ABSTRACTThe Radio Technical Commission for Maritime Service (RTCM) established Special Committee 104 on Differential NAVSTAR/GPS Service in November 1983 to address the methods to be used to provide differential GPS service and to develop standards for the data format. The Special Committee has finished the first phase of its deliberations, which involves data message and format standards, and recommendations for communications bands and a proposed design for pseudolite operation. This paper describes the recommendations of the RTCM SC‐104. The standard message provides range and range‐rate corrections, along with the satellite and station health, and issue of data (which provides satellite message timing). Auxiliary messages are defined, to be interposed occasionally, which provide ground station location, constellation health, tropospheric model parameters, and further range corrections to adjust for satellite message changes. A special message type is defined to support surveying applications. This message provides high‐precision Doppler counts using carrier phase measurements, which make possible centimeter resolution for stationary platforms. Navigation and communication bands are identified for the provision of public and private radionavigation and radiolocation. The use of the marine radiobeacon band, in particular, appears attractive for marine navigation in harbor/harbor approach and restricted wate