How to transmit the obtained energy back to the earth?

Pluto is located in the Kuiper belt on the edge of the solar system, with an average distance of nearly 5.9 billion kilometers from the sun. Even the light from the sun takes nearly 5.47 hours to reach the surface of Pluto. In order to transmit data back to the earth from such a distant place, the New Horizon detector needs to overcome many difficulties.

First of all, the biggest difficulty affecting signal transmission is distance. To better understand this problem, let's give a simple example. Suppose two people are talking, and when the distance is 1 m, one person can hear each other clearly without talking too loudly. But when the distance between two people reaches 100 meters, even if one person speaks loudly, the other person may not listen. Because the signal strength is inversely proportional to the square of the distance, the signal strength decreases exponentially with the increase of the distance. The relationship between the probe and the earth is like two people talking. When the distance between them reaches 4.8 billion kilometers, the signal strength of the detector acceptable to the earth is very limited. It is difficult to detect this signal unless an ultra-high sensitivity receiving antenna is used.

Secondly, the background noise will also directly affect the signal transmission. Back to the previous example, suppose two people are in a noisy stadium. Even if they are 1 m apart, loud noise will affect their communication. If the distance between them is extended to 100 meters, then no matter how loudly you speak, the other party can't hear you clearly. In our? The universe? Many stars, including stars, emit electromagnetic signals of various frequencies. In the solar system, the electromagnetic waves emitted by the sun have become a huge noise source. These noises add up to the background noise of the universe. With the increase of distance, the transmitted signal of the detector is easily submerged in the background noise of the universe, which is more difficult to detect.

To make matters worse, the signal will be distorted due to interference during transmission. Because the distance is too far and the interference is huge, the binary data in the signal is easy to make mistakes. The signal that clearly represents 0 may become 1 due to interference, and the signal that clearly represents 1 may also become 0 due to interference. Once the signal is distorted, there will be serious problems in data recovery.

Although there are many problems in signal transmission, NASA has successfully received the signal from Pluto, and the unprecedented trailer picture announced the success of Pluto exploration program. Now the question comes, how does NASA solve these problems?

Concentrate energy emission as much as possible.

Because the detector is too far away from the earth, any signal that is not aimed at the earth is unacceptable. In order to improve the efficiency of transmitting signals, the transmitting antenna of the detector must be aimed at the earth. Only by concentrating the emitted energy as much as possible can the earth receive the detector signal 4.8 billion kilometers away.

Spread spectrum technology

In order to overcome the influence of cosmic background noise on the signal, the detector uses spread spectrum technology in the process of transmitting the signal. The so-called spread spectrum technology is to change the original signal originally used for transmission into a new signal with wider frequency through spread spectrum technology. When the signal power is much lower than the noise power, the stability of the transmitted signal can be greatly improved by expanding the frequency spectrum of the signal transmission. By demodulating the received spread spectrum signal, the desired signal can be easily obtained from the background noise.

channel coding

Even if a useful signal is obtained, the signal may still be distorted due to interference during transmission. At this time, channel coding is needed for error correction. In order to increase the reliability of communication, people use channel coding to add specific information to the original signal, so as to achieve the purpose of judging and correcting errors at the receiving end. To give a simple example, suppose someone transports a batch of glass. In order to ensure that glasses are not broken during transportation, people usually use cushions such as foam sponge to protect glasses. Of course, there are gains and losses. Because of the buffer, the box that could hold 100 glasses may only hold 80 glasses, and the rest of the space is occupied by the buffer. Similarly, the signal transmission rate will also be reduced due to the addition of specific error correcting codes (90 bits may be error correcting codes when transmitting 100 bits of data, and only 100 bits are really useful data).

By adding specific error correction information to the original signal, even if errors occur in the transmission process, the signal can be found and corrected at the receiving end. Generally, the channel coding method used in deep space communication is LDPC code (low density parity check code). Through this coding method, the signal sent by the detector can be transmitted to the ground receiving station more stably.