Satellite internet: GEO and LEO
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Where no cable reaches, a dish on the roof can bring the internet down from space. Follow one request through a geostationary satellite 35,786 km up, where light alone needs 477 ms for the round trip, then through a low Earth orbit satellite about 550 km up, where it needs only 7.3 ms.
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01Why can a home dish for a geostationary satellite stay fixed in one direction?
At 35,786 km above the equator one lap takes 23 h 56 min, the same time Earth takes to turn once, so from the ground it seems to hang still.
02A request and its reply through one GEO satellite cross how many radio legs of about 35,786 km?
Dish to satellite, satellite to ground station, then the reply goes ground station to satellite and satellite to dish: four legs, 143,144 km in all.
03About how long does light need for those four GEO legs?
143,144 km at 299,792 km/s is 477.5 ms. Real GEO round trips are longer still, usually 550 to 700 ms.
04Why does a LEO dish have to hand over from one satellite to the next?
At about 550 km a satellite laps Earth in about 96 minutes and stays usable for at most a few minutes, so the phased-array dish steers its beam to the next one.
05Why does heavy rain weaken satellite internet?
Rain loss grows with frequency and rain rate (ITU-R P.838), so Ku and Ka band links slow down or briefly drop in a downpour, for GEO and LEO alike.
المسرد
- Geostationary orbit (GEO)
- A circular orbit 35,786 km above the equator whose lap equals one turn of Earth, so the satellite seems fixed in the sky.
- Low Earth orbit (LEO)
- Orbits a few hundred to about 2,000 km up. Starlink's satellites fly at about 550 km and lap Earth in about an hour and a half.
- Ground station (gateway)
- A site with big dishes that talks to the satellites and connects them to the internet over fibre.
- Bent pipe
- A satellite that simply repeats the signal from one side to the other, without routing packets itself.
- Latency
- How long data takes to get from one place to another; here mostly the time radio waves need to cover the distance.
- Handover
- Switching a connection from one satellite to the next as they move across the sky.
- Phased array
- An antenna made of many small antennas; shifting the timing (phase) of each one steers the beam without moving parts.
- Rain fade
- Loss of signal strength when heavy rain absorbs and scatters high-frequency radio waves.
الملاحظات والمصادر
- Heights are not to scale: a GEO satellite is about 65 times higher than a 550 km LEO satellite.
- Light times use c = 299,792.458 km/s and the shortest legs, with the satellite straight overhead; real slant paths are longer.
- Measured round trips add processing, scheduling, queues and the route beyond the gateway: GEO usually 550–700 ms, Starlink usually 25–60 ms.
- Orbit times from Kepler's third law with WGS 84 constants: GEO 23.934 h (one sidereal day), 550 km 95.6 min.
- Rain attenuation follows ITU-R P.838 and P.618: the higher the frequency and the heavier the rain, the larger the loss.
- 192.168.1.0/24 is a private (RFC 1918) home range and 203.0.113.10 an RFC 5737 documentation address.
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