A powerful Proton-K rocket was launched from the Baikonur Cosmodrome Saturday carrying three replacement spacecraft for Russia's Global Navigation Satellite System.

Liftoff of the sixth Proton flight of 2001 occurred at 1804 GMT (1:04 p.m. EST), beginning a four-hour mission to deliver the Uragan satellites into orbit.

Known as GLONASS for short, the navigation system uses satellites to transmit navigation radio signals to military and civilian users.

"GLONASS was set up on the initiative of the Russian Defense Ministry in September 1993," a spokesman for the Russian Space Agency Rosaviakosmos said. "It allows to locate objects on the Earth's and sea surface with an accuracy of one meter."

The positioning information aids air and sea traffic management, cartography, ground transport monitoring and search and rescue operations.

The network is the Russian equivalent of the U.S. Global Positioning System.

Saturday's launch carried two standard Uragan spacecraft and one Uragan-M, which is a next-generation satellite design that promises a longer in-orbit lifespan of 50 to 100 percent. Its instruments are also more powerful and have higher accuracies.

Russian officials say the Uragan-M spacecraft will replace older Uragan satellites in the future, allowing for an improved and more precise navigation system.

GLONASS is supposed to feature 24 orbiting satellites divided into three orbital planes in circular orbits 19,100 kilometers high with an inclination of 64.8 degrees to provide global coverage. However, Russia's economic troubles meant replacement satellites couldn't be launched as needed in the 1990s. The spacecraft have just three-year service lives.

At present, there are only six operational satellites in the orbiting fleet, not counting the three launched Saturday. Three spacecraft from the December 1998 launch are positioned in the constellation's Plane 1, and three satellites launched in October 2000 are flying in Plane 3. The new satellites are scheduled to be placed into operation in Plane 2.

The first GLONASS launch occurred in 1982.