Abstract
When a satellite sensor with a large field of view and wide swath is calibrated, it is not easy to obtain the image when the calibration site is located precisely at the nadir position. If the location of a calibration site is at an off-nadir position in the image, calibration errors will be caused by the inconsistent observation angle between the sensor view and the ground measurement view. The bidirectional reflectance distribution function (BRDF) model plays an important role in solving this problem. In this study, a BRDF measurement system based on an unmanned aerial vehicle (UAV) is developed. This system has the capability of measuring angular data with observation azimuth angle ranging from 0 deg to 360 deg with an angle interval of 30 deg, and observation zenith angle ranging from 0 deg to 50 deg with an angle interval of 10 deg. The directional data of the Dunhuang calibration site were measured using the UAV BRDF measuring system at different solar zenith and azimuth angles, and the spatiotemporal distribution characteristic of forward- and backward-scattering of Dunhuang calibration site was analyzed. A Ross–Li BRDF model, built using measurement data, is used to calculate the directional surface reflectance under any observation geometry of solar and satellite. These calculations are applied to correct the calibration data of the CBERS-04 WFI sensor. Results show that the BRDF model significantly improves the calibration accuracy, especially in the case of large observation angles.
Highlights
Absolute radiometric calibration of satellite sensors is required for the quantitative application of satellite data
All the parameters together with the observed geometric parameters of solar and the satellite sensor are input into the radiative transfer model (6S or MODTRAN) to obtain the top-ofatmosphere (TOA) radiance of the satellite sensors, and the calibration coefficient is calculated according to the TOA radiance and digital number (DN) value of the calibration site in the satellite image.[1,2]
The bidirectional reflectance factor (BRF) curves with higher and lower values concentrate on the observation azimuth angles of 150 deg, 180 deg, 120 deg and 0 deg, 330 deg, and 300 deg, respectively
Summary
Absolute radiometric calibration of satellite sensors is required for the quantitative application of satellite data. Vicarious calibration is an important method for the calibration of satellite sensors. This is done using the reflectance-based calibration method, in which two kinds of parameters are measured—field surface reflectance and atmospheric parameters (such as aerosol levels, temperature, and humidity profile)—while the satellite is passing over the calibration site. During the vicarious calibration process, surface reflectance is the most important parameter since the measurement error of surface reflectance can be completely transferred to the calibration result (calibration coefficient). For the calibration of a satellite sensor with a large field of view (FOV), the surface reflectance error can be caused by the off-nadir observation angle of the calibration site. When the sensor is calibrated, the surface reflectance of the calibration site is manually measured vertically downward (nadir)
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