RTCM (Radio Technical Commission for Maritime Services) refers both to the international standards organization that develops maritime technical standards and to the family of messaging protocols used for transmitting GNSS correction data from reference stations to rovers. The RTCM correction protocol has become the worldwide standard for RTK, DGNSS, and network correction services, enabling interoperability between equipment and services from different manufacturers.

RTCM message standards have evolved through multiple versions. RTCM 2.x, developed in the 1990s, defined messages primarily for GPS differential corrections. RTCM 3.x, current since 2004 and actively maintained, provides a modern binary format supporting multi-constellation, multi-frequency observations and diverse correction types. RTCM 3.x messages use efficient encoding to minimize bandwidth while maintaining full measurement precision, making them suitable for transmission over bandwidth-constrained radio links.

Key RTCM 3.x message types serve specific functions in correction distribution. Message types 1001-1012 convey GPS observations; 1071-1077 provide multi-signal messages (MSM) for GPS; similar MSM families exist for GLONASS (1081-1087), Galileo (1091-1097), and BeiDou (1101-1127). Message types 1005 and 1006 transmit reference station coordinates. Message types 4062 and related SSR messages carry State Space Representation corrections for PPP-RTK services. Network RTK messages convey specialized corrections for specific methodologies.

For organizations deploying GNSS solutions, RTCM protocol support is essential for correction interoperability. Most professional receivers support RTCM input for applying corrections and RTCM output for operating as base stations. NTRIP streams carry RTCM messages over internet protocols. Correction services must document which RTCM messages they provide, and receivers must be configured to correctly process the relevant types. The widespread standardization around RTCM has enabled a competitive market of interchangeable correction services and positioning equipment, benefiting end users with choice and preventing vendor lock-in.