I’m a little confused about how satellite communication works. My understanding is that there is a Line-of-sight wave from the user to the satellite and then a Line-of-sight wave from the satellite to the destination. I also have heard that whether a radio wave behaves like a ground wave or Line-of-sight wave depends on the frequency of the wave. It’s not intuitive to me why this is. It’s also not intuitive why we don’t communicate with ground waves. Is it because they are slower in spite of the fact that you’re substantially increasing the distance that the data has to travel?
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In no particular order, some have been addressed in other answers:
– The ionosphere is called that way because it gets ionized by solar UV radiation, freeing electrons from the atoms. Radio transmissions at frequencies between a few MHz and about 30MHz are reflected by those free electrons. You can take advantage of that to reflect waves back down and overcome Earth’s curvature, but it’s a disadvantage if you want to pierce through the ionosphere.
– On the other hand, frequencies above 30GHz are heavily attenuated by lower atmospheric layers, mainly the air in the troposphere.
– In between lays what’s called the radio window.
– At frequencies even lower than a MHz, waves can bounce back and forth between the ionosphere and the surface, traveling even further.
– In general, higher frequencies are attenuated more by the atmosphere.
– High frequencies also have more room for wide channels. It’s easy to allocate 20MHz wide channels around a 2.4GHz frequency. At 30MHz that doesn’t work well.
– A higher number of wider channels is what allows high data rates. Counter-example – submarine transmissions at a few dozen kHz can have data rates below 100 bits/s.
– Low frequencies need big antennas.
– A satellite has to be very frugal with the amount of energy it uses to transmit data, so the power received at the antenna on Earth’s surface is mindblowingly low. The sensitivity of modern GPS receiver chips is in the order of magnitude of billionths of a billionth of Watt. Any additional obstruction of the path can disrupt the channel.
A combination of the factors above makes authorities decide what frequency ranges to allocate for which uses. All of that is then used by whomever plans an application with radio waves to choose the most fitting transmission, as long as there’s any choice left. Ground waves offer a very limited amount of narrow, slow channels that can reach very far. Very few could use it and they would make those frequencies unusable by anyone else on the same continent, if they took advantage of the possible range.
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