Chang'e-4 relay satellite / Queqiao of China
The Chang'e-4 relay satellite, named Queqiao ('magpie bridge'), is a precursor to an unprecedented attempt to soft-land the Chang'e-4 satellite on the lunar far side in late 2018, when a lander and rover will be send to the Moon. Since the lunar far side does not face the Earth as the moon's orbital period matches its rotational period, a relay satellite is required to facilitate communications between the Chang'e-4 lander on the far side of the moon and ground stations on Earth.
The nickname Queqiao was announced by CNSA (China National Space Administration) on 25 April 2018, China's Space Day. In a Chinese folktale, magpies form a bridge with their wings on the seventh night of the seventh month of the lunar calendar to enable Zhi Nu, the seventh daughter of the Goddess of Heaven, to cross and meet her beloved husband, separated from her by the Milky Way. 1)
This is the main role of the 425 kg spacecraft, developed by the China Academy of Space Technology (CAST), which is being sent into position around six months before the landing mission in order to test and verify is functions.
The 425 kg relay satellite is based on the three-axis stabilized CAST-100 minisatellite bus featuring an 130 N hydrazine propulsion system. It carries a deployable 4.2 m dish antenna for the relay equipment. It provides four 256 kbit/s X-band links between itself and the lander/rover and one 2 Mbit/s S-band link towards Earth.
Figure 1: Illustration of the deployed Chang'-4 relay satellite (image credit: CAST)
Launch: The Chang'4 relay satellite (Queqiao) was launched on 20 May 2018 (21:28 UTC) on a Long March-4C vehicle from the XSLC (Xichang Satellite Launch Center) in China. 2)
Figure 2: Photo of the Queqiao relay satellite, launched ahead of Chang'e-4 lunar mission (image credit: CNSA)
Orbit: Halo orbit of the Earth-Moon L2 (Lagrangian Point 2), around 65,000 km on the far side of the moon, so as to be visible to both ground stations on the Earth and the lander and rover on the lunar far side at all times.
Figure 3: Flight profile of the relay satellite (image credit: CAST)
Secondary payloads: Two Chinese microsatellites, Longjiang-1 and Longjiang-2, were launched with the Chang'e 4 relay mission to conduct astronomical observations from deep space (Selenocentric, elliptical orbit).The two microsatellites were developed by the Harbin Institute of Technology. 3)
Figure 4: Photo of the Longjiang-1 and -2 microsatellites at the launch site, which were launched to the Moon with the Queqiao/Chang'e-4 lunar relay satellite on May 20, 2018 (image credit: Harbin Institute of Technology)
Figure 5: The map of the EML (Earth-Moon Liberation) points (image credit: CAST)
The relay satellite orbiting around the Earth-Moon L2 point is about 60000–80000 km away (halo orbit) from the lander and the rover working on lunar surface. Under the constraints of the launching mass and size, the relay communication link should be optimized in multiple aspects such as the relay transmission modes and the high gain relay antenna development to achieve high-bit-rate communication.
Relay communications: The link between Change'4 relay satellite and lander/rover is designed to work on X-band. The forward link uses unified carrier-wave TT&C regime and the backward uses BPSK. The forward relay signal emitter should be able to scan with complex frequency carrier waves, similar to ground stations. The relay satellite should be able to transmit data to Earth and relay with the lander and the rover simultaneously. To avoid interference, the relay link adopts X-band channel, and TT&C to Earth chooses S-band unified carrier wave regime and the data transmission utilizes S-band and BPSK carrier wave regime. Channel encoding is not used for telecommand transmission and RS+concatenated convolution channel encoding is adopted for telemetry and data transmission. Meanwhile for the purpose of synchronous encoding with the ground, both telemetry and data pseudo-random coded.
Relay communication link design: The link profile among the relay satellite, the lander and the rover and ground stations is as shown in Figure 6. The forward data is emitted from the relay satellite, the lander and the rover receive data via omnidirectional antenna and the signals are modulated in PCM/PSK/PM. The backward data is transmitted via an omnidirectional antenna, a medium gain antenna or a directional antenna and is received by the relay satellite. The backward link data of the rover is transmitted by the omnidirectional antenna or the directional antenna and is received by the relay satellite. The lander backward link adopts omnidirectional antenna, medium gain antenna and directional antenna corresponding to low, medium and high bit rates. The modulation mode of the backward link is BPSK.
During the powered descent process, except for setting up X-band omnidirectional forward/backward relay communication links, the lander and the relay satellite also communicate via a medium gain antenna to return landing camera data back to Earth.
While working on the Lunar surface, the lander and the rover respectively receives forward telecommand signals via an omnidirectional antenna from the relay satellite. The relay satellite can send data at two frequency points at the same time to realize simultaneous control of the lander and the rover. Following the command, the lander and the rover send backward data (including telemetry and scientific exploration data) via an omnidirectional antenna or a directional antenna.
The relay satellite can receive backward data from both the lander and the rover simultaneously.
