ZHANG Lihua,XIONG Liang,SUN Ji,CHEN Luojing

DFH Satellite Co.,Ltd.,Beijing 100094

Abstract:Queqiao relay communications satellite was developed to provide relay communications services for the lander and the rover on the far side of the moon.From entering into its halo mission orbit around the Earth-moon libration point 2 on June 14,2018,it has operated on orbit more than fifteen months.It worked very well and provided reliable,continuous relay communications services for the lander and the rover to accomplish the goals of Chang’e 4 lunar far side soft landing and patrol exploration mission.The on-orbit operation status of Queqiao relay communications satellite is summarized in this paper.

Key words:Chang’e 4 mission,Queqiao,relay communications,far side of the moon

1 INTRODUCTION

The far side of the moon is a unique place for scientific investigations,for example,it is an ideal location for performing low-frequency radio observations.Before Chang’e 4 mission,no spacecraft had landed on the surface of the lunar far side which is never visible from the Earth.Chang’e 4 was a Chinese lunar far side landing and patrol exploration mission,composed of a lander,a rover (Yutu 2) and a relay communications satellite.For the lander and the rover to work on the far side of the moon,communications links with ground stations cannot be established without a relay satellite,as shown in Figure 1.So the development and deployment of a relay communications satellite was the prerequisite for the accomplishment of the Chang’e 4 mission.

Figure 1 Relay communications links for Chang’e 4 mission

As an important and innovative part of Chang’e 4 mission,a relay communications satellite,named as Queqiao,taken from Chinese mythology signifying a bridge formed by magpie birds to reunite lovers in heaven,was developed first and launched into orbit before the launching of Chang’e 4 lander and rover.It can provide real-time and delay-time relay communications to the lander and the rover when operating on the lunar far side to maintain contact with Earth stations.

A halo orbit around the Earth-moon libration point 2 (EML2)is an ideal location for lunar far side relay communications,which permits continuous communications coverage at the cost of longer transmission distances.Queqiao is the first spacecraft in the world to run on a halo orbit around the Earth-moon libration point[1].Up until now,it has operated on the mission orbit for more than fifteen months and provided reliable relay communications services for the Chang’e 4 lander and rover.

This paper summarizes the on-orbit operation status of Queqiao relay communications satellite from the orbit transfer to the current relay communications services for the Chang’e 4 lander and rover.

2 SYSTEM OVERVIEW OF QUEQIAO

Queqiao relay communications satellite is a small spacecraft and was developed based on the CAST100 small satellite platform of DFH satellite Co.,Ltd.and experiences from other Chinese lunar missions.The platform is composed of six subsystems：structure and mechanisms,thermal control,housekeeping and onboard data handling (OBDH),guidance navigation and control (GNC),TT&C,power supply.The payloads include relay communications subsystem,antenna subsystem,and scientific and technology demonstration subsystem.Figure 2 shows the system composition of Queqiao relay communications satellite.Figure 3 shows the on-orbit configuration of the spacecraft.

In order to maintain continuous visibility with the lander and the rover operating at the South-Pole Aitken Basin area on the far side of the moon,a southern halo orbit around EML2 with Z-amplitude of 13,000 km was selected for Queqiao relay communications satellite.The final determination of the specific orbit parameters took the relay communications coverage,eclipse time,etc.into consideration.This orbit has an orbital period of fourteen days,making two revolutions in each lunar“day”.

Figure 2 System composition of Queqiao relay communications satellite

Figure 3 Flight configuration of Queqiao

Due to the unstable dynamics of halo orbits,maneuver navigation and execution errors can cause the actual trajectory of the spacecraft to diverge significantly from its desired trajectory over time.For this reason,if we want Queqiao to stay there for several years,some small incremental propulsive orbit maintenance maneuvers will be necessary to maintain the path.As repeated orbit maintenance has to be performed to keep the Queqiao in the required halo orbit,the cost for orbit maintenance is also an important factor which needed to be considered for the selection of the orbit parameters.

Both real-time and delay-time relay can be provided for the lander and the rover of Chang’e 4 mission during landing and surface operation.However the farthest relay communications distance from the Earth to the Chang’e 4 lander and rover is 79,000 km,so in order to ensure relay communications capability,a large relay communications antenna with a diameter of 4.2 m,shaped like an umbrella,was developed.It is the largest communications antenna ever developed in the world for deep space exploration missions.There are four relay communications links between Queqiao and Chang’e 4 lander or rover.X-band used for these relay communications.The relay communications data can be transmitted at different frequencies and different data rates under different working modes,as shown in Table 1.An S-band data transmission system was used for the data transmission between Queqiao and the ground stations.The X-band data transmission system can be used as a backup so as to increase the reliability of the data transmission.

