Whistling jauntily in brisk coded signals and probing the atmosphere far below with its long antennae-arms, Canada's "Topside Sounder" -- latest of the hard-working satellites -- will be orbiting early next year. The Canadian-built satellite is designed to send back information on the ionosphere -- the all-important shell of ions and electrons surrounding the earth which, by reflecting radio waves, makes long-distance radio communications possible. Results from the 280-pound, yard-high sphere will be collected by Canada, Great Britain, the United States and other interested countries through 10 ground monitoring stations around the world. Canada's intrepid "whistler", due to die after one year in space, will, in its short life, add much to man's knowledge of his world, increase his ability to control it.