PARIS — The U.S. Defense Department and the European Space Agency (ESA) have joined forces to provide seed money for a data-relay satellite system using modified Inmarsat aeronautical-communications terminals to give military and civil users near-real-time access to low-orbiting observation and communications satellites.

In an unusual collaboration, ESA and the U.S. Defense Advanced Research Projects Agency (DARPA) have divvied up work shares and settled on a trans-Atlantic industrial consortium to perform design work for what is being called the SB-Sat, or SwiftBroadband for Satellite, program.

SwiftBroadband is an existing terminal technology used to provide data links at speeds of about 475 kilobits per second to commercial and military aircraft using the Inmarsat-4 constellation of satellites, which provides global coverage, except for the polar regions, from geostationary orbit.

DARPA and ESA want to modify the SwiftBroadband terminals for use on small satellites in low Earth orbit. Using the higher-orbiting Inmarsat satellites as relay stations would permit users of the low-orbiting spacecraft to “fly” their satellites — meaning to send commands and retrieve images or other data — on an “always-on” basis without waiting for them to pass over a ground station.

In contracts announced in August by DARPA, and on Oct. 7 by the ESA team, the two agencies have selected Broad Reach Engineering of Tempe, Ariz., Com Dev Europe of Britain and London-based Inmarsat to perform design work for the SB-Sat system.

The DARPA contract, valued at $18 million, has Inmarsat as the industrial lead and is called the Persistent Broadband Ground Connectivity for Spacecraft in Low Earth Orbit program. Under the contract, Inmarsat will design a space-qualified SwiftBroadband terminal in time for a flight readiness review in early 2012, and a launch in 2014 aboard a still-undetermined satellite.

Inmarsat’s partners in the contract are Com Dev and Broad Reach.

DARPA has placed the program under the auspices of its System F6 fractionated spacecraft research effort, whose early goal was to field a constellation of satellites that would, working together, provide the functional equivalent of one large spacecraft. Fractionating the functions of a single satellite among several smaller satellites limits the risk of a launch failure and, in principle, could reduce overall launch costs and provide a way to repair the system by sending up small replacement spacecraft.

Eyal Trachtman, head of the SB-Sat program at Inmarsat, said the data-relay function is not necessarily a part of the fractionated-satellite scenario, but could be a payload on a satellite whose other function is to be part of a wireless network in space.

In announcing its contract, DARPA said the program goal is to “help establish a persistent communications system for [low-orbit] satellites for time-sensitive spacecraft control for defense maneuvers, rapid transmission of critical mission data such as space weather events, direct-from-theater control of spacecraft, and direct-to-theater data delivery with a small ground-based transceiver.”

The ESA contract, valued at 5 million euros ($7 million), is with Com Dev Europe as prime contractor, with Inmarsat and Broad Reach as partners. The two-year contract is in two phases, the first lasting nine months. The contract includes design work up to production of a qualification model of the ter

minal and a modification to Inmarsat’s existing SwiftBroadband ground network for SB-Sat.

The 18-nation ESA is already studying development of its own data-relay satellite system that would use terminals on satellites in geostationary orbit to relay optical and radar Earth observation imagery to Earth from low-orbiting civil and military observation surveillance satellites.

The European Data Relay System (E-DRS) is backed principally by Germany, whose industry has developed laser optical communications terminals that transmit data between satellites at speeds reaching several gigabits per second.

With Germany giving its full support, the E-DRS program has secured some 170 million euros in funding, which is too far below ESA’s estimated cost of 230 million euros to permit full-scale development. ESA has been negotiating with several prospective commercial companies with a view to selecting a system operator.

In an Oct. 8 interview, Trachtman said SB-Sat should be viewed as complementary to a full-scale data-relay system. He said SB-Sat would permit a ground user to download around 200 megabytes of data per orbit. While this is far less than what E-DRS would do, SB-Sat’s advantage lies in giving ground users nonstop access to their satellites.

“For example, if it is being used to download Earth imagery, SB-Sat could act as a kind of filter, permitting the immediate download of a data sample to determine if the image is clear,” Trachtman said. “You can achieve much more efficiency when you have constant access to the satellite.”

Trachtman said that while Inmarsat would need to adapt several parts of its ground network for SB-Sat, early indications are that the cost would not be a major factor in deciding whether to proceed with full system development.

Robert Goldsmith, business development manager for Com Dev Europe, said in an Oct. 8 e-mail response to questions that the SB-Sat terminals to be adapted for low-orbiting satellites are likely to weigh no more than 5 kilograms and to require a maximum 40 watts of power.

Inmarsat, Com Dev Europe and Broad Reach “will jointly market the terminal and service to [low Earth orbit] satellite manufacturers and operators,” Com Dev said in an Oct. 7 statement. “A number of opportunities are being investigated, with terminal production expected to be divided between Com Dev Europe facilities in the U.K. and at Broad Reach Engineering facilities in the U.S.”

Peter B. de Selding was the Paris bureau chief for SpaceNews.