A spacecraft to link terrestrial U.S. wireless networks with space is awaiting liftoff Sunday on a Proton rocket to begin building a seamless hybrid mobile broadband communications system mixing ground assets and satellites.

The SkyTerra 1 satellite, built by Boeing Co., is the first spacecraft to be launched by LightSquared, a Virgina-based firm formerly known as SkyTerra. The platform, along with an identical satellite due for launch in 2011, will merge with a ground-based network to cover 260 million Americans by the end of 2016.

The 11,900-pound spacecraft is scheduled for launch at 1729 GMT (12:29 p.m. EST) Sunday on a Proton rocket from the Baikonur Cosmodrome in Kazakhstan.

The 191-foot-long launcher rolled to the pad Thursday and was erected vertically atop the complex, then a protective service structure moved into place around the rocket.

The Proton rocket rolled to the launch pad Thursday morning. Credit: Roscosmos
 
Burning toxic storable propellants, the 191-foot-tall rocket will accelerate east from Baikonur, dropping its first stage two minutes after launch and its second stage at T+plus 5 minutes, 27 seconds. The Proton's payload shroud will split open and jettison about 20 seconds later, exposing SkyTerra 1 once the booster reaches the upper atmosphere.

After releasing the rocket's third stage at T+plus 9 minutes, 41 seconds, the Breeze M upper stage will coast for one-and-half minutes and ignite its main engine at T+plus 11 minutes, 15 seconds. It will fire for more than seven minutes to place itself and the payload in a circular parking orbit 110 miles above the planet.

Another four Breeze M burns will raise its altitude and reduce its orbital inclination. The upper stage is scheduled to release the satellite at 0243 GMT Monday (9:43 p.m. EST Sunday).

The launch vehicle is targeting an orbit stretching from a low point of 3,750 miles to a peak altitude of more than 22,200 miles. The planned inclination at spacecraft separation is 19 degrees, according to Khrunichev, the Proton rocket's prime contractor.

SkyTerra 1 is prepared for launch at the Baikonur Cosmodrome. Credit: Khrunichev

SkyTerra 1's launch was postponed from August to replace bearings inside the spacecraft's four reaction wheel assemblies, which control the satellite's attitude in orbit.

According to a LightSquared filing with the Federal Communications Commission, Boeing notified the company there was a contaminant in the grease used to lubricate bearings inside the reaction wheel assemblies.

The spacecraft was delivered to Baikonur Oct. 18 to begin final launch preps.

After deployment from the Breeze M upper stage, SkyTerra 1 will raise its orbit with an on-board liquid-fueled boost engine. The satellite will ultimately be positioned in geosynchronous orbit 22,300 miles above the equator at 101.3 degrees west longitude.

SkyTerra 1's orbital position will put the spacecraft in view of mobile communications customers across the United States, Canada, Mexico and the Caribbean.

Based on the Boeing 702HP bus platform, the satellite will unfurl an expansive 72-foot diameter L-band antenna reflector once in position.

Founded with the help of a $2.9 billion investment from Harbinger Capital Partners, a New York-based hedge fund, LightSquared aims to develop a fourth-generation, or 4G, wireless broadband network combining traditional terrestrial services and satellite-based coverage.

LightSquared also has up to $1.75 billion in additional financing, according to company statements.

Company officials say the service will be the first-ever wholesale nationwide 4G network using satellites. The network will reach underserved rural communities, enhance communications reliability, and increase competition in the explosive wireless market, according to LightSquared.

Nokia Siemens Networks is managing the deployment, installation and operations of the LightSquared network under an 8-year contract worth $7 billion.

LightSquared is also paying Inmarsat, a leading satellite communications firm, to clear a portion of its L-band spectrum to make way for the SkyTerra satellites.