Title of Paper:  Ultra-low power digital correlator for passive microwave polarimetry


Principal Author:  Dr. Jeffrey Piepmeier


Abstract:  The design and application of a high-speed digital cross correlator IC, a critical enabling technology for future ocean surface wind instruments based on passive microwave polarimetry, is discussed in this paper. Global ocean wind vector measurements are key to understanding climate variability and prediction, particularly ocean circulation and its response to changes in surface forcing. Indeed, the value of ocean wind measurements is evident in the NASA Earth Science Vision for post-2002 research missions, which includes the dedicated EOS-5 Ocean Surface Wind Measurement Program. Recent advances in passive microwave polarimetric techniques have demonstrated the feasibility of measuring ocean surface wind speed and direction by observing the full polarization state of microwave emission upwelling from the ocean surface. The high-speed digital cross correlator IC is used to detect the polarization state, or Stokes vector, by measuring the in- and quadrature-phase correlations between two orthogonally-polarized thermal microwave signals. The signals are received by a conventional dual-polarization radiometer, but are sampled at 500MHz and 3-level quantized before detection. The correlator IC computes cross products that are integrated for up to 224 clock cycles. The integration time is programmable with a step size of 28 clock cycles. The design includes a simple bus-oriented interface for use with a microprocessor. Targeted for eventual space-flight use, the correlator chip has been designed with radiation-tolerant architecture and will be fabricated using 0.35mm ULP CMOS logic operating at 1.0V or less (possibly down to 0.5V). Using this correlator IC as part of a future passive microwave imager will enable observations of ocean surface wind speed and direction in addition to many other atmospheric, land, oceanic, and cryospheric parameters.