Validation of Backscatter Measurements from the Advanced Scatterometer on MetOp-A

Abstract

The Advanced Scatterometer (ASCAT) on the Meteorological Operational (MetOp) series of satellites is designed to provide data for the retrieval of ocean wind fields. Three transponders were used to give an absolute calibration and the worst-case calibration error is estimated to be 0.15-0.25 dB.In this paper the calibrated data are validated by comparing the backscatter from a range of naturally distributed targets against models developed from European Remote Sensing Satellite (ERS) scatterometer data.For the Amazon rainforest it is found that the isotropic backscatter decreases from 26.2 to 26.8 dB over the incidence angle range. The ERS value is around 26.5 dB. All ASCAT beams are within 0.1 dB of each other. Rainforest backscatter over a 3-yr period is found to be very stable with annual changes of approximately 0.02 dB.ASCAT ocean backscatter is compared against values from the C-band geophysical model function (CMOD-5) using ECMWF wind fields. A difference of approximately 0.2 dB below 558 incidence is found. Differences of over 1 dB above 558 are likely due to inaccuracies in CMOD-5, which has not been fully validated at large incidence angles. All beams are within 0.1 dB of each other.Backscatter from regions of stable Antarctic sea ice is found to be consistent with model backscatter except at large incidence angles where the model has not been validated. The noise in the ice backscatter indicates that the normalized standard deviation of the backscatter values K p is around 4.5%, which is consistent with the expected value.These results agree well with the expected calibration accuracy and give confidence that the calibration has been successful and that ASCAT products are of high quality.

The ASCAT Soil Moisture Product: A Review of its Specifications, Validation Results, and Emerging Applications

“…d ASCAT backscatter measurements are well calibrated and very stable over time (WILSON et al, 2010). According to ANDERSON et al (2012) the worst-case calibration error is 0.15-0.25 dB and annual changes are in the order of 0.02 dB. This means that ASCAT is very well suited for tracking changes in soil moisture.…”

Evaluating Consistency between the Remotely Sensed Soil Moisture and the Hydrological Model-Simulated Soil Moisture in the Qujiang Catchment of China

“…The ASCAT, flying on the board of the polar-orbiting Meteorological Operational (MetOp) satellite series and using active microwave at a frequency of 5.255 GHz (C-band), was initially designed for monitoring winds over the oceans and was thought to be sub-optimal for soil moisture detection [39], but recent studies have shown that ASCAT soil measurements can be well suited for tracking changes on soil moisture [40] and have a positive impact in practical applications [41]. There are two available ASCAT products currently: the ASCAT-A was launched on the MetOp-A satellite in October 2006, which then became fully operational in May 2007 and continues to operate today, while the ASCAT-B instrument on the MetOp-B satellite became operational in September 2012.…”

“…There are two available ASCAT products currently: the ASCAT-A was launched on the MetOp-A satellite in October 2006, which then became fully operational in May 2007 and continues to operate today, while the ASCAT-B instrument on the MetOp-B satellite became operational in September 2012. In the study, the raw ASCAT Surface Soil Moisture Level-2 (L2) products (ASCAT-A and ACAT-B) given in swath geometry are available from European Organization for the Exploitation of Meteorological Satellites (EUMETSAT) with a spatial sampling resolution of 12.5 km [41]. Retrievals are available at least once a day (in descending orbit and/or in ascending orbit), and a time-series based change-detection method to remove the adverse influence of surface roughness and eliminate the contribution of vegetation is used.…”

Long-term Soil Moisture Time Series Analyses based on Active Microwave Backscatter Measurements

“…Hence, a method was recently developed to monitor the long-term radiometric stability of ESCAT and ASCAT (Reimer, 2014). The developed method closely follows the principle of relative calibration of scatterometers by making use of natural calibration targets on the Earth's surface (Anderson et al, 2012;Crapolicchio et al, 2012;Lecomte and Wagner, 1998). Natural calibration targets are featured by a temporal stable and homogeneous backscatter response over an extensive area like the Amazon and Congo Rainforest.…”