Retrieving crop parameters based on tandem ERS 1/2 interferometric coherence images

One-day interval coherence images derived from pairs of ERS SAR tandem acquisitions are suitable for crop monitoring. Coherence images were analyzed and compared to field measurements of four crops, i.e., winter wheat, sugar beet, potato and maize, taken during the satellite overpass. First, the sensitivity of the coherence to the plant height and the canopy cover was statistically investigated. Regression analyses were computed and the coefficients of determination (R2) ranged from 0.64 to 0.92. The shape of these relationships varied according to the geometric factors which are crop-type dependent. A prediction model of the wheat height was calculated and was able to estimate the plant height with a mean absolute error of approximately 7 cm. While this high performance obtained on the average matched the observed range of the field height for a given date, it was not sufficient for monitoring at the field level. However, such a performance level may meet the information requirements for an operational crop monitoring system at the regional level, which includes a much larger diversity of growing conditions. Moreover, the soil roughness change associated with the sowing practices that occurred between the two tandem acquisitions strongly decreased the coherence signal. This dataset also indicated that variation in the soil moisture influenced the backscattering coefficient more than it influenced the coherence signal. This result enhanced the InSAR coherence potentialities to estimate the crop parameters during the growing season.