Table 1: Relay communication phases
Figure 7: Artist's rendering of the Chang'e-4 relay satellite, launched in May 2018, and lander and rover to set down on the lunar far side in late 2018 (image credit: CAS)
Status of the Chang'e-4 relay satellite / Queqiao mission
• June 18, 2018: Two microsatellites, Longjiang-1 and Longjiang-2, were sent into space on 20 May (21:28 UTC) together with the Chang'e-4 lunar probe's relay satellite from southwest China's Xichang Satellite Launch Center. 4)
- Longjiang-2 successfully reached its destination near the Moon on May 25, and entered a lunar orbit with a perilune at 350 km and an apolune at 13,700 km. However, Longjiang-1 suffered an anomaly and failed to enter lunar orbit, according to CNSA (China National Space Administration).
- With a mass of 47 kg, Longjiang-2 has become the world's first lunar orbiter developed by a university. The Longjiang-2 microsatellite carries an optical camera developed by KACST (King Abdulaziz City for Science and Technology) of Riyadh,Saudi Arabia, as well as a low-frequency radio detector developed by the National Space Science Center of CAS (Chinese Academy of Sciences).
- The camera, which began to work on 28 May , has conducted observations of the Moon and acquired a series of clear lunar images and data, according to Xinhua News.
Figure 8: The released photo shows part of the Mare Imbrium on the moon. On 14 June 2018, China and Saudi Arabia jointly unveiled three lunar images acquired through cooperation on the relay satellite mission for the Chang'e-4 lunar probe (image credit: CNSA, CLEP, KACST)
• June 15, 2018: China has provided an update on its Chang'e-4 relay satellite, launched in May in preparation for a later landing on the far side of the Moon, while also revealing the status of the Longjiang microsatellites intended for lunar orbit. A series of stunning images from a Saudi Arabia-developed camera aboard one of the lunar microsatellites, namely Longjiang-2, were also released. 5)
Figure 9: A demonstration of the Lissajous/halo orbit to be used by the Queqiao Chang'e-4 relay satellite mission (image credit: CASC)
Figure 10: The Earth and Moon imaged on June 8 by the KACST-developed camera on China's Longjiang-2 microsatellite. The image shows Saudi Arabia on the distant Earth, as well as the northern hemisphere of the lunar far side, near the Petropavlovskiy crater (image credit: CNSA/CLEP/KACST)
• June 14, 2018: The relay satellite for the Chinese Chang'e-4 lunar probe, named Queqiao and launched on 20 May 2018, entered the Halo orbit around the second Lagrangian (L2) point of the Earth-Moon system (EML-2), about 65,000 km. from the Moon, at 11:06 a.m. on Thursday, June 14, after a journey of more than 20 days. 6)
- "The satellite is the world's first communication satellite operating in that orbit, and will lay the foundation for the Chang'e-4, which is expected to become the world's first soft-landing, roving probe on the far side of the Moon," said Hongtai Zhang , President of the China Academy of Space Technology (CAST). 7)
- The concept of the Halo orbit around the EML-2 (Earth-Moon L2 ) point was first put forward by international space experts in 1950s. While in orbit, the relay satellite can see both the Earth and the far side of the Moon. The satellite can stay in the Halo orbit for a long time due to its relatively low use of fuel, since the Earth's and Moon's gravity balances the orbital motion of the satellite.
- "From Earth, the orbit looks like a halo of the Moon, which is where it got its name," said Lihua Zhang, project manager of the relay satellite. He said the Halo orbit was a three-dimensional ,irregular curve. It is extremely difficult and complex to maintain the satellite in orbit. "If there is a tiny disturbance, such as gravitational disturbance from other planets or the Sun, the satellite will leave orbit. The orbit period is about 14 days. According to our current plan, we will conduct orbit maintenance every seven days. It's a new type of orbit, we don't have any experience. We ran a number of simulations to make sure the design is feasible and reliable," Zhang added.
- In order to establish a communication link between Earth and the planned Chang'e-4 lunar probe, space engineers must keep the satellite stable and control its altitude, angle and speed with high precision. Next, the team will test the communication function of the relay satellite.
- With a mass of 400 kg, and with a design life of three years, the satellite carries several antennas. One, shaped like an umbrella with a diameter of 4.2 meters, is the largest communication antenna ever used in deep space exploration, this according to Lan Chen, deputy chief engineer of the Xi'an Branch of CAST.
- Tidal forces of the Earth have slowed the Moon's rotation to the point where the same side always faces the Earth, a phenomenon called tidal locking. The other face, most of which is never visible from Earth, is the far side or dark side of the Moon, not because it's dark, but because most of it remains unknown. With its special environment and complex geological history, the far side is a hot spot for scientific and space exploration. The Aitken Basin of the lunar south pole region on the far side has been chosen as the landing site for Chang'e-4. The region is believed to have great research potential.
