What are the flight orbits of the Fengyun satellites that appear in the weather forecast every day?

We are used to checking the weather before going out. But do you know how we can make timely forecasts in the face of unpredictable weather changes? This has to do with meteorological satellites, such as the well-known "Fengyun series" meteorological satellites.

September arrives at the end of summer and the beginning of autumn, and the weather gradually becomes cooler. Coincidentally, the Fengyun satellite was first launched in September.

On September 7, 1988, the Fengyun-1A satellite (FY-1A) was launched; on September 3, 1990, the Fengyun-1B satellite (FY-1B) was launched. Today, the Fengyun satellites have grown into a series.

What are the members of China's Fengyun satellite family? What are their flight orbits? Why were they chosen?

Fengyun satellite cloud image (Source: China National Meteorological Administration)

The four major families in Fengyun satellite

The satellites have realized the integrated detection and application of my country's meteorological, oceanic and space environments for the first time, creating a new era for my country's aerospace industry. Fengyun satellites have names such as "No. 1", "No. 2" and "No. 3", but each name does not refer to a single satellite.

For example, in the Fengyun-1A satellite (FY-1A) and Fengyun-1B satellite (FY-1B), "FY" is the abbreviation of "Fengyun", "1" stands for "No. 1", and A and B are their further codes.

Fengyun-1 is my country's first generation of "sun-synchronous orbit meteorological satellite", mainly used to obtain global meteorological cloud images and ocean data. At the same time, it can also obtain day and night visible light, infrared cloud images, ice and snow cover, vegetation, ocean water color, sea surface temperature, etc., providing the necessary basic data for medium- and long-term weather forecasts, meteorological and oceanographic scientific research, and climate prediction, and also plays a huge role in disaster monitoring and environmental remote sensing.

As early as 1977, the overall preliminary plan for the Fengyun-1 meteorological satellite was formed, and then the research and development work was carried out in an all-round way. In October 1985, the Fengyun-1 meteorological satellite project was included in the key project of satellite model development during the "Seventh Five-Year Plan" period, and Fengyun-1A was successfully launched in 1988.

The Fengyun-1 satellite series includes a total of 4 satellites. About 10 years after the launch of satellites A and B, satellites C and D were launched in 1999 and 2002 respectively. Currently, all 4 satellites have failed and retired. It is worth mentioning that the first launched satellite A was in service for only 39 days, but satellite D served as a "weather forecaster" for 10 years.

Fengyun-1 satellite (Photo source: National Satellite Meteorological Center)

Fengyun-2 is also not just a satellite, but consists of two experimental satellites (FY-2A and FY-2B) and a series of business satellites (FY-2C, FY-2D, FY-2E, FY-2F, FY-2G, FY-2H, etc.).

The Fengyun-2 satellite series can obtain daytime visible light cloud images, day and night infrared cloud images and water vapor distribution maps, broadcast weather maps by fax, and collect meteorological monitoring data from meteorological, hydrological and oceanographic data collection platforms. This series is a "geosynchronous orbit satellite".

Fengyun-2 satellite (Photo source: National Satellite Meteorological Center)

The frequent iterations of FY-2 ensured the continuous operation of the satellite-ground system 24 hours a day, 365 days a year. After FY-2D, a new FY-2 satellite was launched almost every 2 or 4 years, and there was a 2H satellite launched in 2018, which almost spanned the entire teenage years of our young people. No wonder it is often seen on weather forecast programs.

The design and functions of these Fengyun-2 series satellites are very similar. The previous satellite is still working, and the next one has already been launched into space to start observation. It is precisely this kind of "backup" that has ensured seamless connection over the past decade.

Sometimes you will hear "FY-3" in the weather forecast. FY-3 consists of three satellites (FY-3A, FY-3B, and FY-3C). Although they are nicknamed "Olympic stars", only the first one was launched close to the 2008 Olympic Games. The last two were launched in 2010 and 2013.

At that time, its mission goal was to provide meteorological support services for the Olympic Games together with the Fengyun-2 meteorological satellite. The detection objects included three-dimensional, global, all-weather, quantitative, and high-precision data of the Earth's atmospheric environment. Fengyun-3 is a "polar-orbiting sun-synchronous orbit satellite" that succeeded Fengyun-1.

FY-3 Batch 1 (Photo source: National Satellite Meteorological Center)

As for the latest Fengyun-4 satellite, Star A was launched on December 11, 2016, and there will naturally be Star B in the future. They are also geosynchronous orbit satellites, inheriting the mantle of Fengyun-2.

