And so I need a GRANT IMPERSONATE permission

SQL Server 2005 has given us the ability to allow more granular permissions, one of which is the EXECUTE AS clause. This would give the user the ability to impersonate a more privileged user should there be a need to do so. But before a user can do impersonation, it must be given the permission to impersonate. This is where the GRANT IMPERSONATE permission is needed - where a lower privileged user needs to impersonate a higher privileged user and not the other way around, I presume.

I've written an article on MSSQLTips.com to demonstrate how to use the EXCECUTE AS clause with the REVERT clause to impersonate users