Calibration to sigma0 or complex output

I have some questions about radar radiometric calibration.

First, what’s the difference between complex output and sigma0 band?
when I select ‘save as complex output’, the sigma0 box is selected automatically.
I wonder what is different and which one is real ‘sigma0’, normalized backscattering coefficient.

Also, is there any normalized range of sigma0?
How can I define the range to see optimal image of sigma0?

Finally, is it possible to compare the sigma0 from TSX, CSK, and other satellites?
I mean, if there are same objects in two images from different satellites, the signal can be compared as some objects?

Sigma0 is the normalized radar cross section (backscatter coefficient) which is required for analyses of the backscatter intensity. If you calibrate images of different sensors (TSX and CSK are both x band) to Sigma0 you can theoretically compare the results, but the look direction and overall incidence angle makes a difference as well.

Complex calibration is required if you want to make images of different dates (same angle) usable for polarimetric analyses, for example. These require the complex (i+q) information which is removed when converting to Sigma0.

Sigma0 (in power scale) largely ranges between 0 and 1 where 1 represents a fully isotropic scatterer. It can exceed 1 for dihedral or volume scattering.
Sigma0 dB (in log scale) ranges between -40 and +15 db.