The standard is named for the Special Committee 104 of the Radio Technical Commission for Maritime Services (RTCM) that created it.
The format does not define the source of the messages and has been used with systems as varied as longwave marine radio, communications satellite broadcasts, and internet distribution.
The first widely used version of the format was released in 1990 and was based on the 30-bit long packet used by the GPS satellites, known as a "frame".
[1] RTCM Version 2 is based on a set of fixed-length 30-bit "words" which are strung together into longer messages known as "frames".
The format was deliberately modelled on that of the actual GPS messages, in order to maintain familiarity.
The second header word begins with a 13-bit version of the z-count, the unit of time in GPS, a 3-bit sequence number to ensure frames can be sorted if they arrive out-of-order, a five-bit length that counts the total number of words in the frame, including the header, and a three-bit "station health" code, where 111 indicates the station is not working properly.
[3] A total of 64 message types were allowed, although a number of these were deliberately left unused for future expansion, or formats that were rarely used and later abandoned.
[3] Type 1 was a complete DPGS correction set that would be broadcast by a ground station for all of the satellites in its view.
Type 3 is used to periodically send out the location of the ground station, allowing receivers to pick suitable sites.
[3] In 1992 the group met to consider input from users working with phase-comparison GPS (RTK) that produces accuracy on the order of 1 centimetre (0.39 in).
While this was not a problem for most DGPS uses, it made it a poor choice for RTK which has a relatively high message load.
For this reason, Trimble introduced its own Compact Measurement Record (CMR) format in 1996, and an updated CMR+ the next year.
[8] Another major addition to the system are the State Space Representation (SSR) which are used to periodically update information on the satellites, and the Multiple Signal Messages (MSM) which allow data from different sets of satellites to be combined using a single data format.