Chang'e 4's Relay Satellite Bridging Earth and Moon

YU Guobin

Lunar Exploration and Space Program Center, CNSA, Beijing 100048

Abstract: The far side of the moon is a unique place for some scientific investigations. Chang'e 4 is a Chinese lunar far side landing exploration mission. Relay communication satellite, named as Queqiao, is an important and innovative part of Chang'e 4 mission. It can provide relay communication to the lander and the rover operating on the lunar far side to maintain their contacts with Earth. It was launched by LM-4C launch vehicle at the Xichang Satellite Launch Center on May 21, 2018. After five precise orbit controls and a journey of more than 20 days, Queqiao inserted into final halo mission orbit around Earth-moon libration point 2, located about 65,000 km beyond the moon. It is the world's first communication satellite operating in that orbit. Up to now, Queqiao worked very well and provided reliable, continuous communication relay service for the lander and the rover to ensure the mission success of Chang'e 4 exploration mission. Via Queqiao, the lander and the rover were controlled to work by ground stations and obtained a great amount of scientific data. The mission overview, operation orbit selection, relay communication system design and flight profile were introduced in this article.

Key words: Chang'e 4 mission, Queqiao, lunar far side, relay communications, Earth-moon libration point 2

1 PREFACE

Great achievements have been made with the Chinese Lunar Exploration Program attracting worldwide attention since its start in 2004. An even greater target was set by aiming for the far side of the moon, an unchartered domain by worldwide spacecraft. After close to two years, the Chang'e 4 project,composed of lander, rover and relay satellite, feasibility study was approved by the government in January 2016 and commenced engineering development.

Relay communications became the prime key issue to be tackled due to the fact that spacecraft on the far side of the moon cannot establish direct communications with ground stations on Earth. The relay satellite Queqiao aboard a LM-4C was successfully launched from the Xichang Satellite Launch Center on the morning of May 21, 2018. After five precise orbital corrections, the satellite entered its Halo mission orbit at the Earthmoon Lagrange L2 point on June 14. The success of a series of in-orbit tests proved that the satellite was in good shape, with all functions and performance up to mission requirements, ready to perform relay communications services. With the communications bridge set up, all that was left was the arrival of the lander and rover of Chang'e 4.

Figure 1 Chang'e 4 mission sketch (Source from CNSA)

2 WHY THE FAR SIDE OF THE MOON

Humans have carried out a dozen successful lunar soft landing missions including sample return missions. Most of the missions by the U.S. and the former Soviet Union concentrated during the 1960s and 1970s, then China conducted the Chang'e 3 mission in 2013. However all of the missions were to the near side of the moon, which left landing discovery for the far side and the polar areas of the moon a blank page to be filled in. Based upon the results of circling missions of the moon in recent years, South Pole-Aitken basin, the largest and deepest struck basin in the solar system located on the far side and south pole of the moon was selected. Exploration over there will facilitate a better understanding of the history of solar system and key events in the early phase in history of the Earth.Therefore the two poles and far side of the moon boast greater

Figure 2 Relay satellite of Chang'e 4 aboard a LM-4C(Source from CNSA)scientific value from exploration. A number of lunar missions proposed by the U.S. were targeting the Aitken basin. Clear of interference from the ground and ionosphere of the Earth, the far side of the moon is considered to be the ideal place for low frequency radio observation. Among the ten manned landing sites proposed by NASA in their space exploration scenarios, six of them are located in the polar, far side and border areas that are not visible from the Earth. ESA has also proposed the FARSIDE plan launching a far side landing exploration mission.

After the successful soft landing and roving mission of Chang'e 3 on the near side of the moon, as the next step, the Lunar Exploration and Space Engineering Center (hereafter LESEC) of CNSA organized expert panels to carry out an indepth feasibility study and compare multiple proposals on the way ahead. LESEC finally made the decision for selecting the unchartered far side of the moon as the objective, and to land a spacecraft at the South Pole-Aitken basin to perform the first-ever far side soft landing and roving exploration. For a far side mission, lander and rover cannot have direct contact with ground stations on Earth, hence it necessitates a relay satellite supporting the communications between lander, rover and Earth ground stations, transmitting the scientific data acquired by the lander and rover back to Earth, as well as providing tracking and control support during the mission period of the lander and rover on the lunar surface. Additionally, a relay satellite can carry out scientific exploration and new technology testing through onboard scientific and technological experimental payloads,maximizing the benefit of the satellite.

