some ideas:
- have you selected “remove topographic phase” for both maps? I found that it makes a difference for coherence as well.
- if there was heavy rain between the first and second image of the second coherence map, crops might have bent which leads to drastic decorrelation. As soon as the main scattering mechanism is altered (e.g. volume scattering to surface scattering) coherence decreases significantly.
- please also have a look at these considerations: Unexplained differences between different orbits
- harvesting of crops also leads to decorrelation, of couse, but as nearly the entire image has decreased coherence I think it is rather a rain event.
- bad coregistration quality also leads to low coherence from the computational side