X-band Repeat-pass Coherence at Short Temporal Baselines for Crop Monitoring

A one-year time series of X-band images acquired by TerraSAR-X (TSX), TanDEM-X (TDX), and PAZ in the same orbit configuration over an agricultural area has been exploited to assess the potential of X-band repeat-pass coherence for crop monitoring. The combination of TSX, TDX, and PAZ allows working with temporal baselines of 4 and 7 days, i.e., shorter than the 11-day revisit time of the individual satellites. These short temporal baselines are less affected by temporal decorrelation and help increase the sensitivity of coherence to crop growth. The analysis is carried out for 30 different crop types by comparing series of coherence and backscatter at X-band with normalized difference vegetation index (NDVI) extracted from Sentinel-2. C-band coherence from Sentinel-1 is also analyzed with the same dataset for comparison purposes. In addition, a new radar vegetation index for copolar data, called copolar radar vegetation index (coRVI), is defined and evaluated. During the phenological cycle of the crops, coherence decreases at the early stages, and it increases again after harvest, keeping high values out of the growing season. X-band coherences for baselines of 4 and 7 days show the strongest correlations with NDVI, especially for short crops. Regarding the use of coRVI, it outperforms the backscattering coefficient of individual channels. Moreover, coRVI is better correlated with NDVI than coherence for several crop types, hence providing complementary information. The main conclusion is that X-band repeat-pass coherence with short temporal baselines (4 or 7 days) could be a valuable tool for crop monitoring if an adequate acquisition plan were implemented routinely.