Performance Evaluation of Single and Multichannel Microwave Radiometers for Soil Moisture Retrieval

The performance of single-channel, two-channel, and six-channel microwave radiometers is evaluated for the retrieval of soil moisture using parameterized numerical simulations. Commonly used frequencies of 1.4 GHz, 2.7 GHz, and 6.6 GHz at vertical and/or horizontal linear polarizations are considered. The best single-channel retrieval uses 1.4 GHz at vertical polarization (1.4 V). This channel is least sensitive to variable surface roughness and vegetation canopy, but all single-channel sensors, including 1.4 V, are degraded without some level of a priori information about these masking variables. The sensitivity to surface roughness is reduced significantly at 2.7 GHz and 6.6 GHz by increasing the angle of incidence beyond 25°. The best two-channel case uses 1.4 V and 2.7 H to simultaneously retrieve both soil moisture and surface roughness. This improves the soil moisture retrieval and eliminates the need for a priori information about surface roughness. The sensitivity to variations in vegetation canopy is also acceptably small. Simultaneous use of all six channels to retrieve soil moisture, surface roughness, and vegetation canopy provides only marginal improvement over the best two-channel performance. The small amount of additional information contained in the other four channels is largely masked by typical levels of measurement noise.