Shape of a footprint.

[123tim]

I've been studying satellite foot prints, and am intrigued by the way that they
sometimes seem to be shaped so accurately to fit a certain area. Others seem to shoot far into the ocean far away from any land. Also Hawaii sometimes seems to have a small "spot" of high power directed to it with some satellites and not others.
I've also been looking at Atlantic Bird 1 footprints. It seems to be sliced right down the Midwest. (You can see it here: http://www.eutelsat.com/satellites/125e_popd.html#) It seems from the map that at one point you could experience high power (52 dbw), and a mile down the road, you get nothing. Am I reading this wrong? I must be.
Could anyone explain how signals could be shaped in this way? I wonder how and why it's decided where to send the most power?
As you can tell, this isn't really an urgent question. Just something that I wonder about.
Thanks.

 

[PSB]

It has A LOT to do with the curvature of the earth, think about it as pointing a flashlight at a soccer ball!

Of course Spot beams are exactly as you said, little local beams of RF energy pointing at a certain area!

 

[Anole]

Those maps may be idealized pictures, not based on real signal levels in the field.
You'd need ten's of thousands of signal readings just across the US to make those maps.
I'll bet they are built from 100 (or maybe a few hundred) readings, and the rest is based on the projected antenna pattern.

Some of the funny curves to the footprints may be the result of local obstructions. (mountain ranges)
It would be amusing to overlay 'em on topographical maps to see.


two other things -

The maps are often so small you can't really find major cities on them.
The map mentioned above is about the worst I've seen.

As for the "mile down the road" comment, I see what you mean.
The only thing I can think of is that along the line down the US, your view of the bird goes below the horizon.
Of course, in the real world, your height above average terrain would determine where you could see the bird from.

 

[123tim]

Pete and Anole,
Thanks for the answers. Now I know a little more. I guess that would explain some of the drop off at sea level too.
As far as the map being the worst ever seen, I'm not surprised. I have great skill at finding the worst of everything to work with.
Thanks again,
Tim

 

[digiblur]

You haven't seen the pictures of the birds that have dishes shaped like the United States yet?

 

[123tim]

That must be why satellite airtime is so expensive. The U.S. Dishes must be hard to make with Florida sticking way out like that

 

[sharris]

Actually my understanding is that when a TV provider uses a satellite for uplink (time purchased by the major broadcaster of course), any problems with signal and or finding the satellite is communicated to the satellite owner. so in effect there are thousands of signal level readings given back to the provider each year. The projected map (estimation on a new satellite being put into orbit) is based on mathematics, i.e., a beam that starts out this wide at the satellite at this strength with the satellite being at a known altitude will give this result (simple math).