SUMMARY OF CONCLUSIONS
An objective of this project was to determine if an imager using FPA detector technology could be designed to provide performance that would meet the NWS requirements for imaging from geostationary orbit for the year 2000 and beyond. We also examined the potential for including such an imager in the payload of a commercial geostationary communications satellite. The limited pointing accuracy of communications satellites compared with that for satellites such as the GOES I, which are dedicated to the imaging mission, led us to explore means to compensate for pointing inaccuracies and for spacecraft motion by exploiting the imaging capabilities of the FPA instrument. We postulated ground processing concepts to achieve required INR performance given the 3 minute full-earth images provided by the FPA instrument. While many tradeoffs and options still exist, we can present our conclusions based on our work to-date. We summarize the detailed conclusions presented in sections 2 through 5 of this report as follows:
* Second-generation FPA detector technology is the key to improved imaging from nondedicated satellites. We believe that a rapid imager is feasible and can provide the resolution and radiometric performance sought by the NWS as expressed in the NWS' GOES-N requirements [GOESN89].
* A number of technologies now under development are important to FPA imager development. For example: IR detector arrays should be cooled to 70 degrees K or less. A passive radiative cooler to do this would be too large for an imager on a nondedicated satellite. The reverse turbo Brayton mechanical refrigerator is preferred. Mechanical refrigerator developments are well underway with lifetime goals of 10 years.
* For step-stare scanning, the most demanding NEdT requirement is to be able to detect a 0.1 degrees K temperature change in a 300 degrees K scene in channels 4 and 5. This puts a 0.1 percent limit on residual FPA detector nonuniformities, which is within the state-of-the-art for FPA technology. TDI scanning offers superior radiometric performance and reduced requirements for detector production yield compared with step-stare scanning.
* The use of FPA detectors precludes the use of image motion compensation and mirror motion compensation as defined for the GOES I system. This is due to the rigid spatial relationships among the pixels within a single frame of an image, and the consequent inability to alter the positions of individual pixels during image creation.
* The use of overlapping image frames in the step-stare scanning method offers deterministic spatial relationships within and among image frames to a greater degree than TDI scanning. This provides well-coregistered images using ground-based processing for assembling the frames into a whole image. While the predictor-corrector and feedback approaches to INR place the burden of image spatial relationships on the spacecraft and the imager scan system, the overlap approach uses data redundancy and ground-based image processing and results in the simplest space segment. Whole-image assembly from overlapped frames permits compensation for all spacecraft-earth relative motions.
* The proposed instrument can be operated in a free-running mode thereby precluding the need for scheduling special scanning modes or scenarios. The high resolution, improved image quality, and rapid 3 minute update rate for full-earth coverage make the resulting imagery suitable for AWIPS use and for the mesoscale observation objectives of the NWS Modernization program.
* It appears programmatically and legally feasible to deploy FPA instruments on nondedicated commercial communications spacecraft. Costs for such deployment may be sufficiently less than deployment on a dedicated spacecraft. Deployment of an instrument on a non-dedicated satellite could be done as a primary mission or as an adjunct to a primary mission. It also is possible to deploy a developmental instrument in this way to avoid risk to an operational system. Opportunities for deployment on commercial geostationary communications satellites including several Direct Broadcast Service and Mobile Subscriber Service satellites will be available in the post-2000 time frame. There appears to be little opportunity for sharing satellites with currently programmed government geostationary systems.
In the post-2000 time frame, NOAA will need advanced remote sensing technology that includes imaging from geostationary orbit. FPA technology can meet and, in some cases, exceed the performance anticipated in the GOES-N requirements for that era. Technical, programmatic, and legal issues do not appear to present barriers to lower-cost, high-quality imaging instruments to satisfy anticipated needs.
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