Why do we use satellites like these for long distance communication?
Communicating with a satellite ranges from several buildings worth of highly specialized
equipment to a couple hundred dollars worth of handheld amateur radio hardware.
LEOSATs, only about 400 to 2,000 kilometers up, are even close by terrestrial standards.
That means simple ham radios are often used to communicate with them. There's even one
aboard the International Space Station that you can use to talk to the astronauts,
if you catch them at the right time and they're not too busy.
The complexity with communicating with LEO satellites is actually their orbital speed.
LEOSATs are moving so quickly that we have to use antennas that track them as they move
rapidly across the sky. This tracking can be really difficult due to the uncertainty
of the satellite location and how fast they move in and out of view.
A satellite coming in and out of view from horizon to horizon is called a pass. Passes
are the only times that we can actually communicate with satellites. Management of LEO satellites
especially revolves entirely around these concept of passes and maximizing their effectiveness.
For instance, it doesn't help having an imaging satellite over the target location if you didn't
have a pass before it got to that location to tell it to take that photo. Similarly,
you have to wait until the next pass to be able to actually bring that data back down to ground.
Passes are roughly about 5 to 10 minutes for LEO satellites. This duration changes
drastically based on where the ground station is located. Near the equator, the Earth's
rotation actually causes the ground station and the satellite to lose contact with each
other faster. But near the poles, Earth's rotation is negligible and the satellite is
actually in view for almost twice as long, if not more. Essentially, the higher the latitude
of your ground station, the more useful your passes actually end up being. This is the
most ground stations are located in the higher latitudes, even in the very hard to reach
locations. One of the best examples of that is Svalbard, a Norwegian archipelago that's
extremely close to the North Pole. It has over 100 commercial ground station antennas,
making it the largest commercial ground station in the world.