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Completion of Deployment for A Constellation of FORMOSAT-7 Satellites


President Tsai Ing-wen declared in her inauguration speech last year that the space industry is one of the country's six core strategic industries. As an important indicator of Taiwan's space technology, FORMOSAT-7 is also the largest international scientific cooperation program between Taiwan and the United States in recent years. There is good news from FORMOSAT-7 a few days ago that all 6 satellites have been successfully deployed in mission orbits at the altitude of 540 to 550 kilometers on February 3, 2021, and 24-hour evenly distributed atmospheric and ionospheric observation data can be provided for the application of meteorological forecast and space weather monitor.

The FORMOSAT-7 program is jointly implemented by National Space Organization (NSPO) of National Applied Research Laboratories (NARLabs) under the Ministry of Science and Technology (MOST) in Taiwan and the National Oceanic and Atmospheric Administration (NOAA) in the United States. A constellation of 6 FORMOSAT-7 satellites use the occultation technology to observe data and receive signals from the US GPS and Russian GLONASS global positioning satellites. They can provide daily atmospheric vertical profile data at about 4,000 points between 50 degrees north and south latitudes around the world, which can make up for a large amount of observational data that is extremely scarce in tropical ocean areas, which will greatly help the improvement of global weather forecast accuracy. It has been confirmed by international meteorological operation units that the FORMOSAT-7 data can reduce the 24-hour weather forecast error by 10~11% when the observation points are not evenly distributed.

FORMOSAT-7 satellites aboard SpaceX's Falcon Heavy rocket were launched at the Kennedy Space Center in Florida, U.S., on June 25, 2019, Taiwan time, and then they reached a parking orbit of 720 kilometers. After 1 month of satellite health checks and 18 months of orbit-transfer operations, all 6-satellite deployment to the mission orbit was completed on February 3, 2021.

Before the deployment is completed, FORMOSAT-7 has already performed its mission. The atmospheric observation data was released globally in synchronization with NOAA on March 7, 2020. The ionospheric data obtained by receiving GPS satellite signals was released and announced together with the University Corporation for Atmospheric Research (UCAR), USA, on September 15, 2020. The FORMOSAT-7 data is currently being used by many meteorological centers and research institutes around the world. Among them, NSPO, Central Weather Bureau (CWB), National Central University (NCU), and National Cheng-Kung University (NCKU) teams in Taiwan, as well as the scientific teams under UCAR and NOAA in the United States make the most use of the FORMOSAT-7 data. In addition to reducing the overall weather forecast error by 10~11%, it can forecast typhoon generation earlier, improve the typhoon forecast track, and effectively monitor changes in space weather.

In the future, NSPO will continue to work hard to guard FORMOSAT-7, the role of clairvoyance, observing the weather changes thousands of miles away, providing the best service to help CWB and major meteorological centers around the world to grasp the situation earlier, so that the people can make preparation as soon as possible to reduce disaster losses.

FORMOSAT-7 satellite orbit distribution map

1Why does it take 19 months for the six FORMOSAT-7 satellites to be deployed?

The flight speed of the satellite may change the altitude. Satellites at a height of 720 kilometers, thrusting in the opposite direction of travel, slowing the satellite, will cause the satellite orbit to drop.

When the first FORMOSAT-7 satellite is lowered to a mission-orbit altitude of 550 kilometers, its orbital plane will be affected by the oblate gravitational field of the earth. Compared with the orbital plane of the satellite staying at 720 kilometers, it produces about 0.55 degrees of orbital plane drift per day. After 108 days, the orbital plane of the first 550-kilometer-high satellite and the 720-kilometer-high orbital plane will be separated by about 60 degrees. Then, the second satellite began to descend from an altitude of 720 kilometers to an orbit of 550 kilometers. After 108 days, the orbital plane of the second 550-kilometer satellite and the orbital plane of the 720-kilometer satellite are also separated by about 60 degrees. The orbital plane of the first 550-kilometer-high satellite and the orbital plane of the 720-kilometer-high satellite have been separated by about 120 degrees. In this way, the third, fourth, fifth, and sixth satellites are lowered from the orbital height of 720 kilometers to the orbital height of 550 kilometers. It takes 108 x 5 = 540 days from the beginning of the first satellite to lower the orbital height to the end of the sixth satellite lowering the orbital height. In addition, the health check is still being carried out in the first four weeks after the satellite is launched. It takes about 19 months in total. At this time, the first 550-kilometer satellite and the second, third, fourth, fifth, and sixth 550-kilometer satellite orbit planes have been separated by 60, 120, 180, 240, and 300 degrees respectively, and the deployment of a constellation of six satellites is completed.

2What is occultation technology?

When the electromagnetic wave signal emitted by the Global Positioning System (GPS) satellite passes through the atmosphere, it will turn around, slow down, and weaken because it penetrates the air layer of different temperature, pressure or humidity. As long as the characteristics of the signal received by FORMOSAT-7 satellites are analyzed, data such as temperature, air pressure, humidity, or electron density over the earth can be derived in turn.

Schematic diagram of occultation technique