OT: Dumb Physics question

Even geosync communications satellites have fuel to "move around". They require fuel to keep station and a reserve (of fuel) to put them into a higher orbit when they're decommissioned. Of course they also have to get to their nominal orbit, in the first place.

Even military satellites are subject to the laws of physics.

There is a lot of energy in an orbiting satellite and it certainly is not practical to change the orbit "at will". It is possible to make small changes in velocity to slowly move satellites that are in the same orbit to a different relative distance, as is done in the GNSS constellation, or to slowly move a satellite to a different geostatonary position e.g. at a certain phase in its technical lifespan.

However, there is no such thing as "moving to whatever spot of trouble this month". That would consume far more fuel than can practically be carried onboard a satellite.

I believe that's a relatively new thing- they used to just leave them there, but it's getting kind of crowded.

--sp

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Normally the task of getting to the nominal orbit is performed by the final stage of the rocket, which is then detached. In some cases the satellite's own propulsion system is used for this, to finish the job when the rocket stage malfunctioned and underperformed. However, that will always cut into the useful life of the satellite, which ends when the stationkeeping fuel has been nearly exhausted. The owner will often claim a write-off in such cases to cash the insurance money, and the insurer sells the satellite to another (or the same) operator with a reduced projected lifespan.

Note that stationkeeping is required not (only) to correct small errors in the orbit, but also to counteract effects of the gravity of the sun and moon. When nothing is done, a geosynchronous orbit with a zero inclination will perturb during the satellite's lifespan to an orbit with an inclination of about 7 degrees, even when the orbital period remains the same. (satellites are sometimes left in such orbits for some years when the station keeping fuel is exhausted long before they technically cease to function. they can still be useful for purposes where dynamic tracking is not really a problem)

Note that the geostationary orbit has a circumference of over 250.000 km.

When you put 1000 satellites in it, they still are about 250 km apart on the average.

When "it is getting crowded" comes up, what is meant is that it getting more difficult to keep operational satellites at the distance of about

3 degrees that is required before the same frequency can be re-used in satellite-to-home applications.

Seems more than likely.

At will, but not without bounds, obviously.

Then a sensible choice of which of the six orbital parameters to change in order to achieve the required coverage would be a good idea.

--sp

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Best regards,  
Spehro Pefhany 
Amazon link for AoE 3rd Edition:            http://tinyurl.com/ntrpwu8 
Microchip link for 2015 Masters in Phoenix: http://tinyurl.com/l7g2k48

Depends. Changing the orbital plane is always super expensive, but if it's just moving along the one orbit, there's a tradeoff of transfer time vs. delta-V required. You don't necessarily have to get there in one day--that's what SR-71s are for. (No, wait.....)

Cheers

Phil Hobbs

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That is what I tried to explain. Some posts above there was the suggestion that "those military satellites can be moved at will, to the place of trouble this month" and while it may be possible to do that a couple of times, they will quickly burn all their fuel that way, no matter if they are civilian or military. So even military operators probably are smart enough to just send up a couple of satellites in different positions so they don't have to be moved all the time.

(and of course there are many clever orbit types that allow parttime coverage of large areas without having to change the orbit parameters)

On Tue, 26 Jan 2016 14:24:05 -0500, Spehro Pefhany Gave us:

Nope. When a bird gets released from its initial booster, it has to get to its position on its own, and has a reserve to make minor adjustments throughout its life to keep it there.

It is quite recent (15 years) requirement to transfer a geosynchronous satellite into the "graveyard" orbit slightly above the geosynch orbit.

On 26 Jan 2016 19:31:44 GMT, Rob Gave us:

Note where it states that it makes its way to it final orbital position ten days after the initial launch from Baikonur.

It placed itself into orbit and Loral Space Systems has been doing it that way for decades.

If you consider a swinging pendulum, the potential energy when it at rest is equal to the kenetice energy when it is at the lowest level and traveling at the max speed. Why can't the same idea be used to work out the speed of a satellite?

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If forty years is "relatively new", OK. Geosync parking spaces are damned expensive and it doesn't help to have space junk wandering through your parking space.

The swinging pendulum contains a fixed amount of energy, and repeats a cycle. The orbiting kilogram cannot be de-orbited without doing considerable work (like, energy input). Similarly, a falling object has work done on it (by the force of gravity).

So, the pendulum, with no energy input, is conserving energy. Stopping an orbit requires energy input, so it's not a closed system and doesn't conserve energy. The falling object, if it were to penetrate to the Earth's center, is a problem that requires some careful calculating; one issue, is that gravity gets reduced when you fall inside a massive sphere. Another issue, is that a 10cm object doesn't have its entire volume (and mass) at R=0 when it gets to the center.

The final orbit insertion has to be done by the bird itself. It doesn't have a booster at that point.

No, its period will not stay the same (as designed). The Earth's gravity is not uniform. Satellites well all drift in orbit into a gravity well over the Atlantic, IIRC.

They won't stay there and will eventually interfere with other birds.

I found the error. Seems the escape velocity from the earth is 11,186 meters per second which is very close to the 11,200 number I got using the conservation of energy idea. So, it looks like escape velocity can be worked out using the swinging pendulum idea and conservation of energy, and orbital velocity will be somewhat less..

.

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The fuel required to counter-act that effect is much less than the requirement to keep the inclination at 0. So satellites can remain in inclined orbit at the same longitude for much longer than they can be kept in geostationary orbit.

Of course this is only useful for communication satellites, not for direct-to-home broadcast satellites that need to be geostationary because the users don't have steerable antennas. Skimming through lists, it appears to be much less common to operate satellites in such orbits than it was 10 years ago, maybe because of changed use of satellites. (lots of uses have been replaced by internet or private fiber connections)

On Tuesday, January 26, 2016 at 5:54:16 AM UTC-5, DecadentLinuxUserNumeroUno

Depends on the satellite.

Dan

There are some satellites that are in syncrorous orbits. But there are also satellites that are in polar orbits and at much lower orbits than the synchronous satellites.

Dan