Queqiao-2: How does the “postman” on the Moon “chat” with the Earth?

Did you know that the Earth and the far side of the Moon are like two "old stubborn people" who always face each other and cannot talk directly? In order to enable them to communicate, scientists have invented a clever device called a "relay satellite", which is like a "postman" floating in space to help them deliver information.

The Queqiao-2 relay satellite recently launched by my country is one of them. It is like a "communication tower" built in the lunar orbit, dedicated to transmitting information to the probe on the back of the moon. So, how does Queqiao-2 communicate with the earth and the back of the moon?

Sensitive "ears": high-gain antenna

Queqiao-2 has a super cool "golden umbrella", which is not an ordinary umbrella, but a 4.2-meter-diameter deployable antenna, like a sensitive "ear", which can focus weak signals from the earth and the back of the moon and transmit them to the receiver of the probe. Queqiao-2 is also equipped with a 0.6-meter-diameter S/Ka dual-band parabolic antenna. This configuration enables the relay satellite to efficiently send and receive data on different frequency bands.

Frequency band used by Queqiao-2: S/Ka dual-band

Speaking of frequency band, this is a professional term. Simply put, frequency band is the "channel" for signal transmission. Queqiao-2 uses multiple frequency bands, just like we can choose different operators when making a phone call. Different frequency bands have different characteristics and advantages, which can make the signal transmission farther and more stable.

Queqiao-2 uses the S/Ka dual-frequency for communication, with the S band used to communicate with the Earth and the Ka band used to communicate with the probe on the back of the Moon. The S band has a frequency range of 2-4 GHz, a longer wavelength, and a stronger penetration ability, so it is suitable for long-distance communication between the Earth and the Moon. The Ka band has a frequency range of 10.7-12.75 GHz, a shorter wavelength, and a wider bandwidth, so it can transmit more information.

The S band is like the frequency of a slow grandfather telling a story. It is relatively stable and reliable, and is suitable for sending control commands or less urgent scientific data, just like sending a warm reminder to the lunar rover to "remember to take a family photo today."

The Ka band is like a supercar, with super-fast speeds. It is used to transmit large amounts of data, such as high-definition photos or videos of the lunar surface. It is like emergency express delivery, which can quickly send the beautiful lunar scenery captured by the probe to our home on Earth.

The S/Ka dual-band parabolic antenna of Queqiao-2 is like two different "microphones" and "earphones", one for listening carefully to the instructions of Mother Earth, and the other for telling the "explorers" on the moon loudly and quickly, and also bringing the stories on the moon back to Earth. In this way, no matter how much or how little information there is, whether it is urgent or slow, Queqiao-2 can respond flexibly to ensure smooth and efficient communication between the moon and the earth.

Powerful "megaphone": high-power transmitter

In order to clearly transmit information to the probes on the Earth and the back of the moon, Queqiao-2 is equipped with a high-power transmitter that can send out strong microwave signals. Even after long-distance transmission, the signal can maintain sufficient strength.

Error correction "password": ensure information integrity

During the transmission process, microwave signals may be subject to various interferences, resulting in information distortion. To solve this problem, scientists added a special "code" to the signal, called forward error correction code. This code is like a checkpoint that can detect and repair errors that occur during the transmission process, ensuring that the information can be transmitted intact to the detectors on the Earth and the back of the moon.

Conclusion: Keep exploring and you will never be alone

Queqiao-2 will provide reliable communication links for the Chang'e-6, Chang'e-7 and Chang'e-8 lunar far side probes, supporting these probes in conducting scientific exploration and research and helping us uncover more mysteries of the lunar far side.

Source: Chongqing Radio Science Popularization Experience Center

Audit expert: Yang Yaohui

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