ORBCOMM satellites attached to Pegasus rocket during pre-flight preparations. Photo: Orbital Sciences

An air-launched Pegasus XL rocket is being readied to carry seven data relay satellites into space on Saturday for the ORBCOMM communications network.

The Orbital Sciences-built rocket is scheduled to be dropped from the belly of the company's L-1011 carrier jet about 50 miles off the Virginia coastline of the U.S. An available launch window will extend from 1:46 to 2:01 p.m. EST (1846-1901 GMT), with Pegasus release targeted for 1:51 p.m. EST.

Weather forecasters predict favorable conditions for the launch.

The mission will be staged from NASA's Wallops Flight Facility in Southeastern Virgina. Orbital ferried the L-1011 and rocket to the launch site from the Pegasus home base at Vandenberg Air Force Base, California, on Tuesday.

Engineers will use the days leading up to the launch by performing a final round of tests with the rocket and payload.

The disk-shaped ORBCOMM spacecraft will join 28 already in low-Earth orbit providing messaging and data services to subscribers around the world.

Customers of system include the transportation industry, fisheries and oil companies, which use ORBCOMM for two-way remote monitoring, tracking and other communications.

Pegasus is slated to deliver the new craft into a circular orbit 512 miles above the Earth, inclined 45 degrees to either side of the equator.

This mission, the fourth for ORBCOMM aboard a Pegasus rocket, was originally planned to originate from the Kwajalein Missile Range in the Pacific.

However, officials scrapped that idea in favor of Wallops so the new satellites would be placed into a higher inclination orbit, which would improve service to customers in higher latitude regions of the globe.

The additional satellites will shrink the coverage gaps in over such areas as North America, Europe and Japan where ORBCOMM's customer base is developing, said Orbital spokesman Barry Beneski.

"We have plenty of satellite capacity. These (new) satellites will fill in the coverage pattern," Beneski said.

Orbital still plans to launch seven ORBCOMM satellites for the equatorial region, but that mission won't occur until next summer.