Optical Ray Tracing Using Parallel Processors

摘要

One of the instruments on the sun-synchronous Terra (EOS AM) and Aqua (EOS PM) spacecraft, the Moderate Resolution Spectroradiometer (MODIS), obtains calibration data once during every orbit. Observations of the sun permit corrections to observations ofthe earth during the ensuing orbit. Although the in- strument was designed to receive uniform sunlight over the entire surface of its detector, the sunlight was in fact not uniform. While this did not adversely affect the calibration, it nonetheless implied a lack of understanding of how the optical system rcaily functioned. To learn what was wrong, NASA used an optical ray-tracing pro- gram on a DEC Alpha computer. The results correlated well with the observations made by the instrument itself, but it took nearly two weeks to complete the computer simulation, a discouragingly long time. This paper describes the algorithm and its implementation in a system with multiple digital signal processor (DSP) chips operating in parallel. Timing data show a highly linear relationship between the number of DSPs present and the speed of the computation. Administrative overhead is negligible compared to the time taken to compute ray trajectories. This implies that many more thanjust four DSPs could be harnessed before administrative over- head would begin to be significant.