[00:00.000 --> 00:05.480]  Why do we use satellites like these for long distance communication?
[00:15.500 --> 00:19.860]  Communicating with a satellite ranges from several buildings worth of highly specialized
[00:19.860 --> 00:24.280]  equipment to a couple hundred dollars worth of handheld amateur radio hardware.
[00:24.280 --> 00:29.760]  LEOSATs, only about 400 to 2,000 kilometers up, are even close by terrestrial standards.
[00:29.760 --> 00:33.600]  That means simple ham radios are often used to communicate with them. There's even one
[00:33.600 --> 00:37.260]  aboard the International Space Station that you can use to talk to the astronauts,
[00:37.260 --> 00:39.760]  if you catch them at the right time and they're not too busy.
[00:39.760 --> 00:44.020]  The complexity with communicating with LEO satellites is actually their orbital speed.
[00:44.060 --> 00:48.740]  LEOSATs are moving so quickly that we have to use antennas that track them as they move
[00:48.740 --> 00:53.020]  rapidly across the sky. This tracking can be really difficult due to the uncertainty
[00:53.020 --> 00:57.320]  of the satellite location and how fast they move in and out of view.
[00:57.320 --> 01:02.100]  A satellite coming in and out of view from horizon to horizon is called a pass. Passes
[01:02.100 --> 01:06.900]  are the only times that we can actually communicate with satellites. Management of LEO satellites
[01:06.900 --> 01:12.580]  especially revolves entirely around these concept of passes and maximizing their effectiveness.
[01:12.580 --> 01:17.960]  For instance, it doesn't help having an imaging satellite over the target location if you didn't
[01:17.960 --> 01:22.560]  have a pass before it got to that location to tell it to take that photo. Similarly,
[01:22.560 --> 01:27.640]  you have to wait until the next pass to be able to actually bring that data back down to ground.
[01:27.640 --> 01:33.040]  Passes are roughly about 5 to 10 minutes for LEO satellites. This duration changes
[01:33.040 --> 01:37.260]  drastically based on where the ground station is located. Near the equator, the Earth's
[01:37.260 --> 01:41.500]  rotation actually causes the ground station and the satellite to lose contact with each
[01:41.500 --> 01:45.700]  other faster. But near the poles, Earth's rotation is negligible and the satellite is
[01:45.700 --> 01:50.700]  actually in view for almost twice as long, if not more. Essentially, the higher the latitude
[01:50.700 --> 01:55.120]  of your ground station, the more useful your passes actually end up being. This is the
[01:55.120 --> 01:59.700]  most ground stations are located in the higher latitudes, even in the very hard to reach
[01:59.700 --> 02:05.000]  locations. One of the best examples of that is Svalbard, a Norwegian archipelago that's
[02:05.000 --> 02:10.380]  extremely close to the North Pole. It has over 100 commercial ground station antennas,
[02:10.380 --> 02:13.420]  making it the largest commercial ground station in the world.