Whoah! That's very interesting! Any idea how this is done? Is the transmitter horn/dish actually moving back and forth or is there some sort of arrangement of antennas utilizing diversity in some form? You say 'electronically steerable' so I doubt there is anything physically moving.
I saw something today that makes me wonder about this. I caught up to a black dually Chev in some corners along a lake, against a rock cut. I saw the TACC icon lock on and my speed started to reduce. Then the truck drove into a dense shadow and it sort of got lost for a moment against the dark rock, the truck being black and all. The TACC icon disappeared for a moment and the car surged ahead... then the lock was regained and I slowed down again. This all happened in the space of a couple of seconds.
The corners aren't tight enough to lose coverage of the vehicle ahead - at least, it hasn't happened before and I drive this highway regularly. It made me wonder if the camera had been involved with the radar and hadn't adjusted exposure for the shadow quickly enough to retain its visual lock (like my eyes!). However, if the radar is sensitive to direction, perhaps there was some confusion about which way I was going relative to the truck... having only just locked on before unlocking.
It's super-interesting stuff and I wish the average Joe (or pedantic SOB's like myself) could be given access to the finer details of the programming logic (not the code itself), because these sorts of clunks are probably quite explainable with better understanding of the system.
I don't know how much we know about Tesla's radar, but most serious modern radars are Phased Array - including the Delphi automotive unit that I suspect Tesla may be using.
A Phased Array radar has a transmitter/receiver that's made up of a whole bunch of little transmitters/receivers. By starting one edge of the array transmitting before the rest and rippling the signal across the array, you cause the strong part of the beam to head off in the direction of the side you started on - and how fast the ripple proceeds determines how far off to the sides the signal goes. If you assemble a similar sequence of received data, you again get a directional effect.
This is the core technology behind 1970s Ticonderoga/Aegis missile defense cruisers, and it's been moving into more of the market over the years.