The command is only accessible when you are in a *.gcd project and you can only perform analyses when you have at least two overlapping surveys loaded in the GCD Project Explorer. Because the GCD Project Explorer  enforces orthogonality of all rasters loaded, we know that any two surveys will be compatible for analysis without requiring resampling. However, the rasters may have different extents, and the surveys may have different coverages (i.e. the NoData extents) within those rasters. As such, at the time of analysis, the GCD Software creates two temporary concurrent rasters, and only performs the DoD analysis where their data intersect. 

There are a series of video tutorials, which cover how to do various analyses in the Change Detection Panel:

• 04. Basic DoD Analysis in GCD
• 05. Thresholding w/ GCD
• 07. Applying Fuzzy Inference Systems in GCD
• 06. Spatial Coherence & Bayesian Updating