3 LONG HISTORY OF THE CONCEPT OF RELAY COMMUNICATIONS BY THE EARTH-MOON LAGRANGE L2 POINT

As early as in 1966, Farquhar from the U.S. proposed the concept of the Earth-moon L2 point and Halo orbit, as well as the idea of placing a relay satellite providing relay communications for spacecraft on the far side of the moon, as shown in Figure 3. NASA once planned to go ahead with Farquhar's proposal, and had solicited several proposals from different entities,but finally decided not to go through with them due to mission risks. Few countries including the U.S. and European countries have conceived far side of the moon missions, where most of the relay satellites were designed to be around the Earth-moon Lagrange L2 orbit. This orbit permits the relay satellites to maintain constant visibility with lander and rover on the far side of the moon. From thorough study, Queqiao was designed to orbit around south to the Earth-moon Lagrange L2 Halo orbit to provide the relay communications service.

Figure 3 Concept illustration of communications between a relay satellite and far side of the moon from Earth-moon L2 point on the Halo orbit [1]

As no prior far side and polar landing missions were performed for various reasons, Chang'e 4 was the first man-made satellite conducting a soft landing and roving exploration on the far side of the moon, using a realtime communications service from the Earth-moon Lagrange L2 orbit.

4 RELAY COMMUNICATIONS SYSTEM SUITABLE FOR LANDER AND ROVER INTERFACE

The main task of Queqiao is to provide both real-time and delayed relay communications between Chang'e 4 lander, rover and ground stations. Considering the different relay communications requirements between the lander and rover in various phases of the mission, the Queqiao mission is required to take into account these mission requirements in various stages, under different working modes and data rates, and to be compatible with the tracking and control communications system of the lander and rover.

Queqiao relay communications features complex working modes, and stringent requirements on technical specifications.In particular, it is technically difficult to maintain continuous and reliable de-modulation of the return link under such conditions as low signal-noise ratio, low data rate and Doppler shift. Targeting key problems like acquisition and the tracking algorithm,a great deal of simulation, trial and test verification were carried out to realize stable and reliable demodulation.

Since, a strict mass budget was passed down from the project system level, the design scheme and equipment selection of relay communications system was accordingly subject to stringent requirements. An optimal scheme satisfying the requirements was finally worked out by incorporating measures including a reasonable redundancy design, optimized working modes and lightweight product design. A great number of highly compact, lightweight and ministurized equipments were developed, including an X-band 20 W SSPA, waveguide switch,down-converter, UXB demodulator, and lunar demodulator. A regenerative X-band deep space relay communications system was set up as well.

Figure 4 Antenna deployment test before launch (Source from CNSA)

The lunar down-converter was a critical unit for the relay communications return link. A multi-chip packaging design was adopted for all circuits inside. As a result of the highly compact design combing constant temperature crystal oscillators, power,local oscillators, channels, AGC circuits, the total product mass was only 900 g. All chip-based miniaturized frequency source and new generation power units made their maiden flight.Breakthroughs were made in a number of new technologies.Equipments were eventually verified and flight proven, laying the foundation for next generation microwave equipment upgrade.

The selection of a suitable relay communications antenna constituted a key part of the mission. The size of the antenna dictated the performance of the communications link. In particular, the lunar communications return reception capability is mainly decided by the size of the antenna. Queqiao would operate around the Earth-moon L2 liberation point Halo orbit,with a maximum distance with lander and rover of up to 79000 km. Such a long distance communications and constraints in every aspect required a high gain antenna with maximum aperture. To satisfy the requirement of high gain and lightweight antenna, a fixed-plus-mesh, spring-deployed mechanism parabolic umbrella antenna was adopted, with aperture up to 4.2 m when deployed and gain up to 45 dB. The antenna would be the largest of its kind for application so far in deep space exploration missions worldwide, as shown below. The antenna would be stowed during launch and deployed in-orbit. It would achieve relay communication, pointing to lander and rover by attitude maneuver, and data transmission by pointing to the Earth ground stations.

