A directional gamma-ray telescope using coded aperture techniques

A directional detector for γ-ray astronomy has been developed to image sources in the energy range 0.1 to 5 MeV. An array of 35 gain stabilized bismuth germanate detectors, together with a coded aperture mask based on a Uniformly Redundant Array (URA), allows imaging in 4° square sky bins over a 16° X 24° field-of-view. The position of a strong point source, such as the Crab Nebula, can be determined to within ?1°. A complementary “anti-mask” greatly reduces systematic effects arising from non-uniform background rates amongst the detectors. The telescope has an effective area of 190 cm² and an energy resolution of 19.5% FWHM at 662 keV. Results of laboratory tests of the imaging system, including the ability to image multiple sources, uniformity of response over the field-of-view, and the effect of the “anti-mask”, are in good agreement with computer simulations. Features of the flight detector system are described and results of laboratory tests and computer simulations are reviewed. A balloon flight of the telescope is planned for the fall of 1982.     

The Solar Energetic Particle Ionic Charge Analyzer (SEPICA) and the Data Processing Unit (S3DPU) for SWICS,SWIMS and SEPICA

The Solar Energetic Particle Ionic Charge Analyzer (SEPICA) is the main instrument on the Advanced Composition Explorer (ACE) to determine the ionic charge states of solar and interplanetary energetic particles in the energy range from ≈0.2 MeV nucl−1 to ≈5 MeV charge−1. The charge state of energetic ions contains key information to unravel source temperatures, acceleration, fractionation and transport processes for these particle populations. SEPICA will have the ability to resolve individual charge states and have a substantially larger geometric factor than its predecessor ULEZEQ on ISEE-1 and -3, on which SEPICA is based. To achieve these two requirements at the same time, SEPICA is composed of one high-charge resolution sensor section and two low- charge resolution, but large geometric factor sections. The charge resolution is achieved by the focusing of the incoming ions, through a multi-slit mechanical collimator, deflection in an electrostatic analyzer with a voltage up to 30 kV, and measurement of the impact position in the detector system. To determine the nuclear charge (element) and energy of the incoming ions, the combination of thin-window flow-through proportional counters with isobutane as counter gas and ion-implanted solid state detectors provide for 3 independent ΔE (energy loss) versus E (residual energy) telescopes. The multi-wire proportional counter simultaneously determines the energy loss ΔE and the impact position of the ions. Suppression of background from penetrating cosmic radiation is provided by an anti-coincidence system with a CsI scintillator and Si-photodiodes. The data are compressed and formatted in a data processing unit (S3DPU) that also handles the commanding and various automatted functions of the instrument. The S3DPU is shared with the Solar Wind Ion Charge Spectrometer (SWICS) and the Solar Wind Ion Mass Spectrometer (SWIMS) and thus provides the same services for three of the ACE instruments. It has evolved out of a long family of data processing units for particle spectrometers. This revised version was published online in June 2006 with corrections to the Cover Date.     

On-line diagnostics of a self-contained flight actuator

A mathematical model describing the dynamics of a self-contained flight control actuator is established. Typical failures of the actuator are defined in the space of the model parameters in the form of a failure pattern library. A diagnostic model of the actuator, intentionally sensitive to particular failures, is defined. A diagnostic procedure, featuring on-line parameter estimation of the diagnostic model, failure detection, and isolation, is proposed. A failure prediction procedure, utilizing trends exhibited by parameter estimates, is formulated. The results are verified by computer simulation     

Integrated design systems — capturing, reusing and optimizing design methods in new millennium

An important goal of New Millennium is to research new methods of performing spacecraft and mission design. We have completed the first phase of our effort on how to make design tools such as analysis programs more available. We are now embarking with Stanford University on discovering methods to allow more project history and knowledge to be automatically captured and reused and with Ames Research Center on how to use virtual reality to enhance the visualization of new missions before any hardware exists. We are also trying to capture the design process in an electronic form so that computer aided optimization may lead to a vastly greater search of the possible designs which meet the design requirements.