The ionosphere only reflects in the 2-30 or so MHz range. The problem from doing this from space is that there is interference from ground based transmitters. When you do it from the ground it is much easier. The second problem is that from the ground, the returns are true range. From space you have to subtract the satellite altitude. One advantage of the satellite system is that you can do soundings all over the earth with just one satellite which is inclined to cover most of the earth in a number of orbits.