A great deal of radar signature research has been conducted in the past 

 decade and has revealed that C-band or higher frequency radar backscattered 

 signals obtained at high incidence angles are sensitive principally to the 

 water content in a vegetative canopy. Indeed, these higher-frequency radars 

 may be used to distinguish among crop types when measurements are made at 

 periodic intervals through the growing season. These experimental studies 

 have also revealed that a C-band radar operating in the 10°-20° incidence 

 angle range shows a strong sensitivity to soil moisture in the top few 

 centimeters of fields with a wide range of surface roughness and vegetative 

 covers. Significant effects of row structure and row direction have been 

 observed at all frequencies, especially near L-band and near 20° incidence. 

 Most of these experimental studies have been conducted using truck-based boom- 

 mounted radar spectrometers or airborne scatterometers in the 1-18 GHz 

 frequency range. 



The specific radar parameter research issues of interest in the present 

 study are more narrowly focused on the question of the utility of L- and/or C- 

 band SAR imagery for the potential applications listed in Table 2. Key 

 research questions are: 



When considering data from both radar and visible and infrared sensors, 

 what are the best choices for wavelength, incidence angle, and 

 polarization? 



What should the revisit time be? 



What is the best combination of resolution and number of looks? 



What improvement would be realized by using both L- and C-band? 



What improvement would be realized by using two polarizations, e.g., 

 like and cross? 



1. Incidence Angles for Vegetation (Especially Crop) Applications 



Preliminary results suggest that the preferred incidence angles for 

 vegetation canopy identification and condition assessment by SAR are in the 

 45°-60° range due to the fact that this configuration minimizes surface 

 scatter from the soil under the canopy and maximizes volume scattering from 

 water contained in the canopy. However, additional research is needed to 

 establish firmly these results for L- and C-band SARs. Multidate data over 

 several crops, forest types, and wetlands types at L- and C-band for angles 

 from 45°-60° are needed to allow researchers to address this issue. 



2, Dual-Frequency Utility 



The team recommends both C- and L-band based on the approximately 4 to 1 

 wavelength ratio and the importance of wavelength to volume and surface 

 backscattering. The performance of a dual-frequency L- and C-band system 



2-3 



