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The descending Falcon 9 has got to hit (preferably gently) the tiny target of the Autonomous Spaceport Drone Ship (ASDS) in the middle of the ocean. How does the Falcon 9 know where the ASDS is?
Radar seems the obvious solution, but on the Falcon or the ASDS?
Updated: Now that a Falcon 9 first stage has successfully landed on land, there are two cases to consider. Are different systems used for landing on the ASDS and on land?
The issue is interesting, since the Bezos patent is specific about the barge and the rocket handshaking and talking directly to each other. But SpaceX is contesting the patent (obviously, since they are actually doing it).
Almost certainly the initial location is sent by Mission Control, refined the whole time. The ASDS is set to maintain a location, the rocket is aiming for that location.
What would be interesting to know is if the barge tries talking to the booster. It appears that in fact the rocket does the targetting on its own, without Mission Control's help.
During the Iridium-2 launch from Vandenberg Jun 2017, John Insprucker mentioned on the Iridium launch webcast that the ASDS does not send any signals to the returning stage; it only receives telemetry from the stage.
This makes sense as you want a single system for RTLS and ASDS landings where possible.
According to Musk at the post CRS-8 briefing:
Both (ship and rocket) is going to an absolute position. So the ship is holding to the absolute GPS position, with relative GPS and today it was holding to the accuracy of below a meter. It has four engines all of which can rotate 360 degrees and operating continuously to hold the position, to hold both attitude and position in the ocean.
I assume that means that there is no communication between the drone and the rocket and they simply meet at a set GPS location.
I caught following during the THAICOM 8 launch:
Recovery vessel has AOS. AOS means Acquisition Of Signal. That means that the recovery.. the drone ship has reached communication contact with the rocket.
So they do communicate with each other, and that seems to confirm the discussion below that the drone goes to absolute GPS location and that rocket goes to the relative GPS location of the drone by communicating with the drone.
So not sure if they do bother to have relative GPS communication with ground station, since ground doesn't move, rocket may go straight to absolute GPS coordinate.
The stage and ship both navigate to a preprogrammed GPS coordinate. It seems that GPS datum cannot be changed in-flight. Last-minute corrections are done via a radar on the stage.
AOS by the drone ship is one-way communication (stage telemetry going to the ship).
(I know, using a forum post as a source. It's corroborated by another post in the L2 subscription-only section of NasaSpaceflight, by someone with good contacts in SpaceX)
The use of Radar was mentioned by Elon Musk in the post flight interview for the Falcon Heavy Launch. He discusses that the returned side stages were slightly staggered to avoid radar interference between the two. There is likely some use of Radar altimeter or homing on final approach.
During the Thaicom launch they definitely announced AOS. But that doesn't necessarily mean there is two way comm between the booster and the ship. It could simply mean that the drone ship is receiving rocket telemetry for relay to Hawthorne. This makes sense because the booster is over the horizon from Florida and the receivers there will lose signal as the rocket descends.
