In the first stage, the GF-3 satellite broke through the technical foundation and mastered the multi-polarization and multi-mode synthetic aperture radar (SAR) technology. The highest resolution of the satellite reached 1 meter, and the observation width was only 10 kilometers; the lowest resolution could reach 500 meters, and the observation width reached 650 kilometers. Such characteristics are very suitable for ocean observation, usually roughly monitoring a large range of sea areas; in special cases, the radar transmission power is concentrated to observe a small area.

The second stage is to launch two C-band synthetic aperture radar satellites to form a three-star network. The third satellite, launched in April 2022, can achieve a maximum resolution of 1 meter. The "Ocean No. 3" series of satellites inherits the technology of Gaofen-3, and through the co-orbital distribution of multiple satellites, it can monitor islands, coastal areas, and offshore targets all day and all weather, and obtain information on ocean wave fields, wind fields, storm surges, internal waves, sea ice, oil spills, etc., to meet the needs of marine target monitoring, land resource monitoring, etc.

In fact, other types of remote sensing satellites can also play a certain role in ocean remote sensing, including visible light imaging satellites and radar imaging satellites, but they can only play a supporting role. (Author: Chi Huo)