Specifically, the NGST Image Processing Group (IPG)1is developing cosmic ray rejection and data compression algorithms for parallel processors. Real-time cosmic ray rejection would enable us to reduce the downlink data volume by as much as two orders of magnitude by combining multiple read-outs on the spacecraft rather than downlinking them separately. The combined read-outs can be further reduced in size by applying either lossy or lossless data compression algorithms. This paper describes these algorithms, their implementation and optimization for a parallel, fault-tolerant computing environment, and their relationship with the REE project.
The translation of HPCC technology into space requires the Project to address issues of mass, power, fault tolerance, and reliability which are different from the concerns of ground based computing. REE will validate computing efficiencies on the order of 300-1000 MOPS per watt in a multiprocessor architecture that can scale from 1 to 100 watts, depending on the specific mission needs. Software Implemented Fault Tolerance (SIFT) will be developed to use COTS (i.e., non-radiation hardened) components in space without compromising reliability or efficiency.
Radiation affects the reliability of conventional electronic components so hardware faults are likely to happen. Hardening a microprocessor requires either expensive basewafers, more power than equivalent non-hard circuits, or more chip area. The process involves a number of tradeoffs which include radiation tolerance, cost, chip area, electrical performance, and power dissipation that make it very expensive.
REE intends to lower the cost by using COTS technology combined with SIFT algorithms. A major challenge will be to develop a fault detection and recovery schema that assures system reliability without compromising the performance of applications over the intended NGST operation period of 10 years.
In a baseline of exposure, about of the data in the field-of-view will be lost due to cosmic ray hits. To optimize NGST data quality, we should read the detector frequently and discard only suspected cosmic ray events, keeping as much good data as possible. However, the capacity of the current baseline NGST-to-ground communications link (5-6 GB/day) is several orders of magnitude lower than the volume of raw data (320-640 GB/day) required to perform cosmic ray removal on the ground (Fixsen et al. 2000).
We are focusing on two onboard processing approaches, Fowler sampling without cosmic ray rejection (NGST Integrated Science Instrument Module (ISIM) baseline) and uniform sampling with cosmic ray rejection (NGST IPG). Both of them are based in the non-destructive readout capability of detectors as well as the combination of all readouts in one single final image (Fixsen et al. 2000). As it is presented in the cited paper, uniform sampling with cosmic ray removal is good for integration times as long as or even longer. Long observations are desirable because they both maximize the signal to noise ratio and reduce the number of images to be downlinked from the spacecraft.
The ISIM baseline design is for two redundant, flight-qualified computers, each with three 150 MHz PowerPC CPUs. We estimate that the cosmic ray rejection and compression code can run on one of these computers (three CPUs) concurrently with planned house-keeping and management tasks with up to 32 readouts for a baseline of exposures (approximately 1 readout per ). Although the results on this baseline of 32 readouts seem to be satisfactory, REE would provide us with the computational power necessary to process 64 readouts in so the read-noise can be properly reduced. Additionally, appropriate on-board cosmic ray detection followed by either lossless or ``wise'' lossy compression (Nieto-Santisteban et al. 1999) would allow us to downlink not only processed images without cosmic rays but also the whole ramp of suspect pixels. The cost in bandwidth would be the same or even less than the cost of downlinking differences of averaged Fowler samples having cosmic ray contamination.
In order to gain experience with SIFT techniques, several generations of testbeds are being built to perform fault tolerant experiments. These testbeds will be provided with a SIFT middleware layer residing between the operating system and the applications, and between the operating system and the hardware. A software fault injector for simulating of radiation faults has been also generated to test the system and study its performance. The NGST IPG's contribution to these tests has been the development of parallel code for cosmic ray rejection and compression to be run on the testbed under a radiation induced fault environment.
Consultative Committee for Space Data System 1997, CCSDS120.0-G-1 Green Book.
Fixsen, D. J., Offenberg, J. D., Stockman, H. S., Hanisch, R. J., Nieto-Santisteban, M. A., Sengupta, R., & Mather, J. C. 2000, this volume, 539
Nieto-Santisteban, M. A., Fixsen, D. J., Offenberg, J. D., Hanisch, R. J., Stockman, H. S. 1999, in ASP Conf. Ser., Vol. 172, Astronomical Data Analysis Software and Systems VIII, ed. D. M. Mehringer, R. L. Plante, & D. A. Roberts (San Francisco: ASP), 137