On February 27, 2017, Fengyun-4A obtained the first batch of images and data. China obtained and released the world's first geostationary orbit Earth atmosphere hyperspectral map, and also obtained the first color satellite cloud map and lightning distribution map.

Fengyun-4A (Photo source: National Satellite Meteorological Center)

The orbits of these satellites are very easy to remember: the odd-numbered ones, No. 1 and No. 3, are "polar-orbiting" "sun-synchronous orbit" satellites, and the even-numbered ones, No. 2 and No. 4, are "stationary" "geosynchronous orbit" satellites.

“Sun-synchronous”, how to synchronize?

Fengyun-1 and Fengyun-3 are polar orbiting "sun-synchronous orbit" satellites. The so-called "polar orbit" means that the orbit passes over the Earth's North and South Poles, and the orbital plane is perpendicular to the Earth's equatorial plane.

But in reality, satellite orbits usually do not pass through the South Pole or the North Pole exactly, but are slightly tilted. So it is more accurate to say that they are "near polar orbits", with the angle between the orbital plane and the equatorial plane close to 90 degrees.

Let's imagine a globe rotating in place, with the bracket on the side of the globe like an orbital plane. In space, if there is no special force, this plane will be as immovable as the bracket. Then, as the earth rotates, the satellite orbit will traverse all areas of the earth.

Globe (Image source: justglobes.uk)

But, does the satellite orbital plane really not move?

If the earth is a uniform sphere, then the satellite in an orbit with an inclination of 90 degrees will be subjected to balanced forces and the orbital plane will not rotate. This is called "translation" with the earth.

However, the earth is an oblate ellipsoid, and the gravitational force on the satellite is not equal at each point on the oblate ellipsoid, and the earth's axis is tilted. Therefore, the satellite's orbital plane rotates around the earth's axis in the opposite direction of the satellite's movement, which is called the "precession" of the orbital plane. Therefore, the sun-synchronous orbit must choose an orbital inclination slightly greater than 90°.

According to calculations, the precession speed is about 1°, which is just enough to keep the orbital plane and the sun in a fixed orientation. In other words, the satellite's orbital plane is also rotating 360° a year, always perpendicular to the line between the sun and the earth. Then the satellite orbiting around the earth will not be blocked by the earth and can be fully exposed to sunlight, which is "sun synchronization".

Therefore, what the sun-synchronous orbit synchronizes is actually the Earth's revolution.

The Fengyun satellite chose this orbit, which is very beneficial for obtaining weather forecast information. First, it can maximize the use of solar power generation. Second, every time the satellite flies over a certain area, the sun shines on the area from the same angle, and more information can be obtained by comparing the photos taken.

“Geosynchronous station”, can it also be stationary in the sky?

Fengyun-2 and Fengyun-4 are "stationary" "geosynchronous orbit" satellites. So, how are they "stationary"?

In fact, they are not staying still in the universe. It is just that when we on the ground look up at the sky, the satellite is always above our heads and does not fly away, as if it is standing still.

From the perspective of the Earth, the sun rises in the east and sets in the west because the Earth rotates from west to east. The so-called "geosynchronous orbit" is synchronized with the Earth's rotation. The orbital period of the satellite is equal to the Earth's rotation period in inertial space (23 hours, 56 minutes and 4 seconds), and the direction is consistent with it. Therefore, the satellite always rotates with the Earth and its position relative to the ground remains unchanged.

Geosynchronous orbit (Image source: Wikipedia) animation

Since the orbital plane of the geosynchronous orbit coincides with the equatorial plane, that is, the orbital inclination is 0 degrees, the satellite cannot stay directly above any area (such as Beijing), but is always located above a certain place on the equator.

Although China's Fengyun satellites are not directly above the vast land of China, they have chosen longitudes that match the longitudes of the Earth. For example, they have chosen positions such as 105 degrees, 99.5 degrees, and 79 degrees east longitude. If we look up, we can't even capture their figures.

However, it is these satellites that are difficult to see with the naked eye that not only constitute the satellite system serving China's weather forecast, but also play an important role in climate monitoring, disaster monitoring and environmental remote sensing, and have made great contributions to social development.

Produced by: Science Popularization China

Produced by: Wang Zheng

Producer: Computer Network Information Center, Chinese Academy of Sciences