upgrade was achieved through design improvements, parts upgrades, addition of redundancy, and a more robust quality assurance program.

The need to upgrade the bus led to an interesting programmatic trade-off. TRW's STEP spacecraft were being produced in facilities operating efficiently with streamlined processes appropriate to low-cost technology demonstration missions where higher levels of programmatic risk are acceptable. In contrast, processes at TRW's primary Space Park facilities were appropriate to the more demanding requirements of the high-reliability, performance-critical spacecraft produced at that site. Because its design was based on the STEP bus, consideration was given to developing TOMS-EP within the STEP facilities, using processes and controls modified to meet the more demanding TOMS-EP requirements. This plan was rejected, and the Space Park facility was selected for development, primarily to avoid technical and schedule risks to the program that might accrue from imbedding a high-reliability development program within a more informal culture.

Selected Approach

The key technical issues in design approach involved structure, solar array orientation/articulation, battery size, distributed versus centralized architecture for the data system, and the design complexity of the spacecraft safing mode. The finished product resulted in an aluminum structure with fixed arrays, a centralized data system, and a safing system that relied heavily on ground operations for recovery.

Recurring design trade-offs for TOMS-EP were the degree of design flexibility and the type of design margins to incorporate. Flexibility and large margins reduce risk and increase the potential for reuse of the bus design on future missions, but at increased cost for TOMS-EP. Because cost was an important issue on TOMS, most trades were decided in favor of limiting flexibility and margins to that needed to ensure the mission. Nonetheless, the TOMS-EP bus provided the heritage for several later spacecraft buses, including the Republic of China's ROCSAT-1, the Republic of Korea's KOMPSAT, and the SSTI Lewis satellites.

Status and Evaluation

The original plan was to launch TOMS-EP during the summer of 1994. Problems with the Pegasus XL launch vehicle delayed the launch to July 1996. At this time, the launch of the multisensor ADEOS with another TOMS instrument was imminent (it was launched August 17, 1996). The flexibility inherent in dedicated small satellite missions gave NASA the opportunity to reoptimize TOMS-EP to take better advantage of its concurrence with ADEOS. Thus, the TOMS-EP orbit was lowered from 955 km to 500 km where it would provide higher resolution data and augment the ADEOS science data return.

TOMS-EP was successfully launched and deployed on July 2, 1996. By mid-August, the spacecraft had gone through its integral propulsion system firings to get into the correct orbit, instruments were turned on, and TOMS became fully operational with real-time data available to the science community. TOMS-EP continues to be operational as of this writing.

The ADEOS spacecraft failed in orbit on June 29, 1997; lost with it were the data from the TOMS and other instruments it carried. Because the TOMS-EP spacecraft carries on-board propulsion, NASA could raise its orbit closer to that of ADEOS, both to increase coverage of the instrument and to reduce drag (and extend orbit life). The boost maneuver was performed in December 1997 and TOMS-EP was raised from a 500 km to a 750 km orbit. This will extend the mission's orbit life beyond the 2-year requirement and 3-year goal to as long as 5 years.

Lessons Learned

The TOMS-EP project embraced a low-cost, small satellite approach to flying a TOMS instrument over the 1994–1997 time frame as a potential gap filler to ensure continuity of ozone measurements between instruments on the Russian METEOR and Japanese ADEOS satellites. Key programmatic decisions were made, and the program plan was developed, to meet the performance and cost objectives on the desired schedule. TOMS-EP is

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