The SpaceX Autonomous Spaceport Drone Ship is an ocean barge equipped with station keeping engines that has a large platform for landing the first stage rockets for reuse.
Creating a paradigm shift in the traditional approach for reusing rocket hardwareElon Musk
The rockets do not have enough remaining fuel to return to their launch site after boosting the upper stage into orbit. Any Falcon flights going to geostationary orbit or exceeding escape velocity require landing at sea.
The ASDS have precision positioning , within 9.8 feet even in storm conditions. They use GPS positioning and four diesel-powered azimuth thrusters which are marine propellers in pads that can be rotated to any horizontal angle.
SpaceX has two ASDS in service, “Just Read the Instructions II” and “Of Course I Still Love You”. They name their barges after spaceships in the Culture Series science fiction novels by Iain M. Banks.
Just Read The Instructions I upper deck was 170 feet × 300 feet and the span of the Falcon 9 landing legs is 60 feet. This barge was retired with 2 failures and 0 successes. Just Read The Instructions II has had 2 failures and 34 successes. Just Read The Instructions has had 1 failure and 13 successes as of December 13, 2020.
The ASDS use McDonough Marine Service’s Marmac 300 barges. “A Shortfall of Gravitas” is the newest and not used yet. There is a tug boat and support vessel with each ASDS. They get on board quickly after touchdown to lock the landing legs and strap the vessel down for the trip back to port.