Charles B. Cameron, Rosa Nívea Rodríguez, Nathan Padgett,
Eugene Waluschka, and Semion Kizhner, Optical Ray Tracing Using Parallel
Processors, IEEE Transactions in Instrumentation and
Measurement, Vol. 54, Issue 1, 87-97, February 2005.
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 of the earth during the ensuing
orbit. Although the instrument 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
really functioned. To learn what was wrong, NASA used an optical
ray-tracing program 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 than just four DSPs could be
harnessed before administrative overhead would begin to be significant.