10 U.S.C. § 1591

(a) Subject to subsection (b), the Secretary concerned may authorize reimbursement to a civilian employee who is accompanying a Member of Congress or a congressional employee on official travel for actual travel and transportation expenses incurred for such travel. (b) The allowance provided in subsection (a) may be paid— (1) at a rate that does not exceed the rate approved for official congressional travel; and (2) only when the travel of the member is directed or approved by the Secretary concerned. (c) In this section: (1) The term “Member of Congress” means a member of the Senate or the House of Representatives, a Delegate to the House of Representatives, and the Resident Commissioner from Puerto Rico. (2) The term “congressional employee” means an employee of a Member of Congress or an employee of Congress. (3) The term “Secretary concerned” includes the Secretary of Defense with respect to civilian employees of the Department of Defense other than a military department.