Many studies focused on the comparison of soil moisture products from different satellites, such as SMOS, SMAP, AMSR-2, FY-3.
It's acknowledged that the penetration of C and X band is weaker than L-band , especally in the dense vegetated region. So is the comparison between L-band based and C or X-band based soil moisture products fair?
Of course this is fair. Each satellite has advantages and drawbacks.
SMAP filter RFI better than SMOS. Is it fair to compare it to SMOS? Of course it is.
SMOs has multi-angular capabilities and not SMAP. Is it fair to compare it to SMAP? Of course it is.
The user will use the best product and he wants to know which one it is.
Professor Wigneron, thanks for your reply! What I want to ask more is whether soil moisture derived from high frequency band (like X-band) could be compared with the one derived from low frequency band (like L-band). The X-band observation is more affected by vegetation than L-band , is this comparison still fair?
Yes it is fair: even at X band it is sold as a "soil moisture product", it is not sold (defined) as "soil moisture affected by vegetation"
Thanks again.
The penetration of L-band is stronger than X -band ,and it's more sensitive to the surface soil. This gives me an intuitive feeling that accuracy of soil moisture derived from L-band observation is better than the X -band.
The current retrievals are based on tau-omega model, which is a zero-order of RTE. The tau-omega model neglects multiple scattering, and it's more suitable for L band. Multiple scattering contributes more to the observation at X band in dense vegetated region. This brings errors when the tau-omega model-based retrieval is applied to the X band observation. What confused me is how the X band performs better than L band in the vegetated region, except in the RFI area.
A lot of studies showed the SMOS or SMAP soil moisture products performed better than the AMSR-2 soil moisture ，and I think this is also in line with our expectations. The L-band microwave sensors already have advantages than X or C band, and I don't understand the meaning of comparison among their retrieved soil moisture, this is why I asked the fairness of comparison.
Well, for 10 years I have been using satellite soil moisture products for my research... Sometimes C band is better than L band. Why? The band is not the only parameter that determines the accuracy of satellite observations. My suggestion, check the data and use the one performing best for your application, you might be surprised 🤨
@Lu Hu, Note that it is an error to think L-band is less sensitive to surface roughness effects. It is something which is often said, but I could not find any results supporting this. Some analyses showed sensitivity of L-, C and X-bands to soil roughness is ~similar. CfArticle Modelling the passive microwave signature from land surfaces...
Dear Lu Hu,
First of all, thanks for your question. The following is what I concern, and sincerely hope it can give you some ideas.
As you said, there are many remote sensing soil moisture products, which provide us the spatial and temporal characteristic of soil moisture in the global scale.
However, the acquisition of accurate soil moisture content is still limited by the data source (passive or active microwave RS. data, ancillary data, etc.), retrieval algorithm, and time resolution (for multi-temporal retrieval method and change detection method) . At present, we still need to know which products has good accuracy, and which sensor or algorithm has better performance on SMC inversion. Many validation experiment have been implemented to analysis this both for passive or active enhance soil moisture products.
When it comes to your question, I think it is fair to compare the accuracy of these products. Because the soil moisture is dependent on the soil permittivity or dielectric constant, and the depth. For example, when we assess the performance of C-band products, we compare the satellite observations against the site measurement in the depth of 0-3cm; for the L-band, we compare the satellite observations against the site measurement in the depth of 3-5cm. And then we can compare the accuracy (RMSE, R, R2, unRMSE, AE, etc.) of C- and L-band. From this view, the comparison is equitable.
On the other hand, it is also not fair to compare these products with each other. Because the penetration depth is different in different microwave frequencies, and it is largely influenced by the vegetation condition (height, density, VWC, and crop phenology) especially for high frequency signal. When we compare the performance of different sensors on soil moisture inversion in the global scale (with various vegetation conditions) , the X-, C-band will be more influenced by the vegetation than L-band. Regarding this aspect, I think it is not fair.
Best regards,
Hontao
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