The AGSI design permits scan rates slow enough to detect stars as dim as visual magnitude eight in the coarse of normal imaging. This gives many times the number of stars seen with the current Geosynchronous Operational Environmental Satellite (GOES) Imager and can eliminate the need to schedule special star looks. Besides improving image navigation and registration accuracy, the frequency observations enable the Imager to fly aboard a spacecraft with loose attitude control. The slow scan rate is thanks to the long CCD detector arrays and to the time delay integration made possible by the unique windshield wiper scan pattern. The Bremer star detection algorithm describe can be implemented onboard to reduce downlink requirements and so permit star detection across a dedicated full silicon passband. The wide passband increases the number of detectable stars, and cross checking with narrower science passbands eliminates false alarms from high energy particles while preserving low detection thresholds and sensitivity.
The Advanced Geosynchronous Studies Imager (AGSI) system design combines the latest available technologies into an instrument design concept which could deliver the improved performance sought by the National Weather Service at NOAA and meet NASA earth system science goals in a joint program. The instrument could cover the Earth disk every 15 minutes with subsatellite point resolution form 1/2 kilometer in the visible to 2 kilometers in the long wave IR. Simultaneously, it could provide coverage of a 3000 by 5000 kilometer region in 5 minute intervals and 30 second updates of a 1000 kilometer square region containing a weather system of interest. We found that performance margins could be improved even as we drove the design interactions with emphasis on reducing the mass. Scan speed was chosen by maximizing performance while trading off the acceptable impact on the total systems. The resulting 18-channel design could deliver vastly improved performance over the present GOES without great increases in mass or volume, while still paying close attention to control of development cost sand impact on the host spacecraft. The design could be adapted to changed requirements or descoped to have lower data rates and fewer channels.
KEYWORDS: Staring arrays, Long wavelength infrared, Signal to noise ratio, Mid-IR, Short wave infrared radiation, Visible radiation, Mercury cadmium telluride, Sensors, Detector arrays, Signal processing
A focal plane system to generate images in 18 spectral channels ranging from visible to long-wave IR has been designed for the AGSI. The system is a line scan imager, comprised for four focal planes populated with linear detector arrays optimized for operating in the various AGSI spectral regions. Light from the AGSI telescope is diverted to the focal planes by spectrally tuned dichroic beam splitters. At the focal planes narrowband interference filters placed in close proximity to the detector arrays further filter the light. Multispectral silicon CCDs are used for visible and near IR channels. Key to system performance is the ability to use time delay and integration (TDI) in some of the narrower or less photon rich spectral channels. Detector arrays are supported by highly modular and therefore flexible and low risk signal processing and control circuits. Performance predictions have been generated for all of the spectral channels and show that the focal planes will meet or exceed NASA's requirements for an advanced imaging observatory to observe weather and climate processes and NOAA's requirements for an advanced GOES imager.
SWIR (short-wave infrared) imaging technology, phenomena, and applications are described. Commercial SWIR (staring InSb, PtSi, HgCdTe, InGaAs) camera specifications and optimization procedures are discussed. SWIR physics including blackbody distribution, atmospheric MODTRAN predictions, and selected material reflectance measurements are reviewed to illustrate basic guidelines to successful SWIR imaging. SWIR imaging examples of military applications, medical imaging, astronomy, long range observations, plume measurement, and art preservation are included to illustrate the unique properties of SWIR imaging.
A multispectral intensified CCD imager combined with a ring laser gyroscope based inertial measurement unit was flown on the Space Shuttle Discovery from July 13-22, 1995 (Space Transport System Flight No. 70, STS-70). The camera includes a six position filter wheel, a third generation image intensifier, and a CCD camera. The camera is integrated with a laser gyroscope system that determines the ground position of the imagery to an accuracy of better than three nautical miles. The camera has two modes of operation; a panchromatic mode for high-magnification imaging [ground sample distance (GSD) of 4 m], or a multispectral mode consisting of six different user-selectable spectral ranges at reduced magnification (12 m GSD). This paper discusses the system hardware and technical trade-offs involved with camera optimization, and presents imagery observed during the shuttle mission.
Data is presented from several types of infrared spectral imaging systems: narrowband, linear variable filter, transmission grating, and hyperspectral. A comparison review of existing infrared hyperspectral systems is provided.