C-band Polarimetric Upgrade: Digital Sampling and Processing

In order to make a full series of measurements for modeling the active layer of arctic tundra during the summer of 2004, a C-band (6.9GHz) radiometer currently in use must be converted from dual-polarization to fully polarimetric. Whereas dual-polarization measures the H and V planes independently, a fully polarimetric radiometer measures the correlation between the input channels and results in a set of numbers called the Stokes parameters. These numbers can be used for a much more detailed analysis of the radio energy being received. Polarimetry has been used in sea state estimation, but not yet for land remote sensing. A conversion to full polarimetric operation will allow a greater sensitivity to the surface structure of arctic tundra.

Given the short time frame and limited funding available to make this conversion, it is important to design the best possible radiometer system given the practical constraints. The first step in the upgrade was to do a design trade-off study between the various radiometer configurations as described in the 1999 paper by Skou, Laursen and Søbjærg, "Polarimetric Radiometer Configurations..."

It was quickly determined that moving to a fully digital correlating radiometer would be the most fruitful approach that had highest scientific payback and utility. There were ways to completely meet our primary goals using fully analog components, but doing so would expose the project to the pitfalls of analog. A digital system is infinitely more reconfigurable in software vs. reworking expensive hardware. Furthermore, once digitized, noise becomes a non issue and multiple experiments can take place at once in the processor.

So the polarimetric upgrade can be broken down into three main parts: sampling, processing and control. First, the amplified and filtered radiometric signals need to be digitized in an analog to digital converter. Then the signals are processed in the digital domain using simple math functions on a processor. Finally, the data needs to be stored and the system controlled externally.