• June 1, 2018: Queqiao made its lunar swing-by on May 25, performing a braking burn at 13:32 UTC to send the communications satellite towards EML-2, some 60-80,000 km beyond the Moon. 8)
Sensor complement of the Chang'e-4 relay satellite: (NCLE)
On board is a Dutch radio antenna, the NCLE (Netherlands Chinese Low-Frequency Explorer). The radio antenna is the first Dutch-made scientific instrument to be sent on a Chinese space mission, and it will open up a new chapter in radio astronomy. 9)
The NCLE instrument was developed and built by engineers from ASTRON, the Netherlands Institute for Radio Astronomy in Dwingeloo, the Radboud Radio Lab of Radboud University in Nijmegen, and the Delft-based company ISIS — in collaboration with a team from the Chinese NAOC (National Astronomical Observatory of the Chinese Academy of Sciences). With the instrument, astronomers want to measure radio waves originating from the period directly after the Big Bang, when the first stars and galaxies were formed.
Why is it so important for the measuring instruments to be placed behind the Moon? Professor of Astrophysics from Radboud University and ASTRON Heino Falcke: "Radio astronomers study the universe using radio waves, light coming from stars and planets, for example, which are not visible with the naked eye. We can receive almost all celestial radio wave frequencies here on Earth. We cannot detect radio waves below 30 MHz, however, as these are blocked by our atmosphere. It is these frequencies in particular that contain information about the early universe, which is why we want to measure them."
Special about the radio antenna is that it will receive low frequency radio waves with a large frequency range. The instrument passed an important risk assessment review by the Chinese space agency at the end of April.
"In the past this was not possible and therefore a receiver with a narrow frequency band was used, in order to avoid electromagnetic interference of the satellite itself," explains project leader Albert-Jan Boonstra of ASTRON. "We have now succeeded in avoiding the electromagnetic interference and making a broadband receiver. That is, of course, good news for subsequent missions and can, for example, be used for future nano-satellites."
In 2016, the NSO (Netherlands Space Office) and its Chinese counterpart CNSA signed an agreement to cooperate in this project, which was an elaboration of the Memorandum of Understanding the two space agencies signed the year before during a trade mission in presence of the Chinese President Xi Jinping and the Dutch King Willem Alexander.
A digital controller unit, designed and supplied by the Somerset West-based SAC (Space Advisory Company), is part of the NCLE (Netherlands-China Low-Frequency Explorer) as a science payload on the Chang'e-4 relay satellite which lifted atop a Long March 4C Rocket from Xichang Satellite Launch Center at 21:28 UTC on Sunday, 21 May 2018. 10)
While the Chang'e-4 satellite's main mission is to relay messages between Earth and the Moon, the NCLE instrument will ride along and conduct experiments into deeper space.
Duncan Stanton, CEO of SAC said that they are ecstatic to be part of such an unique mission and especially proud of their engineering team who proved themselves to be world-class by meeting the ambitious timeline and performance requirements of the project. They may just have embarked on proudly flying the South African flag the furthest ever.
Stanton continued that the controller unit supplied by them forms a critical part of the digital receiver system for the NCLE instrument. The instrument was built by the Radboud Radio Lab from the Radboud University, the Netherlands Institute for Radio Astronomy (ASTRON), and ISIS (Innovative Solutions In Space) in Delft. The instrument has a primary science objective to detect low frequency 21 cm hydrogen line emissions from the ‘dark ages' period of the universe before stars began to shine.
SAC is a member of the Somerset West-based SCS Aerospace Group (SCSAG), Africa's largest privately-owned group of satellite design and manufacturing companies with more than 25 years of experience in this domain.
Figure 11: The digital controller unit designed and supplied by SAC (Space Advisory Company) integrated into Radboud Radio Labs NCLE Digital Receiver Instrument bound for an orbit beyond the moon (image credit: SAC, RadBoud Radio Labs)
1) "China's Chang'e-4 Relay Satellite Named "Queqiao"," CAS, 25 April 2018, URL: http://english.cas.cn/newsroom/china_research/201804/t20180425_191973.shtml
2) "China preparing to launch Chang'e-4 relay satellite May 21," Space News, 14 May 2018, URL: http://spacenews.com/china-preparing-to-launch-change-4-relay-satellite-may-21/
4) "Microsatellite developed by Chinese university starts to work around Moon," Moon Daily, 18 June 2018, URL: http://www.moondaily.com/reports/Micro_satellite_developed
5) Andrew Jones, "Chang'e-4 update: Queqiao relay satellite in halo orbit, Longjiang-2 returns amazing images from Moon," gbtimes, 15 June 2018, URL: https://gbtimes.com/change-4-update-
7) "Relay satellite for Chang'e-4 lunar probe enters planned orbit," CNSA, 15 June 2018, URL: http://www.cnsa.gov.cn/n6443408/n6465652/n6465653/c6801923/content.html
8) Andrew Jones, "Queqiao update: Chang'e-4 lunar relay satellite establishing halo orbit after approaching Lagrange point," gbtimes, 1 June 2018, URL: https://gbtimes.com/queqiao-update-change-4
9) "Dutch Radio Antenna To Depart For The Moon On Chinese Mission," Moon Daily, 18 May 2018, URL: http://www.moondaily.com/reports
10) "How a South African Company's Hardware Will End Up On the Far Side of the Moon," Satnews Daily, 30 May 2018, URL: http://www.satnews.com/story.php?number=277784253#
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (email@example.com).