Well, they have had a free system that they can see from orbit...I am sure that a cluster of a thousand+ rubber duckies are visiable from orbit!

But it might be an interesting project for oceanographers...a whole bunch of cheap devices which run on solar cells, and can stand up to the blizzards and 50+ C heat...ah...maybe not...sounds kind of expensive just for the base design. I'd go with rubber duckies again! :-)

ttyl