How do satellites distinguish between all the billions of signals emitted by all the devices in the world?

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How do satellites distinguish between all the billions of signals emitted by all the devices in the world?

In: Technology

They’re on different frequencies and spectra. Additionally, signals can have “targeting protocols” or bits of the signal that specify which device is supposed to receive it, so that other devices can filter it out. It’s not that different from how your cell phone distinguishes between calls meant for you versus the other million cell phones in range.

They pay attention to the signals that are aimed right at them.

Very few of the billions of devices in the world have enough power to be picked up by a satellite 23,000 miles up.

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Satellites work on certain frequency ranges that differ from those used by most devices on the ground – usually satellites use higher frequencies and don’t overlap to much with those used on the ground. Additionally, most of the devices on the ground are not even close to powerful enough to send a signal that could get picked up by satellite, those signals are not much more than background noise that gets filtered out easily.

To get around the power requirements and to reduce overall signal noise for everyone often enough signals are somewhat targeted/directed at the satellite so it becomes easy for it to distinguish the strongest signal that is meant for it. Also, certain encoding techniques are used so the satellite can read the ‘signal ID’ to discern whether the signal is meant for it or not.

Most devices don’t actually broadcast directly to a satellite.

Cell phones talk to towers.

I *believe* GPS devices just look for the constellation (and do a bunch of math — it’s complicated but one-way).

As a result, most of the signals that are sent up to a satellite are owned by a large entity. Electromagnetic wave generally have a ‘frequency’ associated with them. It is pretty easy to filter out frequencies.

Analogy time — you can skip this if you already know about the frequency-domain. Think of pushing a kid on a swing — you want to push when the kid is at the top of the swing for maximum effectiveness. It turns out that the time it takes for the swing to go back and forth is only dependent on the [length of the chain](https://en.wikipedia.org/wiki/Pendulum_(mathematics)#Rule_of_thumb_for_pendulum_length), assuming you aren’t doing something crazy like launching the kid around the bar in a full circle. So, let’s assume you very monotonously providing little pushes every n seconds — a set frequency (we’ll say you are willing to run around very quickly to be in the right spot). This frequency of pushing corresponds to the frequency of the wave. The satellite should have some filter that corresponds to the length of the chain. If the chain-length and pushing frequency aren’t matched pretty well, only a little swinging will happen — you might be pushing the kid when they are already at the bottom of the swing, since they are already moving pretty fast your little push won’t help much or it might even be against their current direction.

So it is possible to, say, allocate a bunch of frequencies to a company or particular application. They don’t have to monitor all other signals, they just have to only stick to their frequencies. If an entity is emitting in some frequency that they aren’t allowed to, and they are doing it loud enough, the FCC will likely get involved. To continue the analogy — the only thing that prevents you from going to the playground and pushing other people’s kids on the swings is that eventually somebody will probably call the cops.

There are all sorts of tricks that can be used once you have this selective frequency behavior. For example, if you aren’t sure that only one person will use a frequency (maybe you want to give everybody a frequency list they can pick from, so that we can share more effectively. Maybe you are worried about malicious people trying to drown your signal out — jamming), you can just listen on your frequency and change if you hear somebody else. You just have to arrange that with the receiver.