WASHINGTON — The U.S. Defense Department this month shipped an experimental satellite communications payload and its spacecraft platform to the Kodiak Launch Complex in Alaska, where they were mated in preparation for launch as early as May 5, a Navy official said March 14.

However, the launch date for the Naval Research Laboratory (NRL)-developed TacSat-4 satellite could be affected by an investigation into the March 4 launch failure of a Taurus XL rocket, said Mike Hurley, head of spacecraft development at NRL. The Minotaur 4 vehicle that will carry TacSat-4 to orbit shares some hardware in common with the Taurus XL; both vehicles are built by Dulles, Va.-based Orbital Sciences Corp.

In addition, the military may decide to launch the ORS-1 operational surveillance satellite ahead of TacSat-4 if it is ready in time, the Air Force has said. Although ORS-1 is launching from California’s Vandenberg Air Force Base, both launch campaigns share some personnel.

The Navy started the TacSat-4 program in 2006 to demonstrate an ultra high frequency (UHF) mobile communications capability that would be compatible with many deployed satellite radios. The spacecraft cost $75 million to develop, and the launch cost another $43 million, Hurley said.

The 450-kilogram spacecraft was built by NRL and the Johns Hopkins University Applied Physics Laboratory of Laurel, Md. TacSat-4 will circle the globe six times a day in a highly elliptical orbit with an apogee — the spacecraft’s furthest distance from Earth during each orbit — of 12,050 kilometers. Once on orbit, it will deploy a steerable 4-meter antenna that will provide 10 25- megahertz channels to users inside a coverage area 3,700 kilometers in diameter. TacSat-4’s coverage area can be moved within 24 hours to serve users in a different location, Hurley said.

The Navy currently relies on its fleet of UHF Follow-On geostationary satellites and leased UHF capacity on geostationary commercial satellites for mobile communications links. But geostationary satellites, which operate 36,000 kilometers above the equator, have limitations, and TacSat-4 could offer a unique capability, primarily the ability to reliably cover high latitudes, Hurley said. The satellite could also open up capacity previously unavailable to users with lower priority missions, he said. And because of TacSat-4’s lower orbit, many users will be able to link with the satellite while on the move rather than stopping to point their antennas at a geostationary craft.

The downside of a single satellite in TacSat-4’s orbit is that users will be able to link to it for only about two hours at a time, three times a day, Hurley said. But if the mission proves successful, an operational constellation of three or four satellites in this orbit could provide continuous coverage and augment geostationary satellites, he said.

TacSat-4 will be put through a yearlong military utility assessment led by the Pentagon’s Operationally Responsive Space Office. The Navy has a lot riding on TacSat-4’s success, because it is one element of a strategy the service has developed to stave off a looming gap in UHF coverage.

The Navy’s UHF Follow-On satellites are aging, and the next-generation Mobile User Objective System spacecraft have been delayed by technical troubles. The Navy has done everything it can do to extend the lives of the satellites on orbit, and also has arranged to use capacity from an Australian UHF payload that will fly on the Intelsat 22 commercial spacecraft set for launch in early 2012. Satellite operator Intelsat has added a UHF payload to another satellite that will launch next year with the expectation that the Navy will need its capacity.

Meanwhile, the Air Force is processing both TacSat-4 and ORS-1 for May launches, though one will end up launching later. ORS-1 is the higher-priority mission, but has missed several previously scheduled launch dates because of technical troubles. ORS-1 is launching on a Minotaur 1 vehicle, which for the most part has a separate launch crew from the Minotaur 4, allowing them to be processed concurrently and launched within about 30 days of each other, Hurley said. ORS-1 is tentatively scheduled to launch first, and the final launch order should be determined by April, Air Force spokeswoman Valerie Skarupa said March 10.

But the launch plan could be complicated by the March 4 launch failure of a Taurus XL rocket. In that mission, the rocket’s payload fairing failed to separate from the vehicle, resulting in the destruction of NASA’s Glory climate monitoring satellite. The fairing separation mechanism was a redesigned version of one that failed two years ago, resulting in the loss of NASA’s Orbiting Carbon Observatory satellite.

The redesigned fairing separation system flew on three successful Minotaur 4 flights last year.

Orbital’s TacSat-4 launch team will review the initial findings of a NASA failure review board by the end of March and decide if there is cause to delay the mission.

“That’s the biggest wildcard for both our launch and the ORS-1 launch,” Hurley said.



U.S. Air Force’s TacSat-3 Enters Operations Phase