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Validation of SMOS soil moisture products over the Maqu and Twente regions.

Dente L, Su Z, Wen J - Sensors (Basel) (2012)

Bottom Line: However, the validation analysis resulted in determination coefficients of 0.55 and 0.51 over the Maqu and Twente region, respectively, for the ascending pass data, and of 0.24 and 0.41, respectively, for the descending pass data.Moreover, a systematic dry bias of the SMOS soil moisture was found of approximately 0.13 m(3)/m(3) for the Maqu region and 0.17 m(3)/m(3) for the Twente region for ascending pass data.Several factors might have affected the retrieval accuracy, such as the presence of Radio Frequency Interference (RFI), the use of inaccurate land cover information and the presence of frozen soils not correctly detected in winter.

View Article: PubMed Central - PubMed

Affiliation: Faculty of Geo-Information Science and Earth Observation (ITC), University of Twente, Hengelosestraat 99, 7514AE Enschede, The Netherlands. dente@itc.nl

ABSTRACT
The validation of Soil Moisture and Ocean Salinity (SMOS) soil moisture products is a crucial step in the investigation of their inaccuracies and limitations, before planning further refinements of the retrieval algorithm. Therefore, this study intended to contribute to the validation of the SMOS soil moisture products, by comparing them with the data collected in situ in the Maqu (China) and Twente (The Netherlands) regions in 2010. The seasonal behavior of the SMOS soil moisture products is generally in agreement with the in situ measurements for both regions. However, the validation analysis resulted in determination coefficients of 0.55 and 0.51 over the Maqu and Twente region, respectively, for the ascending pass data, and of 0.24 and 0.41, respectively, for the descending pass data. Moreover, a systematic dry bias of the SMOS soil moisture was found of approximately 0.13 m(3)/m(3) for the Maqu region and 0.17 m(3)/m(3) for the Twente region for ascending pass data. Several factors might have affected the retrieval accuracy, such as the presence of Radio Frequency Interference (RFI), the use of inaccurate land cover information and the presence of frozen soils not correctly detected in winter. Improving the RFI filtering method and the quality of the retrieval algorithm inputs, such as land surface temperature and land cover, would certainly improve the accuracy of the retrieved soil moisture.

No MeSH data available.


Angular pattern of the brightness temperature of one node in the Maqu region on 5 July 2010.
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f10-sensors-12-09965: Angular pattern of the brightness temperature of one node in the Maqu region on 5 July 2010.

Mentions: A preliminary and not comprehensive analysis of the SMOS L1c brightness temperature data was carried out to investigate the possible presence of RFI over the Maqu and the Twente region. A proper rotation was applied to the data, converting the brightness temperature in the antenna reference (X and Y polarization) to that in the Earth reference (H and V polarization). The analysis revealed that for both sites several data are characterized by out of range brightness temperature values, indicating the presence of RFI in the area. Those data were generally filtered out and disregarded by the retrieval process. However, it was also found that several data are characterized by brightness temperatures with acceptable values, but with an unexpected feature in the angular pattern. The angular pattern of both horizontal and vertical polarized brightness temperature was found to have abrupt variations corresponding to small incidence angle increases, instead of the expected smooth variations. All the data collected over the Maqu region, as well as all the descending plus several ascending data over the Twente region, are characterized by this feature. Figure 10 shows an example of the irregular angular pattern of the horizontal and vertical polarized brightness temperature measured at one of the nodes in the Maqu region on July 5, 2010. As explained in [21,22], the brightness temperature angular pattern can be strongly affected by the presence of RFI. Moreover, if the RFI is weak, the observed brightness temperature might range between acceptable values, while the angular pattern is irregular. In this situation, the filtering of RFI-affected observations is not straightforward and studies about RFI detection are ongoing. The use of L1c data improperly filtered might be the cause of less accurate soil moisture retrieval.


Validation of SMOS soil moisture products over the Maqu and Twente regions.

Dente L, Su Z, Wen J - Sensors (Basel) (2012)

Angular pattern of the brightness temperature of one node in the Maqu region on 5 July 2010.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3472810&req=5

f10-sensors-12-09965: Angular pattern of the brightness temperature of one node in the Maqu region on 5 July 2010.
Mentions: A preliminary and not comprehensive analysis of the SMOS L1c brightness temperature data was carried out to investigate the possible presence of RFI over the Maqu and the Twente region. A proper rotation was applied to the data, converting the brightness temperature in the antenna reference (X and Y polarization) to that in the Earth reference (H and V polarization). The analysis revealed that for both sites several data are characterized by out of range brightness temperature values, indicating the presence of RFI in the area. Those data were generally filtered out and disregarded by the retrieval process. However, it was also found that several data are characterized by brightness temperatures with acceptable values, but with an unexpected feature in the angular pattern. The angular pattern of both horizontal and vertical polarized brightness temperature was found to have abrupt variations corresponding to small incidence angle increases, instead of the expected smooth variations. All the data collected over the Maqu region, as well as all the descending plus several ascending data over the Twente region, are characterized by this feature. Figure 10 shows an example of the irregular angular pattern of the horizontal and vertical polarized brightness temperature measured at one of the nodes in the Maqu region on July 5, 2010. As explained in [21,22], the brightness temperature angular pattern can be strongly affected by the presence of RFI. Moreover, if the RFI is weak, the observed brightness temperature might range between acceptable values, while the angular pattern is irregular. In this situation, the filtering of RFI-affected observations is not straightforward and studies about RFI detection are ongoing. The use of L1c data improperly filtered might be the cause of less accurate soil moisture retrieval.

Bottom Line: However, the validation analysis resulted in determination coefficients of 0.55 and 0.51 over the Maqu and Twente region, respectively, for the ascending pass data, and of 0.24 and 0.41, respectively, for the descending pass data.Moreover, a systematic dry bias of the SMOS soil moisture was found of approximately 0.13 m(3)/m(3) for the Maqu region and 0.17 m(3)/m(3) for the Twente region for ascending pass data.Several factors might have affected the retrieval accuracy, such as the presence of Radio Frequency Interference (RFI), the use of inaccurate land cover information and the presence of frozen soils not correctly detected in winter.

View Article: PubMed Central - PubMed

Affiliation: Faculty of Geo-Information Science and Earth Observation (ITC), University of Twente, Hengelosestraat 99, 7514AE Enschede, The Netherlands. dente@itc.nl

ABSTRACT
The validation of Soil Moisture and Ocean Salinity (SMOS) soil moisture products is a crucial step in the investigation of their inaccuracies and limitations, before planning further refinements of the retrieval algorithm. Therefore, this study intended to contribute to the validation of the SMOS soil moisture products, by comparing them with the data collected in situ in the Maqu (China) and Twente (The Netherlands) regions in 2010. The seasonal behavior of the SMOS soil moisture products is generally in agreement with the in situ measurements for both regions. However, the validation analysis resulted in determination coefficients of 0.55 and 0.51 over the Maqu and Twente region, respectively, for the ascending pass data, and of 0.24 and 0.41, respectively, for the descending pass data. Moreover, a systematic dry bias of the SMOS soil moisture was found of approximately 0.13 m(3)/m(3) for the Maqu region and 0.17 m(3)/m(3) for the Twente region for ascending pass data. Several factors might have affected the retrieval accuracy, such as the presence of Radio Frequency Interference (RFI), the use of inaccurate land cover information and the presence of frozen soils not correctly detected in winter. Improving the RFI filtering method and the quality of the retrieval algorithm inputs, such as land surface temperature and land cover, would certainly improve the accuracy of the retrieved soil moisture.

No MeSH data available.