Research Horizons
Miniature Space-Weather Instrumentation
Space-weather situational awareness is hampered by a lack of environmental sensors that could be used by operators and analysts to assess mission hazards and improve future spacecraft designs. Environmental monitors are not routinely flown on all spacecraft because of the significant costs and resources associated with their integration. When anomalies occur on a spacecraft without environmental monitors, the anomalies can only be diagnosed using environmental data from nearby spacecraft. This can give erroneous results because of the highly localized environmental input and differences in spacecraft design.
To improve situational awareness and anomaly diagnosis, there is a need for small, cost-effective, and commercially viable devices to serve as standard housekeeping monitors on all spacecraft. William Crain of the Space Instrumentation Department, along with Dan Mabry, Bern Blake, and Norman Katz, is working on two such instruments: a compact radiation dosimeter and an electrostatic discharge (ESD) monitor.
Aerospace has developed several energetic particle sensors that have flown on Air Force, National Reconnaissance Office, NASA, and commercial satellites, but no radiation dosimeters have been made small enough to enable their placement directly on critical subsystems. "At present," Crain explained, "the radiation dose at one location on a spacecraft can only be inferred from the dosimeter instrument placed at another location." Also, no ESD monitors have been developed with the ability to record a transient waveform, which is critical to determining the discharge source.
Dan Mabry uses a miniature dosimeter to record the total radiation dose absorbed by the silicon detector over the life of a spacecraft mission. The dose is read over standard spacecraft analog housekeeping interfaces similar to simple temperature readouts. |
Crain's team successfully addressed the first problem by developing a small dosimeter that directly measures the radiation dose in a package about the size of a coin. "The new miniature dosimeter is small enough to be placed anywhere on a spacecraft, inside a payload, or on a circuit board," Crain said. "Because its detection element is a silicon test mass, the absorbed dose measured by the device portrays an accurate prediction of the dose being absorbed by other neighboring payload microelectronics." When the dosimeter was tested at the 88-inch cyclotron at Lawrence Berkeley Laboratory (LBL) at UC Berkeley, it was found to be in good agreement with two laboratory control dosimeters.
Two patents were granted to Crain and his team this year, one for the dosimeter device and another for a dosimeter system that profiles the wide area total radiation dose distributed throughout a given spacecraft. A novel method to increase the accuracy of the radiation dose measurement in the presence of low-energy electrons, which can dominate the dose in some orbits and locations on spacecraft, was conceived by Norm Katz of the Space Instrumentation Department. The system integrates detector charge prior to threshold detection and adds the result to the post-threshold charge. "This eliminates the error associated with detection and integration of radiation pulses whose amplitudes are near the energy threshold of the detector," Crain said.
Work continues on the next version of the dosimeter, which will use a new application-specific integrated circuit (ASIC) that includes the improved integrator, logarithmic output range to compress the dose readout, and a threshold compensation circuit to improve the dose efficiency at low particle energies. It will also be designed for single event upset tolerance. The dosimeter will use a smaller detector, reducing the size of the dosimeter package even further.
As the research continues, Crain plans to test the enhanced reduced-size dosimeter devices, including radiation tests at LBL, and develop a high-reliability fabrication process for dosimeter devices. The dosimeter will be upgraded to improve its commercial viability and work will continue on developing a licensing plan for it. A dosimeter will be integrated on the NASA Lunar Reconnaissance Orbiter, which is scheduled to launch in 2008.
Crain, Mabry, and Jim Roeder of the Space Sciences Department have also been working on the ESD monitor. The monitor is based on the SC1-8B transient pulse shape analyzer, which was developed at Aerospace and first flown on the Spacecraft Charging at High Altitude (SCATHA) mission in 1979. This mission provided a fertile proving ground for the measurement technique, detection ranges, antenna selection, and software processing algorithms to be implemented in the ESD monitor. The SCATHA experience also demonstrated Aerospace's ability to detect and identify discharge events throughout the spacecraft, and to discriminate environmentally induced events from those caused by operations (e.g., relay switching, thruster firing). The SCATHA mission also revealed that the transient signature of an ESD event contains vital information to determine whether the discharge was operationally induced, caused by the environment, or the result of hostile action.
The ESD monitor Crain and Mabry are developing will be small and lightweight and will interface with standard spacecraft housekeeping systems. A central element of the device will be an ASIC chip containing an analog transient recorder and readout system. Research is under way to determine the best way to display ESD transients on a ground system terminal in a form operators can use for mission risk assessment. Further study will be aimed at determining the sufficient data set needed from an ESD event to improve anomaly analysis and enhance environmental specifications.
Both the dosimeter and the ESD monitor will provide operators with a visual warning of hazardous environments, data for anomaly diagnosis, improved environmental specifications, and the ability to distinguish environmental phenomena from hostile action.
To Fall 2007 Table of Contents
