TheShadow asks:
"Air to air range 35,000ft to 75,000ft ??"
All other things being optimal it's about 600 miles at 50,000 each end. It would go up another 100 to 150 miles or so if one end was at 75,000'. The distance to a tangent to the earth line of sight is roughly D = sqrt(2*h), where D is in miles and h is in feet. The limiting distance is found by Dt = sqrt(2*h1) + sqrt(2*h2).
As you approach that limiting distance you get some strange effects often called "picket fencing" as the signal and its bounce off the Earth constructively and destructively compete with each other.
That's academic since the range goes to about 300 miles at night at the South Pole in the Northern Summer and the Zephyr becomes Zephyr a glider and then a static object on the surface. You MIGHT get away with near continuous coverage if the Zephyr could be programmed to stay 500 miles North of the pole and follow the tiny patch of Sun it might find. I'm not sure that would be enough Sun to keep it up there with the very oblique angle of the Sun's light on its solar cells.