Eli5: how does voyager send a message 15 billion miles with ~200watts of power and we struggle with cellphones and satellites signals right here on earth?
In: Engineering
You’re comparing things that aren’t really comparable even though they seem superficially similar. They’re just different systems that are designed to do different things.
Cell towers and cell phones talk to each other using frequencies that can quickly send and receive a lot of data but the drawback is the signal strength drops pretty quickly, and they have limited transmission power. Their antennas transmit equally in all directions which means they don’t transmit strongly in any direction. Cell signals can also be blocked by various structures and even just the terrain. Also, if there are a lot of users connecting to the same towers, that can eat up the bandwidth and slow down data traffic.
The Voyager spacecrafts are sending signals through open space. There’s basically nothing between them and Earth to absorb or block a signals – no atmosphere, no buildings, and no hills or mountains. They communicate with Earth using special high-gain antennas that direct all the signal strength in one narrow beam, and here on Earth, we have giant 70 meter dishes to send and receive signals that are specifically designed to communicate with spacecraft in deep space. In other words, we built a dedicated system called the [Deep Space Network](https://en.wikipedia.org/wiki/NASA_Deep_Space_Network) just to talk to far-away probes, so there are no competing signals.
In short, cell phones and space probes aren’t really comparable because they’re not designed to be. Cell phones are designed to send and receive a lot of data short ranges and to balance the needs of a bunch of users in the same area. They have limited transmission power and signals can be blocked by air, buildings, or land.
Space probes are sending and receiving signals through empty space with nothing in the way to block signals and with dedicated antennas designed to transmit strong signals and to receive weak signals.
We have dedicated radio telescopes that point directly at Voyager. The parabolic shape of the telescopes causes the meager radio waves produced by the probe to become concentrated on a receiver which is then able to pick up the signal. The Voyager probes themselves also have a parabolic transmitter which allows them to send a parallel radio signal to Earth instead of having it radiate in all directions.
Cell phones don’t have enormous parabolic receivers, since that would be very impractical to carry around (the Voyager probes are the size of a bus). Likewise, we can’t put those parabolas on cell towers because they have to process hundreds of signals at once, which would require just as many receivers that need to be pointed at every device. Again, very impractical. Same again with satellites that communicate with satellite phones.
In addition to what others said there is the difference of speed. When we do make a connection with our cell phones the round trip is probably in the milliseconds range. Round trip to probes? Minutes. Think for acknowledgments from Mars rovers was like 12 minutes? Imagine calling or texting someone and you don’t receive any reply for 12+ minutes. Very difficult to hold a conversation.
Your phone has an antenna approximately the length of your phone, pointed in a random direction trying to communicate with a tower that is trying to point in all directions. NASA has several radio telescopes up to 70 metres in diameter, which are pointed exactly at what they are communicating with, with the things they’re communicating with pointed at us.
A huge difference is the data rate between your cell phone and Voyager 1. Data is being transmitted from Voyager 1 at about 160 bits per second. By comparison, most cell phones transmit data anywhere between 40Mbps to 10Gbps. In other words, the data being sent to your cell phone is *literally* a million times faster than what’s being transmitted by voyager.
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