3 FLIGHT PROCEDURE

After two and half years’ development,on the morning May 21,2018,the Queqiao relay communications satellite was launched from the Xichang Satellite Launch Center.It successfully separated from the LM-4C launch vehicle after 1530s’flight.One minute later,two solar arrays were unlocked and deployed to provide solar power for the satellite.25 min later,the umbrella-like relay communications antenna was unlocked and deployed to full size.Then the normal fight attitude of the satellite was established and the journey to the moon started.After separating from the launch vehicle,Queqiao was placed on a lunar swing-by trajectory that takes a longer time than a direct transfer but saves energy and reduces the propellantrequired and the mass of the spacecraft.On the evening of May 21,2018,the midcourse correction maneuver was performed to make sure Queqiao was on the right trajectory to the moon.On the evening May 25,as the most critical flight event on the journey to the mission orbit,the perimoon braking was performed successfully at about 100 km above the moon surface,assisted by a 203 m/s impulse burn given by four 20 N thrusters where the fire time of the thrusters was 912 s.This swing-by maneuver threw Queqiao toward the Earth-moon L2 point,at about 65,000 km distance from the moon.On June 14,2018,by means of a 66 m/s Δvmaneuver,Queqiao captured and entered into its final halo orbit according to the designed orbit parameters.It became the first spacecraft to operate in a halo orbit in the Earth-moon system.The perimoon braking and the captured halo mission orbit are shown in Figure 4.

Table 1 Major technical performance parameters of Queqiao relay satellite

4 ON-ORBIT TEST

After entering into final halo orbit,a series of on-orbit tests were performed for the Queqiao relay communications satellite.By July 2018,the tests of the platform were completed,including the tests for GNC,power,TT&C,OBDH,and thermal control.The test results showed the performance of the platform can meet the mission requirements.

As a very narrow beam angle is used,the pointing accuracy of the relay communications antenna is very important for the accomplishment of relay communications mission.From June 16 to July 6,2018,the pointing performance of the relay communications antenna was tested by means of a 65 m diameter ground antenna located in Shanghai,as shown in Figure 5.The onboard relay communications antenna was pointed to the ground test antenna through sweeping controls realized by means of the GNC system.In the meantime,Queqiao’s forward relay communications link sent single-carrier signals to the ground antenna and the ground equipment performed measurements on the received signals,then the pointing error of the relay communications antenna can be calculated.Resultsfrom seven tests showed that the pointing error of the relay communications antenna is less than 0.1°.It can meet the requirement of less than 0.2°.

Figure 4 Flight trajectory and captured halo mission orbit

Figure 5 Pointing performance test of relay communications antenna

Before launching of the lander and the rover,the relay communications links were tested in July 2018.The relay communications links between the Queqiao relay communications satellite on orbit and Chang’e 4 lander/rover electrical models on the ground were established successfully.This situation was very similar to the work status of the lander and rover when on the far side of the moon.The major difference was the communications distance which was compensated for during tests.Detail tests were performed on the forward relay communications link and the backward relay communications link.Both links worked well and test results showed the performance of the relay communications system could meet the mission requirements.

After the Chang’e 4 lander and rover were launched they entered into moon orbit.Relay communications links were tested again during December 2018.The forward relay communications link with 125 bit/s data rate and the backward relay communications link with 1.4 kbit/s data rate between Queqiao in halo orbit and the lander in lunar orbit were established successfully.The results from four tests showed the forward relay link and the backward relay link worked well.It was confirmed Queqiao had the capability to provide relay communications services for the lander and the rover to maintain contact with the Earth stations during their landing,separation and surface operations.

5 ON-ORBIT OPERATIONAL STATUS

5.1 Relay Communications Services During Descent and Landing

As per the schedule,Chang’e 4 probe performed a soft landing on January 3,2019.Before the start of the descent operation,the forward relay communications link and the backward relay communications links with 1.4 kbit/s data rate were established normally.Then the engine for descent of the probe fired and the descent and landing operation commenced according to flight procedure.During the descent phase taking about 700 s,Queqiao had been tracking and pointing its relay communications antenna on the descent trajectory of Chang’e 4 probe and the relay communications links worked continuously.The operational status of the lander was obtained at the ground control center through the 1.4 kbit/s backward relay link in real time.When the descent camera started work,its images were also sent to the ground stations by the backward relay link at a data rate of 50 kbit/s,as shown in Figure 6.

Figure 6 The descent process of Chang’e 4 lander and rover under support of Queqiao(The picture on the left top was taken by descent camera of the lander)

Under the support of Queqiao relay communication satellite,Chang’e 4 lander and rover landed on the surface of the far side of the moon successfully.The landing site was the South-Pole Aitken Basin,one of moon’s most scientifically rich regions.After landing on the moon,the camera on the lander took the first near-distance picture of the lunar far side surface and the picture was sent back to the ground stations via Queqiao,as shown in Figure 7.

5.2 Relay Communications Services During Surface Operations

After the goal of soft landing was accomplished,under the support of Queqiao,the ground stations sent commands to let the rover unlock and separate from the lander.The relay communications link with the rover was also established successfully.Under the control from the ground stations,the rover descended down along the ladder to the lunar surface smoothly,becoming the first rover to explore on the far side of the moon.

Via Queqiao,the operation of the lander and the rover was remotely controlled on the far side of the moon by ground stations on Earth.The images,telemetry and scientific data from the lander and the rover were obtained continuously.Up to now,all the relay communications links at different data rates and frame length have been used.They all work well.

By September 2019,the Chang’e 4 lander and rover had worked on the moon for 10 lunar days.The lander and the rover have already exceeded their design life span of six months months respectively.The rover has traversed nearly 300 m distance on the moon’s surface.During the lunar night,they go into sleep mode.When the lunar day comes,they wake up and start operation via Queqiao relay communications satellite.During the lunar day,all the scientific instruments onboard the lander and the rover are switched on and perform scientific explorations.The scientific payloads include the cameras,a low frequency radio observation instrument onboard the rover and a neutron detector,as well as the cameras,a radar,a spectrometer and a particle analyzer onboard the lander.Over nearly nine months of observations,they have produced a significant amount of valuable scientific data.The amount of scientific data from the lander totals about 638 Gbits and the data from the rover is about 166 Gbits.All the data was received first by the X-band relay communications links of Queqiao and then sent back to ground stations via the S-band data transmission link.Both Chinese and international scientists have started to analyze the data and have published their initial results.As the rover roams around the Von Karman crater in the South-Pole Aitken Basin,more and more scientific results will be obtained,processed and analyzed.

Figure 7 The first image from the lander on the far side of the moon

Figure 8 Relay communications links used in lunar surface operations.

6 ON-ORBIT OPERATIONAL MANAGEMENT

On April 2019,the Chang’e 4 mission transferred into the routine on-orbit management phase.During the lunar day,Queqiao relay satellite provides relay communications services for the lander and the rover,including support for awakening and sleeping,performing exploration and conducting scientific observations.The ground station sends commands and uplink data to the Queqiao relay satellite first at a data rate of 1,000 bit/s.Then the relay satellite transfers the commands to the lander and the rover respectively through forward relay links with a data rate of 125 bit/s.Queqiao receives the telemetry and scientific data from the lander and the rover through backward relay links with data rates of 555 kbit/s and 285 kbit/s respectively.After demodulating the received data,the encoding and framing are performed together with the data produced by Queqiao itself,then the combined data is sent to the ground stations via the S-band data transmission system with a data rate of 2 Mbit/s or 4 Mbit/s.This process can be seen in Figure 8.

During the normal on-orbit operation phase,command,ranging and telemetry for Queqiao are performed by means of Chinese deep space TT&C network.With the use of a ground station in South America,in most cases,near continuous tracking (about one hour blank at most) and control of the Queqiao relay satellite can be realized.Up to now,more than 11,700 commands for the lander and more than 10,600 commands for the rover have been sent via the Queqiao relay communications satellite.The data from the onboard S-band data transmission system is also received by the ground stations located in Beijing and Kunming respectively.These belong to the ground application system of the Chang’e 4 mission,and are operated by the Chinese Academy of Sciences.

In addition to being a relay communications satellite,Queqiao also carries some additional payloads to perform scientific exploration and technical demonstration.The Netherlands-China Long-wavelength Explorer (NCLE) is one of them.The NCLE is an instrument performing astrophysical studies in the radio spectrum from 100 kHz to 80 MHz,to explore and to study the radio sky at very long wavelengths,particularly radio emissions from the Earth,the sun,and the large planets in our solar system and from the Milky Way.The payload is also a pathfinder experiment for developing new astrophysical observational techniques and radio telescopes in this frequency spectrum in space.

After the major mission objectives of Chang’e 4 lander and rover were accomplished,during the lunar night,the on-orbit tests for NCLE payload were also performed.The instrument was switched on and worked according to its designed scientific observation modes.The low frequency radio signals of the background environment have been obtained.

There are two cameras onboard the Queqiao relay communications satellite,one for the monitoring of antennas of the satellite,the other for the imaging of the Earth and the moon.These two cameras work normally.Figure 9 is a picture taken by the camera onboard Queqiao.

