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?

Falcon 9 landing attempt on ASDS enter image description here


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.

  • $\begingroup$ Musk definitely said that, but what he said was contradictory. Relative GPS means you are not going to an absolute position. $\endgroup$ May 9 '16 at 16:28
  • 2
    $\begingroup$ I wonder what he meant to say was that the ship goes to a position with absolute GPS, and then the rocket goes to that coordinate with relative GPS. In that case, yes there is communication between the rocket and drone to do relative GPS positioning. $\endgroup$
    – hshib
    May 9 '16 at 17:35
  • $\begingroup$ AOS doesn't imply two-way communications :) $\endgroup$ May 31 '16 at 22:07
  • $\begingroup$ I'd be unsurprised if AOS was acting as a backup for GPS. $\endgroup$
    – SF.
    Jun 29 '17 at 14:26
  • $\begingroup$ No GPS system on Mars, how would it land? $\endgroup$
    – seccpur
    Sep 9 '20 at 13:30

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.

  • 5
    $\begingroup$ Hadn't even considered the Blue Origin patent. I always assumed that the ASDS knows its own position using GPS, and that gets sent to the stage from Mission Control. $\endgroup$ Apr 16 '15 at 18:59
  • $\begingroup$ @NateBarbettini My guess is, yes to all of the above. Everybody reports as much as possible. More info is better than less. Line of sight is hard though for a rocket and a ship. $\endgroup$
    – geoffc
    Apr 16 '15 at 19:11
  • 2
    $\begingroup$ The patent is here: google.com/patents/US8678321 -- it calls for the landing platform to broadcast its present or future position to the stage. $\endgroup$ Apr 20 '15 at 4:05
  • 4
    $\begingroup$ @RussellBorogove I sure hope that patent is defeated. That's trivially obvious! $\endgroup$ Dec 24 '15 at 21:19
  • 2
    $\begingroup$ Blue Origin cancelled the '321 patent in September of 2015 and apparently started trying to issue a new (possibly narrower) patent -- I don't know if they completed that process. $\endgroup$ Apr 11 '16 at 16:22

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.

  • $\begingroup$ On Iradium-2, they announced AOS with drone at T+6:53, about 50 seconds before the touch down: youtube.com/watch?v=ei3nGWD4d5A&t=21m52s $\endgroup$
    – hshib
    Jun 30 '17 at 16:15
  • $\begingroup$ Piecing together many comments over the years -- AOS is in regards to telemetry relay. Stage is below the horizon from the cape during landing, and the ASDS contains equipment to capture the telemetry feed and relay it via satellite to Hawthorne. $\endgroup$
    – Saiboogu
    Apr 3 '19 at 13:18

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.