An Effective Emission Depth Model for Passive Microwave Remote Sensing

Knowing the exact penetration depth for passive microwave will enable us to gain a better understanding of the characteristics of the targets. But now, there is only a penetration depth model that can calculate the specific depth while it is suitable for active microwaves. However, due to the completely different working mechanisms, the penetration depth model is most likely limited when applying to passive microwaves. Therefore, an effective emission depth model for passive microwaves was first proposed based on the radiative transfer theory, assuming that a homogenous double-layer (soil and atmosphere) medium exists between the passive microwave radiometer and a certain soil depth. With simulated data from the advanced integral equation model (AIEM) and subsequent analysis, the effective emission depth model was further simplified to make it more practical with a root-mean-squared error (RMSE) of 0.46 cm and a bias of 0.032 cm. For the simplified model, only the soil moisture content and the noise-equivalent differential temperature and center frequency of the radiometer are required. By comparing the penetration depth model and simplified effective emission depth model, a certain soil moisture content that makes both the models reveal the same results is found.