How do space probes transmit data back to Earth across vast distances?
In: Planetary Science
By radio, very slowly. A space probe like Voyage has a radio transmitter that emits a fairly narrow beam, sort of like a laser but for radio waves. This beam is pointed at Earth, where a large receiving dish picks up the signal. Each bit is transmitted for a longer period of time to ensure that the faint signal is picked up clearly; Voyager has a bitrate of ~160 bits/sec. Aside from the scales involved, its the same technology as your car radio.
Radioeaves, with a few special tricks. Bigger dishes on both the transmitting and receiving radios to increase the amount of power effectively received, more sensitive receivers with lower internal noise to hear s cleaner signal, and using lower symbol rates to increase the amount of energy per bit received which can negate some of the random noise.
The last is sort of like how you can communicate across a further distance by yelling slowly rather than talking at a normal speed and volume – is harder for the background noise to mess up words that are functionally much longer in duration than a given random “noise”.
Imagine you and your friend were on opposite sides of a valley and wanted to communicate in Morse code, which consists of patterns of long and short beeps to indicate letters. Since you’re so far away, you probably will miss most of the beeps and will likely miss most of the message. Instead of using short and long beeps, you decide to play 1 minute of Beethoven’s 9th for 1 minute to mean a short beep and 1 minute of Death Metal to mean a long beep. If the receiver is knows when to start listening, they will likely be able to tell which type of music is playing at least once per minute. Thus they will be able to receive the message, just much much slower.
This technique is known as forward-error correction and is one of the simplest methods to reduce data transfer errors over a noisy channel. There are many many other methods, founded in math, to detect and compensate for errors that are all collectively known as Error Correction techniques.
There’s a few ways first they use a frequency that has a long transmission wavelength. For example AM radio has a wavelength that measures about 2 meters in length.
They also use high power transmitter the more power you put into the transmission the longer it can go.
Here on Earth with with all the interference that we have a long wire antenna can transmit a hundred Watts across the world. NASA talks to Mars with about 250 Watts. The Voyager probe has a 1,000 Watts transmitter and it doesn’t have to deal with any interference.
The other thing is it sends all data in a more simplified form slow but long transmissions. So a simple status update could take 2 hours to transmit.
Add to that it sends the same signal at minimum of three times. The reason for that each listening station could hear a different part of the message. Or have errors due to interference and sending the transmission three times those errors don’t line up so you will get at least one clean signal out of it.
The other way is on the receiving end. Having multiple stations listening for that satellite all over the world and transmitting to that satellite from all over the world means that any signal that is received can be picked up in chunks and then processed so that the signal becomes clear and understandable.
An amateur radio we do this. Where we will ask for information to be sent tripled that way we get a clear reception of every part of the message since atmospheric and other interference won’t show up at the exact same space every time.
And just kind of put the point home, think of these radio communications like light. In the old days ships would communicate by flashing Morse code a simple on and off signal across the water. Now when the ship was communicating this way to shore it was easy to understand because there’s only one signal of that style and the person on shore was looking for that specific signal. Now if you did the same thing communicating using flashlights in a city it would be harder to understand because of all the interference from the other lights.
In short using radio waves and multiple transmissions of the same signal on the transmitter side. And multiple receivers on the planet side looking for a very specific frequency in a very specific region of space. All the other interference kind of goes away cuz it’s filtered out. Leaving just the original signal with errors which get corrected by the multiple transmissions.
As radio. Radio, being electromagnetic radiation, travels as fast as it’s possible for any information to– the speed of light. The probe is designed to transmit as narrow a broadcast as possible and to keep each bit of data going for a while so that our receivers on Earth have as good a chance as possible of picking up the signal, which means it’s still a pretty slow connection.
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