Microwave remote sensing: a perspective from the last few field experiments

There have been numerous field experiments which have tested the effectiveness of microwave remote sensing, both active and passive under varied land surface conditions. The Southern Great Plains Experiment 1999 (SGP99) was held in Chickasha Oklahoma where winter wheat and rangeland was the predominant land surface type whereas at the Soil Moisture Experiment 2002 (SMEX02) in Walnut River watershed in Ames Iowa it was a mixture of corn, and soyabeans. In the SMEX03 (Soil Moisture Experiment in 2003) in Little River Watershed, the land surface was a mixture of peanuts, vegetables, cotton and pasture and for SMEX04 (Soil Moisture Experiment in 2004) in Walnut Gulch, Arizona, the land surface cover is primarily brush and grass covered rangeland vegetation. Given that microwaves have low sensitivity to soil moisture in the presence of vegetation, these field experiments offer an opportunity to examine observations of sensitivity in the presence of varied (and varying with time) vegetation densities. In addition, in each of these experiments, there were different instruments on aircrafts and satellite sensors that were deployed. In SGP99, we had observations from the PALS (Passive Active L and S band Radar and Radiometer), PSR (Polarimetric Scanning Radiometer) from the C130 aircraft and the TMI (TRMM Microwave Imager) and SSM/I (Special Sensor Microwave Imager) from space. In SMEX02, we had PALS, PSR, AIRSAR (Airborne Synthetic Aperture Radar) from the aircraft platforms and in SMEX03 PSR only. Satellite sensors in SMEX02 and SMEX03 included AMSR (Advanced Microwave Scanning Radiometer), TMI, and SSM/I. In the recently concluded SMEX04, PSR was used from the aircraft and AMSR, TMI and SSM/I satellite observations were available. We will use these sensors and observations in the microwave channel in conjunction with ground observations of vegetation characteristics and soil moisture to study the sensitivity of microwaves to soil moisture under varied land surface conditions.