The umbrella antenna featured large aperture, high foldability and high accuracy. This type of antenna had no previous in-orbit flight experience. It was with great development difficulty and high application risk. As the antenna parts that were away from the body of the satellite were exposed to temperatures below 230℃, the adaptability to the low temperature environment was the first issue to be tackled. High requirements were was also required for the electrical performance such as maintaining the in-orbit shape accuracy, while satisfying the gain requirements remained another challenge. Considerable validation tests were conducted by the development team in response to the low temperature adaptability challenge to make sure that the antenna would survive the low temperature with performance meeting the requirements. In order to have the antenna achieve the specifications in a complex space environment, great effort was exerted to increase the reflector shape accuracy and stability. The shape accuracy met the mission requirements after rounds of meticulous adjustment and time to met the environment.

5 QUEQIAO IN PLACE AFTER PERFECT ORBITAL FLIGHT

Queqiao had a complex orbital flight sequence. When separated from the launcher, the satellite had first to correct the attitude variance induced by the separation, unlock and deploy its solar array, unlock and deploy the parabolic umbrella antenna and establish a normal flight attitude. Three corrections were conducted on its journey to the moon. Queqiao reached the vicinity of the moon after four days of flight and conducted a perilune break 100 km above the moon and by leveraging the force flew to Earth-moon L2 liberation point with several orbital corrections. It took a few days to reach the nearby Earth-moon L2 point. After orbital acquisition and correction, Queqiao entered the Halo mission orbit, which circles Earth-moon L2 point with an amplitude of 13000 km in Z direction. A series of in-orbit tests were conducted for both the platform and payload in preparation for the following relay communication mission after the launch of lander and rover.

At 05:28 Beijing time on May 21, 2018, Queqiao was successfully launched on board a LM-4C launch vehicle from the Xichang Satellite Launch Center. The satellite was firstly separated from the launcher, then smoothly deployed its solar cell panels, parabolic umbrella antenna and established a normal flight attitude. Owing to the high precision orbital injection provided by LM-4C, the relay satellite had only one correction during its flight to the moon.

Figure 5 Queqiao captured by camera on LM-4C after separation (Source from CNSA)

On the night of May 25, Queqiao arrived in the vicinity of the moon and fired a perilune break around 100 km above the moon. Four 20 N attitude control engines burned for 912 seconds and shut down as per precedure. Telemetry data confirmed the result of the orbit control process and that the relay satellite had entered the transfer orbit from moon to Earthmoon Lagrange L2 point, thus marking a complete success of this perilune break.

On June 14, the relay satellite conducted an acquisition control maneuver prior to entering into mission orbit. The relay satellite made its way to the Halo mission orbit that circled around the Earth-moon Lagrange L2 point. The success of this process symbolized that through five accurate orbital maneuvers, Queqiao achieved a brilliant performance during its long journey from Earth to the Earth-moon Lagrange L2 point with a perfect ending, and became the first spacecraft worldwide in achieving the Halo orbit.

In-orbit tests took place right after the relay satellite entered the mission orbit up to July 20. All tests were completed including platform, payload, relay communications satellite pointing calibration, and space-to-ground closed-loop test between the relay satellite, Chang'e 4 lander and rover. The test results indicated the sound working status of the relay satellite, with every function and performance up to the specification requirements,and the ability to provide relay communications in the wake of lander and rover reaching their orbital positions. The communications bridge was ready, with the relay satellite standing by in its mission orbit, awaiting the arrival of the lander and rover on the moon to commence relay communications services.

6 CONCLUSION

Chang'e 4 has made the first-ever soft landing and exploration on the far side of the moon as a human-made spacecraft, and Queqiao has conducted its first relay communication around the Earth-moon Lagrange L2 point orbit, both with great significance and worldwide attention. Queqiao, being a critical and precondition for making the Chang'e 4 a success,had gone through all in-orbit tests since its launch in May, 2018,with all functions and specifications up to mission requirements,ready to serve the lander and rover.

It was in the design scheme of Queqiao to be equipped with an extended application capability apart from just providing a relay communications service for lander and rover on the far side of the moon. For one thing, it has piggybacked scientific and technological experimental payloads to perform in orbit scientific exploration and technological test missions. For another,the design life of the satellite is more than five years, therefore it may be capable of providing other relay communications services for other countries' on the far side and for polar missions of the moon, in addition to the original plan for the Chang'e 4 lander and rover.

Queqiao, conquering a new territory as lunar relay communications satellite, presents great significance for future development. There is another relay communications satellite envisaged in the fourth phase of Chinese lunar program as well as in manned lunar missions. Moreover, the development and application for Queqiao has laid a well-founded technological foundation for future exploration and application about the Earth-moon Lagrange point.