A Multibaseline Pol-InSAR Inversion Scheme for Crop Parameter Estimation at Different Frequencies

A novel oriented volume over ground (OVoG) inversion scheme is developed and tested on a data set of simulated agricultural scenarios and real SAR acquisitions. The algorithm makes use of multibaseline measurements to estimate the whole set of the OVoG structural parameters (e.g., crop height, differential extinction between eigenpolarizations, and ground-to-volume ratios) and is significantly robust against non-volumetric decorrelation contributions. The theoretical assessment points out that, in the dual-baseline case, the vegetation height hV can be estimated with a relative root-mean-square deviation (%RMSD) of 7.8% if the selected baselines fulfill the condition 1.2 <; κ
_z
h
_V
<; 2.8 rad (κ
_z
is the vertical wavenumber). Furthermore, the variance of the estimates is inversely related to the number of baselines Nb. Compared with the dual-baseline case, the RMSD of the differential extinction is reduced by 45% (from 1.1 to 0.6 dB/m) when Nb = 5 baselines are employed, whereas its mean bias is independent of Nb. The proposed scheme has been assessed using a set of repeat-pass F-SAR acquisitions at L-, C-, and X-band of an agricultural area in Germany. Using two baselines, the height of maize and rape fields is estimated with an average 10% %RMSD if the inversion is carried out over L-band acquisitions. On the other hand, when X-band data are employed, one can obtain reliable estimates of wheat and barley height, with a %RMSD better than 24%. The study also indicates the existence of differential wave propagation effects through maize (Δσ = σ
_VV
- σ
_HH
between 0.7 and 1 dB/m) and rape (Δσ = -0.8 dB/m) canopies at L-band.