International Space Station astronauts Don Pettit of NASA and Andre Kuipers of the European Space Agency have replaced a UHF communications unit aboard the station that will be required for the upcoming berthing of the unpiloted SpaceX Dragon cargo capsule.

The Dragon is scheduled to lift off from Cape Canaveral Air Force Station, Fla., atop a Falcon 9 rocket on May 19 at 4:55 a.m. EDT, initiating the first U.S. commercial resupply mission to the orbiting science lab.

One of two Commercial Orbital Transportation System (COTS) Ultra High Frequency Communication Units, an internal electronics device developed by SpaceX to permit the station’s astronauts as well as the company’s flight control team to transmit commands and receive signals from Dragon, failed on May 10. Both must function as part of the commit criteria for the Dragon mission launch. The replacement took place May 14.

“We want both systems up and operating for redundancy,” says Mike Horkachuck, NASA’s SpaceX COTS project executive. The exchange of GPS signals through the system is critical to the relative navigation that permits Dragon to approach the ISS and the station crew to issue commands aborting a rendezvous, if necessary.

If the Dragon test mission unfolds as planned, the capsule and a cargo of just over 1,000 lb. of nonessential supplies will approach the station early May 22. Pettit and Kuipers will grapple Dragon with the station’s Canadarm2 and berth it to the U. S. segment for an 18-day stay.

Meanwhile, a three-man Soyuz crew raced toward docking with the station early May 17, following a flawless climb to orbit from the Baikonur Cosmodrome in Kazakhstan. The TMA-04M transport carrying Russians Gennady Padalka and Sergei Revin and American Joseph Acaba will restore the station to six-crew operations for the first time since April 27 when they dock at the Russian segment at 12:39 a.m. EDT.

Their Soyuz 30 mission lifted off on May 14 at 11:01 p.m. EDT, or 9:01 a.m. at the launch site.

The newcomers will be greeted by Pettit, Kuipers and Expedition 31 commander Oleg Kononenko of Russia.

During a four-month mission, Padalka, Revin and Acaba expect to greet U.S., Russian and Japanese replacements for Kononenko’s crew as well as Japanese, Russian and perhaps additional U.S. commercial supply ships. In late July, Russia’s 47 Progress freighter will depart the station and return in a flight test of new automated rendezvous hardware.

Padalka and cosmonaut Yuri Malenchenko, a future crewmate, have trained for a spacewalk to prepare the station for the arrival of Russia’s Multipurpose Laboratory Module, possibly next year. If time permits, the spacewalkers will install protective orbital debris shielding on the Russian segment.

The station’s crew is serving as operators or subjects in about 200 science experiments and engineering evaluations as well. “That’s our main focus,” Acaba says.

Padalka, who served as the station’s commander in 2009 and 2004, will become the first person to lead a station crew for a third time when Kononenko, Pettit and Kuipers depart in early July. Revin, an NPO Energia flight-test engineer, is flying for the first time. Acaba, one of NASA’s educator astronauts, is a former Florida middle and high school science and math teacher, and a U.S. Peace Corps volunteer. He served aboard a shuttle space station assembly mission in 2009.