[Denvercoder9]

The thrust-to-weight ratio of an empty Falcon 9 is far too high to hover. Burn the engines too long and it'll just ascent again.

[mhandley]

Their F9R test vehicle could hover. Isn't that essentially a mockup Falcon 9 first stage, but with only the centre engine?

https://www.youtube.com/watch?v=ZwwS4YOTbbw

[Intermernet]

I wonder how feasible / useful it would be to have realtime best-guess altitude from accelerometers / bouys / laser line-of-site relays etc to give the rocket up to date info to adjust thrust / vector to the drone-ship.

Or do they already do this?

[jccooper]

Almost certainly they have really good GPS and inertial sensors on both. That should be enough. Any line of sight like laser or visual can't be trusted due to weather.