Abstract
A wind direction retrieval algorithm using spaceborne global navigation satellite system reflectometry (GNSS-R) in nonspecular geometry is presented. This article first focuses on investigating the influence of wind direction on the scattering strength purely in specular geometry and nonspecular geometry. The sensitivity of the scattered GNSS signals in nonspecular geometry to wind direction is demonstrated despite the weaker power level. Therefore, a wind direction retrieval algorithm based on the geophysical model function (GMF) whose parameters are parameterized as a look-up-table is proposed. Then, the spatial resolution and power link of the spaceborne GNSS-R in nonspecular geometry are analyzed. The incidence angle of the scattered signals ranges 15° to 35° to ensure the sensitivity to wind direction and the power level of scattered signals. The retrieving results using simulated data show that the ambiguity of the measurement geometry can be eliminated by using the multisatellite observation, and two possible wind directions for the same corresponding observable are obtained in the wind direction retrieval algorithm. The accuracy of wind speed importantly impacts on the retrieving performance of wind direction, therefore, it is needed to improve the measuring accuracy of wind speed. These results provide information to guide the wind direction retrieval using spaceborne GNSS-R in nonspecular geometry.
Highlights
G LOBAL navigation satellite system reflectometry (GNSS-R) that was primarily proposed for ocean mesoscale altimetry by Martin-Neira [1] has been significantly extended to a wide range of applications as an emerging earth remote sensing technology
Wind direction was found to correspond to the anisotropy in probability density function (PDF) of ocean surface slope, which can be estimated from the airborne reflected GPS waveforms [18]
This suggests that wind direction observation may be more accurate for the spaceborne GNSS-R in nonspecular geometry when the incidence angles range between 15◦ and 50◦
Summary
G LOBAL navigation satellite system reflectometry (GNSS-R) that was primarily proposed for ocean mesoscale altimetry by Martin-Neira [1] has been significantly extended to a wide range of applications as an emerging earth remote sensing technology. The possibility of wind direction observation using spaceborne GNSS-R observables was investigated theoretically, and two metrics of DDM were proposed to retrieve wind direction from the DDM asymmetry, which were explored using the simulation tool P2EPS [23]. The abovementioned research works on wind direction retrieval mainly focused on the signals reflected off the surface in the general specular geometric observation configuration. Based on the previous description and the published research works mentioned above, it is more likely to retrieve wind direction from spaceborne GNSS-R observables using the slightly nonspecular observation geometry between the purely specular and backward. The feasibility of retrieving wind direction from the spaceborne GNSS-R observables in nonspecular geometry is explored, and the retrieving performance is assessed by the theoretical simulation.
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