[lostdog]

You can do even better with radar. If you place a set of radar reflectors around the tower at known locations, then you can detect them from the booster and triangulate the distances to a precise position. Plus, radar gives you relative velocities, so your speed and roll rate estimatimates get even more precise. I bet you could get down to millimeters with a setup like this.

[aeyes]

Is this really viable given all the electromagnetic interference the rocket motor exhaust plumes are generating?

[generuso]

Falcon-9 uses radar altimeters for determining vertical "distance to go" during landing.

While a sideways position error of even ten meters is not fatal, it is critical for the rocket to be quite close to zero altitude when deceleration brings the velocity to zero. (Any residual error must be dealt with by the shock absorbers, and their capability is modest.)