ERS, ENVISAT and Sentinel-1 are C-band satellites at quite similar spatial resolution. This makes them potentially comparable and usable within the same scope.
- https://www.sciencedirect.com/science/article/abs/pii/S0034425712000612
- https://www.tandfonline.com/doi/abs/10.1080/15481603.2016.1269404
- https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2015JC011149
However, most classification approches need a multi-dimensional feature space. Optical satellites acquire information at multiple wavelengths (visible, infrared, thermal), but SAR data is restricted to one wavelength. Its potential for classification purposes is therefore limited.
Have a look at the possibilities to increase feature space for SAR data: