Communication with the far side of the moon - satellite relay "Tseyutsyao" (Magpie bridge)

The main thing in a complex technical project is to organize a stable connection between all components.

So the first part of the Chang'e-4 mission was the organization of a communication channel with the devices on the far side of the moon. Six months ago, the era of relay radio communications began with the far side of the moon.

How this was done, what data are transmitted between the devices on the moon (the landing module and the rover) and the relay satellite beyond the moon, how communication with the MCC on Earth (via the space communication center) is organized further and with what data rates are described here.

The communication channel is a thin invisible thread connecting the back of the Moon and the Earth, which should be easy and easy to implement, but also reliable in terms of node failures, since in the absence of this only communication channel, the devices on the back of the Moon will be lost and provided themselves (for some time, automation).


It all started with a legend.




Problem

One of the main problems in the study of the far side of the moon is the problem associated with the organization of communications, since the devices on the far side of the moon are not available for communication directly from the Earth (the reverse side is never visible from the Earth due to the phenomenon of tidal capture), therefore, for transmission signals on the channel "Earth <-> the reverse side of the moon" requires a separate and special satellite relay for communication.

With the help of a repeater satellite, it was planned to accomplish the following tasks:

- organize (the first in the world) data transmission channel “the reverse side of the Moon <-> Earth”;

- to organize tracking of the Chang'e-4 descent vehicle and data transmission when the apparatus performs lunar maneuvers and the landing procedure on the far side of the moon;

- transfer to the MCC on Earth full control of the devices launched to the surface of the lunar side of the Moon (the Chang'e-4 module and the Yutu-2 rover) using the tracking, telemetry and transmission command subsystems (TT&C - tracking, telemetry and command subsystem) ;

- receive scientific data independently via separate communication channels from the Chang'e-4 module and the Yutu-2 rover, send this data to the MCC on Earth;

- conduct their own scientific experiments (using an onboard low-frequency spectrometer) and transmit the obtained scientific data to the MCC on Earth;

- to shoot on-board space camera and transfer photos to the MCC on Earth;

- maintain the data channel “the reverse side of the Moon-Earth” in an operational mode for at least 5 years after the satellite began to operate in orbit beyond the Moon (the maximum estimated service life is up to 10 years);

- the software and hardware of the satellite is designed to work with equipment not only for one Chang'e-4 project, since the lifespan of the Chang'e-4 launch module is one year, and the Yutu-2 rover is designed to work for three months but it has already almost doubled this time, so that the repeater satellite, after the end of the Chang'e-4 project, will be further involved in new research and for organizing communication channels with new devices on the far side of the moon.

Decision

A unique repeater satellite was developed, which was planned to be put into halo orbit around the special gravitationally stable Lagrange point Earth-Moon L2, from which it will maintain direct visibility with the Earth and the far side of the moon at any time, withstanding a temperature drop of up to 300 degrees Celsius.

Engineers at the Chinese Academy of Space Technology (CAST) had only 30 months to develop a relay satellite.

In December 2015, design work began, two years later the final prototype of the satellite was already produced, which, after testing and testing, was prepared for launch into outer space.

The space communications team worked together with a group of scientists and engineers who developed and sold lunar vehicles for the Chang'e-4 project - the Chang'e-4 landing module and the Yutu-2 rover, the communication with which was the main task of their project.

In 2015, the team of space communications engineers of the Chinese Academy of Space Technology already had experience in the development of satellites, long-range space communications equipment and the management of lunar spacecraft:

• November 7, 2007, the first Chinese lunar satellite Chang'e-1 was launched;
• On October 1, 2010, the Chang'e-2 research satellite was launched, which worked on lunar orbit until July 9, 2011, and then left it to reach the Lagrange point L2 of the Sun-Earth system (in 1.5 million kilometers from Earth) for conducting scientific experiments;
• Chang'e-3 landing module and Yutu rover, which were successfully landed on the visible side of the moon on December 14, 2013, and Chang'e-3 landing module is still communicating with the MCC on Earth.

However, the communications engineers of the Chinese Academy of Space Technology had a tight deadline, the launch dates of the repeater satellite and its entry into the working orbit could not be disrupted, since in any emergency or abnormal operation of the satellite elements - the launch of the Chang'e-4 spacecraft with the lunar rover "Yutu-2" on the far side of the moon would not have been possible at the planned time, delayed or even canceled.

In general, 2018 was a very busy year for Chinese space signalmen.

