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At L-band, the vegetation scattering albedo ω is assumed to be close to zero and independent

Surface roughness effect: assuming the surface scattering over the interface between soil and air,

ffiffiffiffiffiffiffiffiffiffiffiffiffiffi εs� sin <sup>2</sup>θ p

� � � � 2

> � � � � 2

ffiffiffiffiffiffiffiffiffiffiffiffiffiffi εs� sin <sup>2</sup>θ p

> ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi εs� sin <sup>2</sup>θ p

> ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi εs� sin <sup>2</sup>θ p

� � � exp �Hr cos Np ð Þ <sup>θ</sup> � � (37)

<sup>2</sup> (39)

(38)

the rough soil reflectivity Rp was obtained from the smooth surface reflectivity rp:

rh <sup>¼</sup> cos <sup>θ</sup>�

rv <sup>¼</sup> <sup>ε</sup><sup>s</sup> cos <sup>θ</sup>�

� � � �

� � � �

cos θþ

ε<sup>s</sup> cos θþ

The parameter Q quantifies the polarization mixing degree due to the surface roughness and is neglected at L-band [42, 43]. Np represents the dependence of roughness on incidence angle. Furthermore, the effective surface roughness parameter Hr is associated to the measured

> Hr ¼ 4k 2 s

with wave number k and surface RMS height s. However, the clear general relationship between Hr and measured surface roughness is still uncertain. In the literature, different empirical relationships were established to link the Hr parameter to surface RMS height and the autocorrelation length [44]. The Hr parameter is also found to be influenced by soil

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

Rp ¼ ð Þ� 1 � Q rp þ Q � rq

moisture, but it is reported to be mainly valid for the sandy soils [42].

6. Joint active-passive microwave for soil moisture estimation

where rp is the Fresnel coefficients for h and v polarizations:

of the polarization and incidence angle [41].

50 Soil Moisture

surface roughness in a conventional way:

Hongquan Wang1,2\*

