Title of Paper: On-Orbit Calibration of Correlation Radiometers
Principal Author: Ed Kim
Abstract: Many of NASA's earth science requirements for the next decade, especially in the Earth Science Enterprise themes of the Global Water & Energy Cycle and Climate Variability & Change, are based on observations using passive microwave instruments due to the strong microwave signal from water in its various phases. In particular, two types of microwave radiometry---synthetic thinned array radiometry (STAR) and fully-polarimetric (FP) radiometry---have received increasing attention during the last several years. STAR radiometers offer a technological solution to achieving high spatial resolution imaging from orbit without requiring a filled aperture or a moving antenna, and FP radiometers measure extra polarization state information upon which entirely new or more robust geophysical retrieval algorithms can be based. Several instrument concepts for the post-2002 earth science notional missions employ one or even both of these two technologies.
Radiometer configurations used for both STAR and FP instruments share one fundamental feature which distinguishes them from more "standard" radiometers, namely, they measure correlations between pairs of microwave signals. The resultant increase in instrument complexity creates a variety of additional technological requirements including tight matching and stability of the radiometer receivers themselves, as well as a more complex yet robust in-flight calibration subsystem. Calibration is a critical aspect of any instrument design and operation. However, it is often considered to be of secondary importance to the design of the instrument itself. As instrument complexity increases this approach becomes more risky, potentially increasing the chances of finding problems late in the development cycle and increasing cost. The complex nature of correlation radiometer calibration, coupled with the inherent similarities between STAR and FP instruments, suggests significant leverage in addressing both problems together. We descibe efforts to develop a compact low-power subsystem for in-flight STAR and FP receiver calibration including a signal source with precisely generated correlation properties, along with a laboratory correlation receiver testbed to provide realistic test conditions, to guide optimization, and to generate recommendations for the design of a flight-qualifiable on-board calibration subsystem.