But they did it all - the 425-kilogram repeater satellite called “Tseyuqiao” (translated as “The Magpie Bridge”) at the Chinese Academy of Space Technologies designed, produced at its own facilities (engineers from the Netherlands were connected for a joint scientific additional load - installation on the satellite-repeater of a special low-frequency telescope-spectrometer) and launched at the scheduled time and with full functionality.

The CAST-100 platform from the Chinese Academy of Space Technologies was used to create the Tseyuqiao repeater satellite. Specialists from the Chinese company DFH Satellite Co., Ltd were involved in its design and production . (DFHSat), which works closely with CAST and is owned by China Spacesat.

The satellite platform CAST-100 includes:

• three-axis stabilization system, navigation control system, thermal monitoring system;
• one-component power plant with 100 kg of anhydrous cesium fuel (hydrazine), with a total thrust of 130H (Newtons), 12 engines - 8 engines of 5N each (two on each of the lower sides of the cube) and 4 central engines of 20N each;
• a power supply system for on-board systems consisting of two solar panels (the maximum output power of the solar battery is about 800 W) with an area of ​​3.8 m2 and a high-energy lithium-ion battery pack of 45A / h.

The total weight of the repeater satellite is 425 kg, it has a cuboid shape with a size of 1.4 m × 1.4 m × 0.85 m, its body consists of an aluminum honeycomb sandwich structure.

The following systems were added to the CAST-100 satellite platform as a payload (primary and secondary):

1) the main load is a radio relay.

The Tseyuqiao Satellite Repeater Radio Repeater System operates in the X-band and S-band.

The X-band is used to communicate with the Chang'e-4 landing module and the Yutu-2 rover - four channels are organized with a data transfer rate:

• direction "rover \ landing module <-> satellite-relay" 256-280 kilobits / s;
• direction “satellite-relay <-> rover / landing module” 125 bit / s.

The S-band is used to transmit data to Earth - one channel is organized with a data transfer rate of 2 megabits / s.

TTC & C telemetry data (USB + VLBI) is transmitted at a speed of 1000/2048 bps.

The structure of the radio relay includes a unique parabolic antenna with a diameter of 4.2 meters, which opens like an umbrella after the satellite-relay enters a working orbit.

2) additional load:

• The Netherlands Low-Frequency Experimental Radio Telescope (NCLE) with three five-meter antennas, with which low-frequency radio emission from the early Universe will be recorded to study its structure;
• A wide-angle laser reflector for measuring the distance between a spacecraft and a ground station, developed by Sun Yat-sen University in Guangdong Province in southern China and will be used to take the longest laser distance measurement in the world between a relay satellite and an observatory on Earth;
• a camera that is also planned to be used for shooting asteroids falling on the back of the moon;
• To interest the public in space exploration and moon exploration projects, the China Space Agency (CNSA) invited everyone to write down their wishes for moon and space exploration, and the relay satellite carries the names of tens of thousands of participants in this event and their messages.

Scientific load:



Here is a camera mounted on a repeater satellite:



Example of a photograph from a repeater satellite:



Laser reflector (drawing):



Tseyuqiao satellite repeater:



How the elements of a repeater satellite (antenna, batteries and spectrometer) are revealed:



With engineers in the laboratory (for scale):



Antenna (antenna unit itself on the left, already mounted on the satellite on the right):



Unfolded in tests:



Scientific equipment (three antennas of a low-frequency radio telescope, each of them 5 meters long when sliding):







Copy 1 to 3 relay satellites in the space museum:





About the parabolic umbrella antenna and its creation

Engineers at the Chinese Academy of Space Technology have developed several antenna options for the repeater satellite, including in the form of an umbrella with a diameter of 420 centimeters in open form.

In the design and manufacture of such an antenna were involved ... textile technologists and watchmakers.

Only by the joint work of communications engineers and specialists in the watch and textile industry in the laboratory of the Chinese Academy of Space Technologies were able to solve the difficult task of grouping the smallest elements of the metal mesh of the antenna and its 18 edges so that it could be folded to the right size for transportation and launch , and in outer space she could turn around like a beach umbrella.







Antenna elements withstand temperature changes of more than 300 degrees Celsius.

Dozens of tests and testing programs for antenna components and its general assembly were carried out in special bench laboratories of the Chinese Academy of Space Technologies before installation on a satellite-relay.

The antenna includes a special drive mechanism to control direction tracking, which allows you to control the direction of the antenna in the design range in increments of 0.2 °.

Visualization of the deployment of an antenna in outer space:



Antenna elements are affected by low-temperature environmental conditions. The temperature of some ribs, tension cables, wire mesh and other components on the antenna will drop below -200 ° C, which had to be taken into account in its production.





Problems, limited budget for development and production

During the development of the repeater satellite, the engineers as much as possible inherited the design of the Chang'e-3 telecommunication system, so there were almost no problems in implementing the shoulder of the “Satellite-relay <-> devices on the far side of the Moon” communication channel.

By the repeater satellite, the received and demodulated data from the Chang'e-4 landing module and the Yutu-2 rover are combined in accordance with the communication protocol and transmitted to the MCC to Earth via a direct space communication system.

The main problem in the implementation of the shoulder of the communication channel "Satellite-relay <-> devices on the far side of the Moon" was that the maximum distance of this channel is about 80,000 km, and the signal attenuation at this distance reaches 210 dB. Therefore, engineers had to find a balance between the bandwidth of the communication channel, the dynamic change in the positions of three devices (satellite, landing module and rover), as well as the radio relay power control system.

The optimal working scheme for them was as follows: telemetry data is transmitted at any distance from the devices on the surface to the relay satellite, but the transfer of scientific data (large amounts of data) is organized when the orientation of two devices (satellite-rover or satellite-landing module) is relatively stable and the power of all devices is enough to organize a channel with the required bandwidth.

For example, Yutu-2 rover antennas need to be configured to point to a repeater satellite to correctly send and receive control signals, while the rover's solar panels should be optimally tilted to receive a lot of sunlight so that maximize power generation at the time of data transfer.



In the shoulder of the communication channel "Satellite-relay <-> MCC on Earth" after the satellite-relay enters into a halo-orbit around the Lagrange point L2 of the Earth-Moon system, the accuracy of pointing the relay antenna is calibrated (distance of 480,000 km).

During the calibration process, the relay antenna of the relay satellite is directed to the Shanghai Astronomical Observatory of the Chinese Academy of Sciences. From the Earth, the signal is tracked using a ground antenna with an aperture of 65 meters. The test results show that the relay antenna with a high gain has a pointing deviation of less than 0.1 °, which meets the requirements for this project.

The repeater satellite conducts a daily self-test of its systems - it checks the key functions and performance indicators (RF modulation characteristics, acquisition time, transfer delay and data format) of the relay system. Test results are sent to the MCC on Earth, where they are analyzed for compliance with design requirements.

Calibration and testing are necessary, since the relay satellite located in the working halo orbit, due to thermal deformation and other factors, the actual orbital orientation of the relay communication antenna will have deviations in the guidance, which must be corrected and their changes checked.

Estimated thermal deformation of antenna elements (in mm) at different temperatures:



The costs for the design, manufacture and launch of the Chang'e-4 mission apparatus were strictly limited. And the possibility of even slightly exceeding the costs was not that it wasn’t, but on the contrary, the engineers were motivated to minimize parts and elements of the project, refine and expand their functionality in order to reduce the total cost of production and reduce costs.

Therefore, the repeater satellite was originally designed with a relatively low weight (425 kg) so that the costs of its production and launch were minimal.

Have backup relay satellites been made? This is an interesting question - as an option, several prototypes were made, but only one, the most tested one, was launched.

What will happen if in outer space the relay satellite goes down? Of course, in its composition there are several overlapping elements that are most critical for the project - parts of the on-board computer, power supply system and radio relay.
If the satellite reaches its working orbit beyond the moon, then its operability will be maximum and its service life can be up to 10 years.

The biggest problem that a repeater satellite can have is fatal damage to the antenna, so it was made in the form of a giant umbrella with a mesh inner coating, which micrometeorites can damage without disabling the functional part. And the chance that a large meteorite will collide with a relay satellite is very small.

However, if this happens, then within 30 days it will be possible to restore the communication channel “Earth on the other side of the Moon” by launching a new relay satellite and putting it into working orbit beyond the Moon.

The engineers of the Chinese Academy of Space Technology had the following new tasks after the creation of the relay satellite:

• prepare the relay satellite for launch on a launch vehicle and accompany the launch;
• track its trajectory and direct the relay satellite to the moon;
• perform a lunar maneuver to transition to a working halo orbit around the Lagrange point L2 of the Earth-Moon system;
• test the communication channel with the satellite until the lunar devices of the Chang'e-4 mission enter the orbit of the moon at the end of December 2018;
• hold the first communication session with the Chang'e-4 lander at the end of December 2018, the Chang'e-4 lander is in the orbit of the moon;
• January 3, 2019 to receive data from the Chang'e-4 landing module, which will begin the landing procedure on the far side of the moon;
• conduct the first communication session with the Chang'e-4 landing module and the Yutu-2 rover, which are located on the surface of the far side of the moon.

Launch and operation in outer space



on May 21, 2018: the satellite relay Tseyuqiao (Magpie Bridge) was launched from the Xichang Chinese Cosmodrome.

Start at MCC on Earth:








Payload compartment:





Tseyuqiao repeater satellite flight path:







June 14, 2018: Tseyuqiao satellite repeater entered a halo orbit around the Lagrange point L2 of the Earth-Moon system, approximately 65,000 km from the Moon, becoming the first in world communications satellite operating in this orbit.


This photograph was taken from the Tseyuqiao repeater satellite:



Where the Moon, Earth and satellite elements are visible:



The repeater satellite can remain in its orbit for a long time due to relatively low fuel consumption, since the gravity of the Earth and the Moon balances its orbital traffic.

Being in its orbit, the relay satellite can “see” both the Earth and the back of the moon. From Earth, the orbit of a repeater satellite looks like a halo of the moon.

The concept of deploying a relay satellite in halo orbit was first put forward by American space experts in the 1960s (the main contribution to the calculation of such an orbit was made by Robert Farquhar, NASA mission planning specialist more than 50 years ago - in 1968), but was first implemented by Chinese space engineers only in 2018.

Libra points of the Earth-Moon system:















Establishing communications with the Chang'e-4 lander and the Yutu-2 rover

Six months later, after the Tseyuqiao relay satellite reached its working orbit beyond the Moon, the second working phase of the “ Chang'e-4 "- launch of the spacecraft" Chang'e-4 "with the lunar rover" Yutu-2 "on board into outer space.





December 8, 2018: The Changzheng-3B booster rocket with the Chang'e-4 station was successfully launched from the Sichan space center in China.

Chang'e-4 Station Flight Path:



After 110 hours, Chang'e-4 Station reached the Moon and entered its orbit.

It was then that the first combat testing of the Tseyutsiao repeater satellite began by organizing a communication channel with Chang'e-4 station when it flew over the far side of the moon:





Test modes and operation of the Tseyuqiao repeater satellite and the Chang'e-4 spacecraft "(The descent module and the rover)



When the Chang'e-4 station started the landing procedure on January 3, 2019, here at the MCC on Earth, they switched to full-fledged work with the Tseyuqiao relay satellite to receive telemetry and photos from the descent module" Chang'e 4".

January 3, 2019: Chang'e-4 lander makes landing in the Karman crater on the far side of the moon. The Chang'e-4 lander contains the second Chinese lunar rover Yutu-2, a modernized analogue of the Yutu rover.

The first images of the far side of the moon in the landing zone, as well as thousands of frames from the Chang'e-4 lander’s landing camera, are obtained through the Tseyuqiao satellite relay in the MCC on Earth, and combining these wonderful videoes of landing on the far side of the moon:

Video landing procedures on the far side of the moon:


After completing all stages of the successful landing procedure and installing independent communication channels with Chang'e-4 devices (the landing module and the rover), the era of exploration of the far side of the moon began.





But all this could not have been possible without the Tseyuqiao relay satellite and the communication system organized with the help of it:





Chang'e-4 project communications organization scheme:







Telemetry data from the Chang'e-4 landing module and the Yutu-2 lunar rover to the Tseyuqiao repeater satellite, which then transmits them to Earth at the Beijing Aerospace Control Center, and then to the MCC, which delays operators from receiving data on Earth for up to two to three minutes.

At the Space Communications Center:





At the Mission Control Center of the Chinese Academy of Space Technology:







Question: Is it possible to transmit live from the surface of the far side of the moon with the help of the satellite repeater Tseyuqiao?

Answer: Theoretically, this is possible, but the current communication channels cannot meet the requirements for streaming video in real time.

The Chinese Academy of Space Technology is doing everything possible so that the repeater satellite can work as long as possible, providing future communications for probes and devices from other countries if they intend to explore the back of the moon during the satellite’s life.

This is a peaceful scientific project, which everyone can join.

Moreover, during the maneuvers by the repeater satellite to achieve their working orbit, the engineers of the Chinese Academy of Space Technologies were able to optimize the number of maneuvers by engines, which saved 16.8 kg of fuel, which can now be used later on if necessary to correct its orbit and extend service life.



To understand that there will still be new studies on the Moon - the current scientific satellite constellation on May 5, 2019 .