Finding offset using AM

I believe this might work, but you may want to go up a bit in frequency to keep the antennas smaller than the craft. You only need a single frequency and antenna on the leader. On the follower you have two slightly diverging yagis and two receivers. Detect and take the difference of the receiver amplitudes. When it's zero you're lined up, otherwise the polarity will tell you which way to steer. Don't follow too closely, the near field will mess things up. Something like 20 wavelengths or more should do it. The amplitude might be used to keep a constant distance from the leader. Complication: In an aircraft you also have to consider the up-down dimension.

This was the basis for the German Junkers JU-88 night fighter radar, the pilot centered a spot on a CRT until he came into visual range. The spot was actually a circle, the nearer to the target, the smaller it got. Ever seen a photo of them with that cumbersome hardware in the nose?

Come to think of it, a modulated LED or laser and cross-eyed optical detectors might work too.

- YD.