Figure 9 Earth and moon image taken by the camera onboard Queqiao

As a promising technique for deep space mission ranging applications regenerative pseudo-noise ranging capabilities were demonstrated successfully in May and June,2019.The digital USB transponder onboard the Queqiao relay communications satellite has the functions of regenerative pseudo-noise ranging.Regenerative pseudo-noise ranging test results showed that the ranging sensitivity increased more than 20 dB and the ranging performance improved significantly compared to traditional transparent pseudo-noise ranging.This demonstration laid a firm foundation for future applications.

7 MISSION ORBIT MAINTENANCE

Mission orbit maintenance or station keeping is a very important operation for the on-orbit operation management of the Queqiao relay communications satellite.Queqiao operates on a halo mission orbit around Earth-moon Libration point 2.The libration point orbits near collinear locations,including quasi-periodic halo and Lissajous trajectories,are inherently unstable and must be adjusted after operating for some time.It was already known that any change in energy from an unstable Earth-moon libration point orbit will result in a departure from this orbit,either towards the moon or in an escape direction towards the Earth or the sun-Earth regions.For the Earth-moon applications the stationkeeping of an orbit about libration point is more challenging than in the sun-Earth system,in part because of the shorter time scales,the larger orbital eccentricity of the moon,and the fact that the sun acts as a significant perturbing body both in terms of the gravitational force and solar radiation pressure.

While a variety of station keeping strategies have previously been realized for other missions,most notably for applications in the sun-Earth system.Only the ARTEMIS spacecraft[2]and Chang’e 5T1 spacecraft have entered into trajectories near the Earth-moon libration point,but the orbit type they used is the Lissajous orbit.The orbit type selected by Queqiao is the halo orbit and it is the first time use in the Earth-moon system.The station keeping hence is more complicated.Moreover,in order to ensure the performance of the relay communications,there are more strict position and orientation requirements for the halo mission orbit.

The propulsion system of the Queqiao relay communications satellite is a monopropellant hydrazine blow-down system.The propellant is stored in two equally-sized tanks of 70 L volume and either tank can provide propellant to any of the thrusters through a series of latch valves.Queqiao was launched with a dry mass of 343.7 kg and 105 kg of hydrazine propellant.The transfer to the halo mission orbit spent about 52.6 kg propellant.At the beginning of the station keeping phase,the remaining propellant mass was 52.4 kg.

Up to now,the Queqiao relay satellite has operated in the halo mission orbit for more than fifteen months.By September 10,2019,47 orbit maintenance maneuvers in total had been performed.On average,the orbit maintenance maneuver was performed every 9.6 days.The total propellant used during these orbit maintenance burns was 4.92 kg,only about 109 g per burn.All the orbit maintenances burns were completed successfully ensuring the halo orbit can be maintained to meet the requirements.

According to original design,theΔvbudget for maintenance maneuver requirements is about 36 m/s each year.During the on orbit operation phase,in order to achieve a longer operational life for Queqiao,the strategy of orbit maintenance was made with the goal of minimizing the propellant cost.A continuous-improvement control strategy was used for the orbit maintenances of Queqiao[3],as shown in Figure 10.By means of this control method,the maintenance maneuvers were performed to minimize theΔvrequirements while ensuring the continuation of the orbit for several revolutions downstream.

Figure 10 Continuous-improvement control method of halo mission orbit maintenance

According to the real orbit maintenance results in first year,the spentΔvwas about 18.1 m/s per year in total.It was only about half of the anticipated maintenance cost.Currently the remaining propellant is 47.5 kg in mass,which can support the Queqiao relay satellite operation in its halo mission orbit for more than 8 years.

8 CONCLUSIONS

As a critical part of the Chang’e 4 mission,Queqiao works very well and has provided a reliable,continuous relay communications service for the lander and the rover to ensure the success of Chang’e 4 exploration mission.Via Queqiao,the lander and the rover were controlled from ground stations,and a considerable amount of scientific data from the lander and the rover has been obtained.With the further investigations into the far side of the moon,particularly the Von Karman crater,the Chang’e 4 mission will enable scientists to discover more about the moon and deepen their knowledge about the early history of the moon and the solar system.

Under the support of Queqiao,the Chang’e 4 mission has already been seen as an amazing success and it is expected it will continue to achieve more scientific successes in lunar exploration in the coming years.

The design life of the communications relay satellite is more than 5 years.It could stay in its halo orbit for a longer time because less propellant was used for routine orbit maintenance.The longer the operational life of Queqiao,the more scientific results can be achieved by the Chang’e 4 lander and rover.

Apart from providing relay communications services for the Chang’e 4 mission,it is possible to provide relay communications services for other lunar far side exploration missions in the future.