By Irene Klotz

CAPE CANAVERAL, Fla. March 1 (Reuters) - Two U.S. astronauts left the International Space Station on Sunday for a seven-hour spacewalk to install communications and navigation aides for new commercial space taxis.

Station commander Barry "Butch" Wilmore and flight engineer Terry Virts floated outside the station's Quest airlock shortly after 7 a.m. EST/1200 GMT to begin their third spacewalk in eight days, a NASA Television broadcast showed.

The purpose of the outings is to prepare berthing slips for spaceships being developed by Boeing and Space Exploration Technologies, or SpaceX.

The astronauts are scheduled to install more than 400 feet (122 metres) of cables and a pair of antennas that the new vehicles will use to navigate toward and dock with the station, a $100 billion laboratory that flies about 260 miles (418 km) above Earth.

Sunday's outing follows two spacewalks last week to rig power and data cables for a pair of docking port adapters that are due to arrive later this year. One adapter will be installed at the berthing slip once used by NASA's space shuttles, which were retired in 2011. The second docking system will be located at an adjacent hatch on the Harmony connecting node.

Since the shuttles' retirement, the United States has been dependent on Russia to fly crew to and from the station - a joint project in which 15 nations are involved. NASA aims to break Russia's monopoly before the end of 2017 by buying rides from Boeing and SpaceX.

(Editing by Clelia Oziel)