Do you think that the best solutions for ground monitoring and maps updating in the areas of high clouds cover, most months of the year are the use of SAR Images?
SAR images do not read surface topography very well, since the signal is an integration of surface to penetrating depth returns. In fact, this aspect is utilized to infer total soil moisture from SAR data. Therefore, using it as stand alone for replacing vis observations may not be very good idea, at least for the time being. If one can think of sub-mm wave SAR, situation may be some what different.
Furthermore, clouds are dynamic system and filtering their impact in mm and sub-mm wave SAR will be very tricky.
According to the literature SAR is the current best solution for imaging land in areas with cloud cover during most part of the year. Unfortunately using any SAR systems for monitoring could be expensive because you do not have a systematic coverage. You need to ask to the mission control and contract a survey for a specific date.
SAR techniques are suited for moniting the ground land with slowly deformation, this is a limitation because of the verious ground deformation patterns
Can you tell us what you want to monitor? This would help us giving you a more precise answer.
The question is very open ended and depends on the application, SAR system properties and type of processing. If you want to do deformation mapping interferometric SAR is very good. If your interested in vegetation mapping (my particular area) shorter wavelength SAR data such as X- and C-band provide canopy information whereas longer wavelengths such as L- and P-band provide more information on trunks and surface properties such as moisture and roughness.
One response to the comment about systematic SAR coverage is that JAXA have been putting a lot of effort into this and are continuing to do so. During the years ALOS-1 was operational worldwide gamma0 mosaics were produced annually. From these forests/non-forest maps were produced, which shows one good application area of L-band SAR data for monitoring. The maps and SAR data are available to freely download from:
http://www.eorc.jaxa.jp/ALOS/en/palsar_fnf/fnf_index.htm
In order to estimate soil moisture and surface roughness YES. However, some limitations exist according to characteristics of SAR sensors (radar wavelength ...) and soil characteristics (it is difficult to map soil moisture if the values are heigher than about 40%, also, it is difficult to provide more than two or three roughness calsses according to radar wavelength).
SAR techniques have several limitations. They can detect deformation in limited range. Also, effects as foreshortening, layover, shadowing lead to restrictions of estimation of deformation in any places.
Let me please add some hints, the explanation of the question I have written "Do you think that the best solutions for ground monitoring and maps updating in the areas of high clouds cover, most months of the year are the use of SAR Images?" So all what you would is really true, but again what is the solution in cloudy area most of the year.
I guess e.g. ESA Sentinel-1 C-band SAR is a good starting point, since (continuous) monitoring of large areas should be economically feasible then. But that data should be combined with "constant-everything" you can gather, perhaps even with some primitive water dynamics model. Also, combining SAR with very sparse stationary ground-based measurement points would be good. It seems that there is no simple one-solution-fits-all approach. Optimizing such a ground-based sensor network is a nice subject in itself.
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