The Correction Signal
The agreed upon protocol for communication between base stations and rovers was first designed for used in marine navigation by an organization known as the Radio Technical Commission for Maritime Services (RTCM). RTCM is an independent not-for-profit organization that is supported by an international membership that includes both governmental and non-governmental institutions. Its goals are educational, scientific, and professional. Toward those ends, it provides information on maritime radio-navigation and radio communication policies and associated regulations to its members. It is also involved in technical standards development.
In 1985, the, RTCM Special Committee (SC-104) created a standard that is still more used than any proprietary formats that have come along since. RTCM is open. In other words, it is a general-purpose format and is not restricted to a particular receiver type. The message augments the information from the satellites. It was originally designed to accommodate a slow GPS data rate with a configuration somewhat similar to the navigation message. The data format has evolved since its inception. For example, RTCM 2.0 supported GPS code only. However, when it became clear in 1994 that including carrier phase information in the message could improve the accuracy of the system, RTCM Special Committee 104 added four new message types to Version 2.1 to fulfill the needs of RTK. RTCM 2.1 supported both code and phase correction, but still GPS only. Version 2.2 became available in 1998. RTCM 2.2 added support for GLONASS, and version 2.3 included antenna corrections, and the changes continued. In 2007, the Radio Technical Commission for Maritime Services Special Committee 104 published its Version 3. RTCM 3.0 utilizes a more efficient message structure than its predecessors which proves beneficial in the RTK data heavy real-time communications between a base and a rover. Version 3.0 still provides both GPS and GLONASS code and carrier messages, antenna and system parameters. RTCM 3.1 adds a network correction message and version 3.2 announced in 2013 introduces a feature known as Multiple Signal Messages (MSM). MSM includes the capability to handle the European Galileo and the Chinese Beidou GNSS systems in the RTCM protocol.
The GPS constellation along with the Russian GLONASS system, the European Galileo system, and other systems comprise GNSS currently. It is likely that more systems will become included under the GNSS label in the future. It is also likely that more accuracy of autonomous positions will be available from GNSS than GPS alone. However, in GNSS, as with GPS, even better accuracies can be achieved by broadcasting corrections from reference stations at precisely known locations. And by utilizing RTCM 3.2, it is not only possible to use receivers from different manufacturers together, but also to incorporate signals from satellites other than GPS.