47 CFR § 87.471 - Scope of service.
Stations in the aeronautical radiodetermination service provide radionavigation and radiolocation services.
(a) Transmission by radionavigation land stations must be limited to aeronautical navigation, including obstruction warning.
(b) Radionavigation land test stations are used for the testing and calibration of aircraft navigational aids and associated equipment. When used as radionavigation land test stations (MTF) signal generators must be licensed as radionavigation land test stations (MTF). Transmission must be limited to cases when radiation is necessary and there is no alternative.