more than one sat in a position

[JulienPDX]

Newbie question here

I understand that DIRECTV has more that one satellite at 101W.

How can my dish antenna see more than one satellite at that location? What I mean is, wouldnt' you only be able to put the satellite "Right behind" another one if they are in the exact same position? Also, have any of the sat companies thought about putting satellites all around the earth so that people with all different views of the sky can get a reading?

:confused:

[Mainstreet]

Newbie question here

I understand that DIRECTV has more that one satellite at 101W.

How can my dish antenna see more than one satellite at that location? What I mean is, wouldnt' you only be able to put the satellite "Right behind" another one if they are in the exact same position? Also, have any of the sat companies thought about putting satellites all around the earth so that people with all different views of the sky can get a reading?

:confused:


The satellites are out there at about 22,000 miles from earth. At that distance, they can be a few miles apart and still be within the same orbital 'slot'. Someone else can elaborate more.

[ericha]

The satellites are out there at about 22,000 miles from earth. At that distance, they can be a few miles apart and still be within the same orbital 'slot'. Someone else can elaborate more.

That's right. The antenna "sees" a certain angle in the sky, and anything at the right frequency with enough power get's received. It doesn't matter if the satellite transponders are all on one "bird", or on separate satellites.

For a given frequency, the larger the antenna, the narrower the "look angle". Engineers call this "antenna gain", as it concentrates the transmitted power in a narrower area (for transmitters), or it intercepts more of the transmitted power (for receivers). Note that as the frequency increases, the same size antenna will have a narrower beamwidth, and more therefore more gain.

As to the original question of why not put satellites all around the earth, there already are satellites in just about every direction! But there are some limitations about how close you can make them:
-When we think of geosynchronous satellites, it's easy to think that they are just "hanging" in space. But they aren't, they are really travelling at about 1000 miles per hour--the same as the equator of the Earth (the circumference of the Earth is just about 24,000 miles, and it takes 24 hours :) ). The orbit isn't perfectly circular, and there are other bodies with a gravitational pull (most notably the Moon & the Sun) that pull the satellites a bit. So you can't put the satellites too close, or they might block each other's signals, or worse yet hit each other!

-For DBS, you want to have a fairly small receive antenna, for both aesthetic & cost reasons. The smaller the antenna, the larger the "look angle" into the sky. So other satellites that are too close can cause interference. It's actually a bit more complicated than that, depending on the transmitted power levels, transmitter antenna gain, frequencies, expected receiver antenna gain, and expected receiver filter selectivity. The people at the FCC & similar regulatory bodies in other countries work pretty hard to try to keep this from being a problem.

-Satellites are used for many other things besides DBS. Telephone calls, broadcast video, satellite news gathering, weather forecasting, mapping, and defense are just a few of the other uses. So all of the "slots" can't be used for DBS.

-Putting up a satellited is pretty expensive, too. I don't have exact numbers, but I would expect that around $100 million is about the minimum. I would think that the DBS satellites are pretty close to the maximum, since they are very high power & have lots of transponders; I seem to remember an recent Echostar satellite costing around $500 million or so. So for one company to put up a DBS satellite every 2 degrees around the planet would cost around $90 billion. Assuming that the insurance costs are negligable (they aren't), and that you could get issue junk bonds at 10% for all of this, you'd have to have a cash flow of at least $750 million a month, just for the satellites. And then you have to pay for programming, promotions, marketing, warrenty, salaries, & profit!
From the Echostar DBS SEC form 10-Q (see: http://ccbn.tenkwizard.com/filing.php?repo=tenk&ipage=2423493&doc=1&total=26&TK=DISH&CK=0001042642&CK2=1042642&FT=0&FC=000000&BK=FFFFFF&SC=ON&TC=FFFFFF&TC1=FFFFFF&TC2=FFFFFF&AL=0033FF&VL=6699CC#004), Echostar's business model works reasonably well with about 10 million subscribers & 5 satellite locations (or is it 6 now? :) ). To have 180 satellite locations, they'd have to have 360 million subscribers, each willing to pay about $50/month. That's about 3 times the number of households in the US. So if you got 100% of everyone in the world that could afford satellite TV, perhaps you could do it--but then competition (or regulators) would screw it up!