I am wondering how a very long range line-of-sight (LOS) broadband link can be maintained using directional tracking antenna at both ends, i.e. onboard and ground station. I am interested in RSSI (received signal strength indicator) instead of GPS based tracking, specifically with 15 to 20dBi onboard tracking antenna, preferably in C or X band.
Look into tracking antennas, particularly monopulse trackers often seen in radar applications. Arrays and reflectors with sum and difference patterns will be helpful. A great introduction is the Airborne Radar Handbook by George Stimpson. I would recommend running down a copy and adding it to your reference library.
You might also want to pay attention to the size of your tracking antenna on the airborne side. Gain practical is often limited by the available space on the vehicle. Here, X band is preferred because antennas for a particular gain are smaller. Also pay attention to the one way radar range equation so you have enough signal to make the link work.