It has been approximately 6.4 years since the Gyroscopes on HST have been replaced. During this time two Gyroscopes have failed and two have developed problems, but operational work-arounds are available. Further Gyroscope replacement will not occur until the anticipated Shuttle Servicing Mission scheduled for November, 2007. To extend the science mission life of HST up to an additional 15 months, the control system has been modified from a three/four-Gyro, to a two-Gyro control law. This paper describes the new two-Gyro Guide Star acquisition strategy to enable science observations, including the integration of Astrometry commanding with the Acquisition Logic.
Older spacecraft missions, especially those in low Earth orbit with telemetry intensive requirements, required round-the-clock control center staffing. The state of technology relied on control center personnel to continually examine data, make decisions, resolve anomalies, and file reports. Hubble Space Telescope (HST) is a prime example of this description. Technological advancements in hardware and software over the last decade have yielded increases in productivity and operational efficiency, which result in lower cost. The re-engineering effort of HST, which has recently concluded, utilized emerging technology to reduce cost and increase productivity. New missions, of which NASA's Transition Region and Coronal Explorer Satellite (TRACE) is an example, have benefited from recent technological advancements and are more cost-effective than when HST was first launched.
During its launch (1998) and early orbit phase, the TRACE Flight Operations Team (FOT) employed continually staffed operations. Yet once the mission entered its nominal phase, the FOT reduced their staffing to standard weekday business hours. Operations were still conducted at night and during the weekends, but these operations occurred autonomously without compromising their high standards for data collections. For the HST, which launched in 1990, reduced cost operations will employ a different operational concept, when the spacecraft enters its low-cost phase after its final servicing mission in 2004. Primarily due to the spacecraft’s design, the HST Project has determined that single-shift operations will introduce unacceptable risks for the amount of dollars saved. More importantly, significant cost-savings can still be achieved by changing the operational concept for the FOT, while still maintaining round-the-clock staffing. It’s important to note that the low-cost solutions obtained for one satellite may not be applicable for other satellites. This paper will contrast the differences between low-cost operational concepts for a satellite launched in 1998 versus a satellite launched in 1990.
Space-based interferometry already exists. We describe our experiences with on-orbit calibration and scientific observations with Fine Guidance Sensor 3 (FGS 3), a white- light interferometer aboard Hubble Space Telescope. Our goal, 1 millisecond of arc precision small-field astrometry, has been achieved, but not without significant challenges. These included a mechanically noisy on-orbit environment, the self-calibration of FGS 3, and significant temporal changes in our instrument. Solutions included a denser set of drift check stars for each science observation, fine- tuning exposure times, overlapping field observations and analyses for calibration, and a continuing series of trend- monitoring observations. HST FGS 3 will remain a competitive astrometric tool for faint targets in crowded fields and for faint small-separation binaries until the advent of large- aperture, ground-based and longer-baseline space-based interferometers.
The three fine guidance sensors on-board the Hubble Space Telescope are the first white-light amplitude shearing interferometers to be used for space platform guidance, control, and astrometry. Two fine guidance sensors (FGS) under fine lock control now maintain spacecraft pointing precision to within 7 milliseconds of arc rms over the majority of each orbit. Fine guidance sensor control optimization techniques have yielded significant improvement in tracking stability, integrated performance with the pointing control system, loss-of-lock statistics and astrometric accuracy. We describe the optical interferometer, based on the Koester's prism design. We include a discussion of the instrument calibration status, the FGS fine lock performance design enhancements, pointing control system design enhancements, and ground software techniques appropriate to jitter removal in astrometric data. The combination results in marc sec precision astrometry.
The control system requirements for the next generation space telescope are discussed, based on the authors experience with Hubble Space Telescope (HST), Advanced X-Ray Astrophysics Facility (AXAF) and Space Infrared Telescope Facility (SIRTF). Since the HST design phase, there have been significant strides in the guidance and control domain (i.e., fiber optic gyroscopes, solid state star trackers and non-linear control algorithms). The control system design will be determined by the predicted spacecraft configuration, mirror geometry (6 to 8 meters will be considered) and science requirements. Spacecraft dimensions have been estimated for the telescope aperture range of interest. Presently, the Energiya rocket can only accommodate a 6 m telescope, the proposed Heavy Lift Launch Vehicle apparently can accommodate a 7 m telescope. A low Earth orbit (600 Km) has been adopted for this study, the advantage of Shuttle servicing and an accompanying long spacecraft life, weighed heavily in this decision. However, the possibility of a long spacecraft life in a high altitude orbit, with the requisite attitude control redundancy and fault tolerance, may be feasible.
The Hubble Space Telescope (HST) is an orbiting astronomical observatory, designed to operate as close as possible to ground based instrumentation, given the limitation of operating in a low earth orbit. The spacecraft design had to accommodate an absolute pointing accuracy of 0.01 arc seconds, a relative pointing stability of 0.007 arc seconds rms, the capability to maneuver 90 degrees in 18 minutes, and operate autonomously in a safemode control scheme for up to 72 hours. Furthermore, the design had to provide for a flexible, stored command methodology, and real-time command capability. This paper briefly reviews the spacecraft engineering hardware and software design. A detailed critique of the on-orbit performance of the spacecraft is provided. Enhancements and work-around, which have enabled HST to continue implementation of a successful science plan, are explained.