Ultra-low Power Digital Correlator for Passive Microwave Polarimetry

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. 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 inand quadrature-phase correlations between two orthogonally-polarized thermal microwave signals. The signals are received by a conventional dual-polarization radiometer, but are sampled at 500 MHz and 3-level quantized before detection. The correlator IC computes cross products that are integrated for up to 2 clock cycles. The integration time is programmable with a step size of 256 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 is being fabricated using 0.35um ULP CMOS logic operating at 1.0V or less (possibly down to 0.5V). INTRODUCTION 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 and the EOS-2 Climate Variability and Trend Mission. Passive microwave polarimetry, a complimentary technique to active microwave scatterometry, is one This development was supported by the NASA Earth Science Technology Office’s Advanced Technology Initiatives Program. of the most important and recent developments in the remote sensing of ocean wind vectors (e.g., [1]). Microwave polarimeter technology is specifically identified within the NASA/ESE Capability and Needs Assessment (CNA) matrix as an enabling technology for future ocean-wind measurement missions. This paper discusses the design of a high-speed digital correlator integrated circuit for microwave polarimetry and its application to ocean surface wind velocity remote sensing. Recent advances in passive microwave polarimetric techniques have demonstrated that significant geophysical information is contained not only within the vertical and horizontal polarized brightness temperatures, but also within the full polarization state of the microwave emission upwelling from the ocean surface (e.g., [2]). Aircraft observations strongly suggest that a space borne multi-frequency passive microwave polarimeter would be capable of measuring ocean vector winds with an rms error 2 m s in speed and 20 in direction [1]. In addition, such an instrument would be capable of all the observations currently made using conventional radiometers such as the upcoming EOS AQUA AMSR-E (e.g., sea surface temperature, precipitation, and sea ice). A distinction is drawn between a polarimeter and a radiometer by considering a complete description of the radiation field as defined by the modified Stokes vector: