6. Joint active-passive microwave for soil moisture estimation

The radar signal comprised of the amplitude and phase is coherent and more influenced by the surface roughness and vegetation. In contrast, the radiometer signal is incoherent, reducing the influences from the surface roughness and vegetation. In addition, the radar signal is acquired with high spatial resolution at the cost of narrow swath range, while the radiometer signal has a frequent revisit cycle but coarse spatial resolution. In order to combine the advantages of the radar and radiometer signals, recent studies go into the soil moisture retrieval by a joint activepassive approach. In this context, the original objective of the SMAP mission is to monitor the soil moisture by the active-passive combination, although the radar component failed.

For the airborne platform, the active and passive signals can be obtained with a similar spatial resolution. The optimization process is conducted to match the microwave signals to the model output. For instance, the following cost function was constructed [45] by using both the radar and radiometer signals:

$$\mathbf{C(X)} = 0.5 \left[ \sum \left| \frac{\sigma\_{pq}^{data} - \sigma\_{pq}^{simu}(\mathbf{X})}{\sigma\_{pq}^{data}} \right|^2 + \chi \cdot \sum \left| \frac{TB\_p^{data} - TB\_p^{simu}(\mathbf{X})}{TB\_p^{data}} \right|^2 \right] \tag{40}$$

where σdata pq and TBdata <sup>p</sup> are the real data from the radar and radiometer, respectively. σsimu pq ð Þ X and TBsimu <sup>p</sup> ð Þ X are the simulated radar and radiometer signals. The γ is a tuning parameter to balance the radar and radiometer signals in the optimization process. The increase of γ represents the enhanced contribution of the radiometer signals for the soil moisture retrieval. The airborne Passive-Active L-band Sensor (PALS) data were collected during the SMAPVEX12 and SMAPVEX16 campaigns, providing an opportunity to develop the active-passive soil moisture retrieval approaches.

For the spaceborne platform, such as the condition of the original SMAP mission, the radar and radiometer signals have different spatial resolutions. In this case, the radar signal with fine spatial resolution is used to disaggregate the radiometer signal with coarse resolution to obtain TB data with moderate resolution, considering the correlation between the radar and radiometer signals. Then, the emission model was applied to the disaggregated brightness temperature to retrieve the soil moisture at a moderate spatial resolution.
