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Company Supports Extended Lifetime of the FUSE Mission with Enhanced Command and Control Software

(Dulles, VA 22 July 2003) - Orbital Sciences Corporation (NYSE: ORB) announced today that it has completed the in-orbit delivery and initial checkout of its new gyroless software for the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite for its customer, the Johns Hopkins University (JHU). Based on the company's MidStar spacecraft bus, FUSE was launched in June 1999 and last year completed its three-year primary mission. The satellite is now operating in an extended mission lifetime, which the new gyroless software will significantly enhance.

With the success and importance of the FUSE science, extended mission operations are highly desirable. Given that the FUSE satellite's Inertial Reference Unit's have life-limiting elements, the FUSE program team embarked upon a software development program to produce a new system to enable gyroless control of the satellite in order to extend the space science mission should the spacecraft encounter future gyro failures. Orbital integrated a new "safe mode," developed by NASA's Goddard Space Flight Center, with the Orbital-developed coarse and fine pointing gyroless software to provide a substantially enhanced capability to the FUSE spacecraft and maintain its sub- arcsecond pointing capability. This new operations scheme also required significant changes to the instrument's software and the Fine Error Sensor's software, which were modified by the JHU and the Canadian Space Agency, respectively.

"We are thrilled that we are able to provide the scientific community with assurance of extended mission capability and continuity with the implementation of this new software" said Mr. Michael Miller, Orbital's Vice President of Science and Technology Satellite Programs.

The new gyroless software provides FUSE with better stability than the original design with only a minor impact to observational efficiency. A significant amount of new Fault Detection and Correction software was also added and modified to provide for autonomous "fail operational"

paths associated with the new operations and to speed up recovery back to the science timeline in the event of anomalies. Combined with the changes made to the instrument's software, this new system will allow the extended mission operations to be handled with fewer operations staff through greater automation.

Mr. Brian Class, Orbital's lead designer of the original control system and the new gyroless upgrade, said, "The ground testing of this new system, prior to implementation aboard the satellite, posed a significant challenge to the FUSE team. We developed a virtual interface that connected the JHU test laboratory in Baltimore. Maryland with the Orbital test laboratory in Dulles, Virginia. The telemetry and command interface was then linked with the Spacecraft Control Center in Baltimore and the Orbital's Dulles test laboratory. This methodology allowed us to perform closed-loop testing of the Attitude Control Subsystem and the Instrument Data System software and to test scripts with the new Spacecraft Control Center databases and displays in a very realistic way, thereby significantly reducing the risk associated with these major modifications."

Mr. Class concluded, stating, "The FUSE gyroless control software is Orbital's second operational system to provide full gyroless capability to a science mission. Earlier this year we launched NASA's Solar Radiation and Climate Experiment spacecraft for the University of Colorado's Laboratory for Atmospheric and Space Physics, which we designed and built without gyroscopes. The FUSE system, however, is significantly more complex due to the finer pointing requirements and the need to incorporate data from the FUSE Fine Error Sensor."

Orbital develops and manufactures small space and missile systems for commercial, civil government and military customers. The company's primary products are satellites and launch vehicles, including low-orbit, geostationary and planetary satellites for communications, remote sensing and scientific missions; ground- and air-launched rockets that deliver satellites into orbit; and missile defense boosters that are used as interceptor and target vehicles. Orbital also offers space-related technical services to government agencies and develops and builds satellite-based transportation management systems for public transit agencies and private vehicle fleet operators.

Contact: Barron Beneski, 7034065000, beneski.barron@orbital.com

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