In addition, the critical frequency for the ionosphere is ~10MHz (that is, above that frequency, you can't really bounce a meaningful amount of energy off the i

In addition, the critical frequency for the ionosphere is ~10MHz (that is, above that frequency, you can't really bounce a meaningful amount of energy off the ionosphere, most passes through). 10MHz = a wavelength of 30m. So in general, you'll only be able to detect things about 100ft or larger in size (and that's a lower limit-- practical is probably bigger). I'm not at all surprised that you'd need line of sight to track a plane. Knowing that there's something there is easier, but tracking is trickier.