32 U.S.C. § 503

(a) (1) Under such regulations as the President may prescribe, the Secretary of the Army and the Secretary of the Air Force, as the case may be, may provide for the participation of the National Guard in encampments, maneuvers, outdoor target practice, or other exercises for field or coast-defense instruction, independently of or in conjunction with the Army or the Air Force, or both. (2) Paragraph (1) includes authority to provide for participation of the National Guard in conjunction with the Army or the Air Force, or both, in joint exercises for instruction to prepare the National Guard for response to civil emergencies and disasters. (b) Amounts necessary for the pay, subsistence, transportation, and other proper expenses of any part of the National Guard of a State, the Commonwealth of Puerto Rico, the District of Columbia, Guam, or the Virgin Islands participating in an exercise under subsection (a) may be set aside from funds allocated to it from appropriations for field or coast-defense instruction. (c) Members of the National Guard participating in an exercise under subsection (a) may, after being mustered, be paid for the period beginning with the date of leaving home and ending with the date of return, as determined in advance. If otherwise correct, such a payment passes to the credit of the disbursing officer.