A general schema for the Remote Sensing Extensions. Please note that the RSE schema has not been fully tested and therefor might contain some discrepancies from the actual stardard. When in doubt please consult the standard at http://www.fgdc.gov/standards/status/csdgm_rs_ex.html and report any errors to the sourceforge bug tracker.
Most Schema validators allow you to enter a schema by URL. If you are using one of those validator, you can use http://fgdcxml.sourceforge.net/schema/fgdc-std-012-2002/fgdc-std-012-2002.xsd
You can also download the files via Sourceforge.