Orbital Focus - International Spaceflight Facts and Figures
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Launches and Orbital Operations

Tyneside, UK
2024 Jun 15
Saturday, Day 167

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Yaogan 31-02 Formation



These three satellites, Launched 2021 Jan 29, make up the second group of satellites with the 'Yaogan 31' name. There have been previous groups in similar orbits and acting in similar ways but they simply received the next available number in the ongoing sequence of Yaogan numbers.

Shortly after launch, one satellite moved to a higher orbit and the other two dropped by a similar amount. The singleton in the higher orbit started falling back from the other two. Previous mission profiles told us this would continue until the satellites came together again after drifing apart by the equivalent of whole circuit of the Earth and then going through the cycle a second time. They would then slide into formation for the active part of the mission.

apsides plot

The plane of a posigrade satellite orbit moves slowly westwards with time, manifested as a decreasing value for the Right Ascension of the orbit's Ascending Node as seen in the Space-Track data. The effect of being at two slightly different orbit heights is that the two orbit planes move at different rates with the lower orbit moving faster. By the time the three satellites of this mission come together, the orbit of the singleton will lie about 110 kilometres to the east of the pair.

By spacing the pair about 125 km apart from each other along the same track and having the outlier travelling at the same speed, the three vehicles will spend most of their life travelling in a near-equilateral triangular formation. It allows them to detect radio transmissions from the Earth's surface and then pinpoint the location by measuring the slight differences in the Doppler effect as observed from the three vehicles.

The three satellites of the Yaogan 31-02 group came into close proximity February 12, and then went into their second cycle of chasing each other down as can be seen by the plot at the top of the page. Next time they caome together was February 23 when all three satellites adjusted their orbit height to drop into formation travelling at the same velocity.


The illustration above shows the Yaogan 31-02 formation only hours after the three satellites had made the final manouvres to bring them together. Less that 30 minutes after this pass above the Gulf of Mexico, the next Yaogan 31 group reached orbit.

The USA has been using satellites in similar orbits to Yaogan 31 to do this job for many years - the 'NOSS triplets'.

The first Yaogan 31 launch was in 2018 April. On that occasion, it was the pair that went to the higher orbit and the singleton moved lower taking it ahead of the others and coming up on them from behind. The on-orbit 'catchup' process for that mission took the 'usual' four weeks to perform before the three vehicles settled into close formation.

apsides plot

Launch Announcement from Xinhua News agency

JIUQUAN, Jan. 29 (Xinhua) - The second group of China's Yaogan-31 remote sensing satellites were sent into space from the Jiuquan Satellite Launch Center in northwest China on [Jan 29] at 12:47 p.m. (Beijing Time).

The satellites were carried by a Long March-4C rocket. It was the 359th flight mission of the Long March carrier rocket series, the launch center said.

Having entered their planned orbits, the satellites will be used for electromagnetic environment surveys and other related technology tests.

China launched the first group of the Yaogan-31 remote sensing satellites in 2018.

Page Date - 2021 Feb 10
Updated - 2021 Feb 18
Updated - 2021 Feb 24

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