Why must communication satellites travel 35000 kilometers above earths surface?

Answer 1

The requirement is to place the satellite in such a way that it appears motionless in the sky, as seen from the surface of the earth. This is a great advantage in satellite communication, because if the satellite appears stationary, then the dish antenna on the ground doesn't have to move to follow the satellite ... the dish can be aimed once, and can stay in the same position permanently. In order to have the satellite appear motionless, it has to follow the 24-hour rotation of the earth. The orbital period of any satellite's revolution (around a much larger body) depends only on the average orbital distance. For the earth, the period of a 238,000-mile orbit (where the moon is) is about 27 days, and the period of an orbit that averages about 22,400 miles is 24 hours. So a communications satellite in an orbit with this average distance will complete one revolution around the earth in 24 hours. Wherever it is right now, it will appear in the same exact place 24 hours from now. This is a "geosynchronous" orbit. But that's not good enough yet. The orbit may have the right average distance, but it may still be very eccentric, ranging from close-in to way-out in the course of 24 hours. If that's the case, then it will move faster when it's closer in, and slower when it's farther out. Watching it from the earth, it'll appear to move back and forth like a pendulum, returning to the same position every 24 hours but making a complete left-right swing every day. This still wouldn't be useful for stationary ground-based dish antennas. So another restriction on the orbit is that it must not only be at the correct average distance, but it must also be very close to a circular shape, so that the satellite's speed in the orbit is nearly constant. And there's yet one more requirement that the orbit has to satisfy. Consider this in your imagination: There can't be an orbit where the satellite circulates over, say, a little 20-mile circle around the North Pole. A satellite orbit has to revolve around the "whole earth", which is a clunky way of saying that the center of the earth has to be in the plane of the orbit. The orbit can "incline" as much as you want ... the satellite can stay over the equator all the time, or swing from North pole to South pole and back again, but the center of the orbit always has to be at the center of the earth. Now you can see the final requirement for a communications satellite: If the orbit is inclined to the equator, then the satellite will appear to swing above and below its average location in the sky every 24 hours, which also makes it hard for a stationary antenna on the ground. The orbit has to be oriented at 'zero inclination', meaning it lies directly above the equator at every point. Now, finally, with a nearly circular, equatorial orbit, of exactly the right size, the satellite appears motionless in the sky, and all those little 18-inch TV dishes on the neighborhood rooftops can be pointed once at the satellite and never need to move. A satellite in this orbit is not only "geosynchronous" (24-hour orbital period), but also "geostationary" ... motionless relative to a point on the earth.

Answer 2

They are positioned in orbits that are

-- over the equator

-- as perfectly circular as possible

-- 22,236 miles above the surface of the earth.

In that kind of orbit, the satellite appears to remain motionless above one point on the equator,

and ground stations can use stationary antennas that don't have to be constantly moved to

follow the satellite.

Answer 3

An artificial satellite has a purpose, something that it is designed to do. So to fulfill its purpose, it has to be launched into an orbit that will allow it to work. So many communications satellites are in 23,000 mile high geosynchronous orbits, so that they orbit the Earth at exactly the same speed that the Earth turns -thus remaining "motionless" in the sky relative to a point on the Earth.

But that's pretty far up; if the transmitter has a very low power, like a cell phone, the satellite needs to be much lower, which means that the orbit will be faster. So you may need a series of satellites so that a satellite will swoop down near the atmosphere and scoot across the Earth, and then soar high into space and get back into position for another pass.

Answer 4

They DON'T have to; they can orbit at any altitude above the Earth's atmosphere.

However, the math for a 35,000 KM orbit (22,500 miles) is kind of neat. At that altitude, a satellite will orbit at the same speed that the Earth turns, and so a satellite will seem to hover in a fixed position in the sky. This is VERY convenient, because we can built small expensive satellite dishes that have motors and gearing to track the apparent motion of a low satellite across the sky, and to reacquire the next satellite after that one sets, OR we can build big, cheap dishes that do not require any tracking ability - because the satellite seems to hang motionless in the sky. Which one of those do you think might be cheaper?

Answer 5

They don't. The satellite, and the communications link, can work just fine, with the satellite

in any old orbit at all.

But the geostationary orbit is done as a great convenience to you for your little TV dish on the

corner of the house, and anyone else down on the surface who wants to communicate through

the satellite.

If the satellite does not appear stationary in the sky, then every antenna that wants to use

that satellite must move to follow it. And if it's in a truly pesky orbit, then the satellite rises

and sets periodically, and isn't even available for some part of the time.

Geostationary orbit means fixed, stationary antennas down on the ground, which ... trust me ...

are way simpler and cheaper.

Answer 6

Satellites are placed in geostationary orbit at 35,786 kilometers above the Earth's surface to match the rotation of the Earth, allowing them to remain fixed relative to the ground. This orbit provides continuous coverage for communication purposes without the need for the satellite to constantly adjust its position.

Answer 7

Satellite patterns are the result of Gravity. Particular patterns are "designed" to cover applications.