冬小麦冠层光谱的方向性特征分析

Analysis of Directional Characteristics of Winter Wheat Canopy Spectra

二向性反射因入射和观测的角度变化而变化,合理选择遥感观测角度、太阳天顶角等为提高遥感应用精度提供可能。通过采用各向异性因子和各向异性指数定量分析了冬小麦冠层窄波段二向性反射率及NDVI的方向性特征。分析结果表明:主平面内二向性反射率的各向异性最强,垂直主平面最弱,其他平面介于前两者之间;可见光波段反射率随观测天顶角的敏感性大于近红外波段;反射率对太阳天顶角的敏感性随着观测天顶角的增大而增大。前向NDVI整体上大于后向,且NDVI随着太阳天顶角的增大呈增大趋势。为了减少观测方式所带来的不确定性,应尽量选择近红外波段和小太阳天顶角,估算结构参数时应尽量选择主平面进行遥感观测,在利用NDVI估算生物物理参数时尽量远离"热点"区域。

The bidirectional reflectance factors vary as the incidence directions and the view angles change.At present the remote sensing is almost at nadir,therefore it is possible to improve the accuracy of remote sensing application by reasonably selecting the looking angle,solar zenith angle,and so on.Based on the multidirectional spectra of winter wheat canopy at several critical growth stages,the paper quantitatively analyzed the sensitivity of narrowband bidirectional reflectance to view planes,view zenith angle,solar zenith angle,growth stage,and band by using anisotropy factor （ANIF） and anisotropy index （ANIX）.The change of NDVI with view zenith angle,solar zenith angle and growth stage was also studied.The results show that the anisotropy characteristics of bidirectional reflectance factors at solar principal plane was stronger than that at the other planes,and orthogonal principal plane was the weakest.The ANIX at solar principal plane was the biggest.The reason was that the shadow of canopy changed more dramatically at solar principal plane than at the other planes.The sensitivity of bidirectional reflectance factor at visible bands to zenith angles was stronger than in near infrared regions,the reason for which was that the shadow effect in visible regions was stronger than in near infrared regions.The ANIX in visible regions was bigger than in near infrared regions.The sensitivity of bidirectional reflectance factor to solar zenith angles increased as the view zenith angle increased.The NDVIs at every looking zenith angle all increased with the leaf area index increasing.The NDVIs at forward direction were larger than at backward direction,which resulted from that the shadow effect in visible regions was stronger than in near infrared regions.The solar principal plane implies rich internal structure information on object.In order to reduce the uncertainty from the observing method,the near infrared bands and small solar zenith angle should be chosen.The retrieve of structure parameters ought to select solar principal plane,and avoid hot spot region when inversing biological parameters using NDVI.