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

The performance of single-channel, two-channel, and cal variables that play an important part in agricultural six-channel microwave radiometers is evaluated for the scheduling and water resource management (Jackson, retrieval of soil moisture using parameterized numerical 1993). Recent studies have shown the effects of soil simulations. Commonly used frequencies of 1.4 GHz, 2.7 moisture on the dynamics of the atmospheric boundary GHz, and 6.6 GHz at vertical and/or horizontal linear layer and hence on weather and climate. Such studies polarizations are considered. The best single-channel rehave also shown the influence of soil moisture on the trieval uses 1.4 GHz at vertical polarization (1.4 V). This feedback between land surface and atmospheric procchannel is least sensitive to variable surface roughness esses that lead to climate anomalies (Shukla and Mintz, and vegetation canopy, but all single-channel sensors, in1982; Delworth and Manabe, 1989). The improved charcluding 1.4 V, are degraded without some level of a priacterization of surface soil moisture, vegetation, and temori information about these masking variables. The sensiperature in numerical weather prediction models can tivity to surface roughness is reduced significantly at 2.7 also lead to significant forecast improvements (Beljaars GHz and 6.6 GHz by increasing the angle of incidence et al., 1996). For these reasons, the lack of a global hybeyond 258. The best two-channel case uses 1.4 V and drologic land surface-observing capability has been rec2.7 H to simultaneously retrieve both soil moisture and ognized as a limitation on improved climate forecasting surface roughness. This improves the soil moisture reand monitoring. Soil moisture has become a key meatrieval and eliminates the need for a priori information surement priority (Martin and Manu, 1999). about surface roughness. The sensitivity to variations in Passive microwave remote sensing has long been vegetation canopy is also acceptably small. Simultaneous recognized as a viable alternative to ground truth meause of all six channels to retrieve soil moisture, surface surements of soil moisture (Schmugge, 1978; Njoku, roughness, and vegetation canopy provides only marginal 1996). Groundand aircraft-based microwave radiomeimprovement over the best two-channel performance. ters have been used in numerous field experiments to The small amount of additional information contained in validate the approach and quantify its performance (e.g., the other four channels is largely masked by typical levJackson, 1993; Wang and Choudhury, 1995; Jackson et els of measurement noise. Elsevier Science Inc., 2001 al., 1999). Previous attempts to retrieve soil moisture from space-borne radiometer measurements have had limited success due to the use of frequencies at 6.6 GHz INTRODUCTION or above (Neale et al., 1990; van de Griend and Owe, 1994). Lower microwave frequencies are better suited to Soil moisture is of major importance to the hydrological soil moisture retrieval, and there are now several spacecycle. It is one of the few directly observable hydrologiborne mission designs being conducted that use them (Le Vine et al., 1989; Martin-Neira et al., 1994; Njoku et al., 1999). The two most commonly considered lower * Department of Electrical Engineering, The Pennsylvania State University, University Park, PA frequency options are 1.4 GHz and 2.7 GHz. Adequate Address correspondence to Christopher S. Ruf, University of Michspatial resolution from orbit at 1.4 GHz or 2.7 GHz igan, Dept. of Atmospheric, Oceanic & Space Sciences, Ann Arbor, MI would require much larger antenna apertures than have 48109, USA. E-mail: [email protected] Received 14 March 2000; revised 16 June 2000. been flown in the past. One compromise that has re-