It was eight months before launch when my second Flight Operations Team lead said he was leaving the project for another job. Six months earlier, the original lead had said he was leaving. I was stunned--but I remained confident that we would recover. I didn't expect to lose the second lead. After all, lightning is not supposed to strike twice in the same place. This time, with only eight months until launch, I was very much concerned. No, 'concerned' is probably too mild a word. Let's get it right: I was sweating. Losing a lead at any stage presents problems, but two losses within 6 months of each other can definitely shake the confidence of an inexperienced Flight Ops Team.
NASA, at the direction of its Administrator, is undertaking a grand challenge of change for faster, better, cheaper; strong emphasis is now being placed on staying within cost
projections while still meeting schedule and performance goals. This led NASA to mandate fixed-price, capped-cost programs. The Advanced Composition Explorer (ACE) is the first mission to be funded in this manner. Consequently, the ACE Project has sought novel approaches and techniques to constrain costs without compromising schedule or science goals. A key approach that has been adopted is novel, multiple reusage of the ACE control center subsystem.
ACE will use a version of the Transportable Payload Operations Control Center (TPOCC) for its mission operations. It was determined in Phase 8 of the ACE Project that a potential existed for substantial savings if the adaptation of the TPOCC for ACE Mission Operations could include adapting it as well for use as the primary component in the Ground Support Equipment for
Integration and Testing of the ACE Spacecraft and, at the same time, also adapting it be the basic component in the ACE Science Center; thus, realizing three separate uses for essentially the same system. Implementing this approach required enhancing the TPOCC requirements, significant
changes in its development schedule, and changes in the allocation and activities of personnel responsible for development of ACE operations. This paper discusses how these issues were addressed, the unforeseen problems that have been encountered, how these problems have been
resolved, and an evaluation of what this approach portends for application to future missions.
Information necessary to understand the Autonomous Attitude Determination System (AADS) is presented. Topics include AADS requirements, program structure, algorithms, and system generation and execution.
The Gamma Ray Observatory (GRO) spacecraft needs a highly accurate attitude knowledge to achieve its mission objectives. Utilizing the fixed-head star trackers (FHSTs) for observations and gyroscopes for attitude propagation, the discrete Kalman Filter processes the attitude data to obtain an onboard accuracy of 86 arc seconds (3 sigma). A combination of linear analysis and simulations using the GRO Software Simulator (GROSS) are employed to investigate the Kalman filter for stability and the effects of corrupted observations (misalignment, noise), incomplete dynamic modeling, and nonlinear errors on Kalman filter. In the simulations, on-board attitude is compared with true attitude, the sensitivity of attitude error to model errors is graphed, and a statistical analysis is performed on the residuals of the Kalman Filter. In this paper, the modeling and sensor errors that degrade the Kalman filter solution beyond mission requirements are studied, and methods are offered to identify the source of these errors.
The primary scientific objective of the High Energy Solar Spectroscopic Imager (HESSI) Small Explorer mission selected by NASA is to investigate the physics of particle acceleration and energy release in solar flares. Observations will be made of x-rays and (gamma) rays from approximately 3 keV to approximately 20 MeV with an unprecedented combination of high resolution imaging and spectroscopy. The HESSI instrument utilizes Fourier- transform imaging with 9 bi-grid rotating modulation collimators and cooled germanium detectors. The instrument is mounted on a Sun-pointed spin-stabilized spacecraft and placed into a 600 km-altitude, 38 degrees inclination orbit.It will provide the first imaging spectroscopy in hard x-rays, with approximately 2 arcsecond angular resolution, time resolution down to tens of ms, and approximately 1 keV energy resolution; the first solar (gamma) ray line spectroscopy with approximately 1-5 keV energy resolution; and the first solar (gamma) -ray line and continuum imaging,with approximately 36-arcsecond angular resolution. HESSI is planned for launch in July 2000, in time to detect the thousands of flares expected during the next solar maximum.
The Upper Atmosphere Research Satellite (UARS) has two definitive attitude determination requirements: the definitive attitude of the Modular Attitude Control Subsystem (MACS) and the definitive attitude of the gimbaled Solar-Stellar Pointing Platform (SSPP). The onboard computer (OBC) will compute the MACS attitude using a Kalman filter and will transform this attitude solution through the SSPP gimbals to calculate the SSPP attitude. The attitude ground support system (AGSS) will compute the MACS attitude using a batch least-squares differential corrector algorithm and will also transform this solution through the gimbals to obtain the SSPP attitude. This paper reports the results of a prelaunch study to predict the accuracy of the OBC attitude solutions and the accuracy of the AGSS attitude solutions. The OBC and AGSS solution accuracies are then compared to establish the relative quality. The effects of star observability, sensor noise, and sensor misalignment uncertainties on attitude determination accuracy are analyzed for each case.
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)
