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OpenGTS offers a lot of features out of box. But as 100yan pointed out, the code is very complicated to change. I suppose the complication is deliberate to prevent users from extending the system! XML files provide many customizations and someone makes the desired change by just modifying the xml files. However if you want to change the code itself, it is a waste of time and effort.
Its complex to configure and setup. For ease and smooth GPS tracking operations try trackforever.blogspot.com.