Over the next several months, the avionics, propulsion and electrical subsystems will be integrated into the two FDS spacecraft buses. Much of the process will be completed by early next year, when the sensor payload built by Raytheon Systems Co. is delivered and mated with the spacecraft.

The FDS spacecraft are scheduled for launch in late 1999 aboard a single Delta II launch vehicle. TRW will operate the satellites for one year after launch and has total system program responsibility (TSPR) for providing the Air Force with spacecraft test data, satellite health and mission status information.

"Integrating and testing the spacecraft software represents one of the largest challenges we face," said Jim Burnett, TRW's SBIRS-Low program manager. "SBIRS-Low spacecraft are very complex, software-intensive spacecraft. They are capable of processing large amounts of mission data in space that older programs must process on the ground."

To improve the quality and reduce the risk associated with the software integration process, TRW has built an electrical engineering model test bed. Consisting of all the spacecraft avionics/electronics and some actual flight units, the test bed is currently being used to validate and verify all spacecraft functions. The test bed is colocated with the spacecraft and will be used during A,I&T to resolve anomalies.

TRW is building a ground station at its Redondo Beach facility that will also be used during A,I&T as a software validation and risk reduction tool. Data will be sent between the ground station and the spacecraft via a high-speed telephone line, enabling engineers to debug the ground station database and software while at the same time gaining experience in understanding how the FDS spacecraft will operate on orbit.

The ground station hardware and software will be transitioned to serve as the operational command and control segment of FDS.

SBIRS-Low is the low Earth component of the SBIRS system, the next-generation surveillance system. It will consist of a constellation of advanced spaceborne sensors geared to track the threats of the 21st century -- and will provide continuous observations of ballistic missiles in the boost, midcourse and re-entry phases of attack.

A TRW/Raytheon team is designing, building and launching two SBIRS-Low FDS spacecraft and the infrared sensor payload under contract to the Air Force Space & Missile Systems Center.

Concurrently with spacecraft A,I&T, Raytheon is developing, integrating and testing a total of six infrared payload sensors and associated mission data processing hardware and software at its facilities in El Segundo, Calif. Four sensors will be integrated into the spacecraft next year; the other two are considered "pathfinders," or extremely high-fidelity engineering prototypes.

The sensor payload includes a scanning short wavelength infrared target acquisition sensor and a staring multispectral infrared tracking sensor. The acquisition sensor provides high-resolution horizon-to-horizon coverage, plus coverage a few degrees above the horizon, to search for, detect and track missiles in the boost phase.

The track sensor can receive a handover from the acquisition sensor and continue to track the missile with greater precision through midcourse and re-entry.

FDS will demonstrate all key operational SBIRS-Low's capabilities, including the ability to:

• detect and track theater and strategic ballistic missiles in boost, midcourse and re-entry;
• autonomously hand over acquisition sensor tracks to tracking sensor;
• hand over track sensor target data to a second satellite via crosslinks;
• show flexibility of the system design for additional missions such as space surveillance, theater missile defense, technical intelligence and battlespace characterization; and
• exhibit overall feasibility of a low Earth orbit infrared missile tracking system as part of the SBIRS architecture.