[londons_explore]

You don't need to make a finely focussed beam... You simply need to be able to put nulls at each of the planes antennas.

If the attacker has two antennas on the ground, say 1km apart, and the plane is 1km up, then no real precision is required - there exists a phase offset between your two antennas where only one of the planes antennas picks up your signal, and the other antenna picks up nothing. If the plane where stationary, this could be found by a simple sweep of possible phase offsets.

If the plane is moving, it becomes harder to find and track the necessary offset, but if the plane is flying half way between the attackers ground based antennas, the offset is ~constant, so a sweep again starts to look doable...

[knorker]

So TX1 sends data headed for RX1, plus the TX2 signal phase-modulated to cancel out TX2 at RX1?

I don't know how much an aircraft shakes, but if I understand what you're saying then this is possibly even harder. You'd need to predict the positions in fractions of a wavelength, don't you? And atmospheric changes could possibly affect it too.

> If the plane is moving

In the air, they tend to. On the ground accurate positioning is less important to protect.