An Evaluation of the Xilinx Virtex-4 FPGA for On-Board Processing in an Advanced Imaging System

The multi-angle spectro-polarimetric imager (MSPI) is an advanced camera system currently under development at JPL for possible future consideration on a satellite based Aerosol-Cloud-Environment (ACE) interaction study as outlined in the National Academies 2007 decadal survey. In an attempt to achieve necessary accuracy of the degree of linear polarization of better than 0.5%, the light in the optical system is subjected to a complex modulation designed to make the overall system robust against many instrumental artifacts that have plagued such measurements in the past. This scheme involves two photoelastic modulators that are beating in a carefully selected pattern against each other [1]. In order to properly sample this modulation pattern, each of the proposed nine cameras' in the system needs to read out its imager array about 1000 times per second, resulting in two orders of magnitude more data than can typically be downlinked from the satellite. The onboard processing required to compress this data involves least-squares fits of Bessel functions to data from every pixel, effectively in real-time, thus requiring an on-board computing system with advanced data processing capabilities in excess of those commonly available for space flight. In order to meet the MSPI onboard processing requirements, we evaluated a Xilinx Virtex-4FX60 field programmable gate array (FPGA). In addition to configurable hardware capability, this FPGA includes PowerPC405 microprocessors, which together enable us to pursue a combination hardware/software processing system. This approach allows us to efficiently partition the processing system, using software for algorithmically complex tasks and hardware for more computationally intensive ones. In order to increase performance and/or meet data flow requirements we have developed a system that consists of hardware-based data-capture and least-squares Bessel function fitting and software-based transcendental function computation. We compare the capabilities of this system, and the V4FX60 accordingly, with the requirernents of the MSPI camera system.(2,3)