RTCM v2 - Version 1 had problems and was replaced by version 2.0. Version 2.1 added support for RTK, version 2.2 added support for GLONASS, and version 2.3 added some messages to improve accuracy in RTK mode. This is an open standard used by many manufacturers, but since it is old, it is used only on legacy equipment that cannot use version 3.

RTCM v3 is a new data format from RTCM, which has better data compression and message integrity than version 2.X. Version 3.1 added support for network RTK through the Master-helper concept. Still an open standard, this format is used by most modern receivers. ^[1] .

RTCM formats Traditionally fall into three categories: Compact Measurement Record (CMR), Observation State Representation (OSR) and State Space Representation (SSR). These groups use different methods, delivery mechanisms, and basic technologies to solve the same problem ^[2] .

Compact Measurement Record (CMR) - “Compact measurement indicator” - the original Trimble format was developed in 1992. The format was developed as a method of transmitting code and carrier phase correction data in a compact format from GPS base stations to GPS rovers for shooting RTK GPS. Dr. Nicholas Talbot of Trimble publicly introduced the CMR format at the 1996 Institute of Navigation (ION) conference. It was originally a proprietary format, but other receiver manufacturers figured out how to disassemble it, and Trimble eventually published its specifications so everyone could use it. Open and used in most receivers, the format is quite old, it contains information only from GPS satellites L1 / L2 ^{[3] [4] [5]}

Application: Single Base RTK

Before version 2.1, a different format was used; after the release of version 2.1, the Compact Measurement Index (CMR) format is used; earlier versions are considered obsolete.

Observation State Representation (OSR) translated from English. - “Representation in the observation space” is considered relatively outdated. since the main methods are focused on transmitting corrected GNSS observations from the nearest reference station (Base) to the Rover.

OSR, as a rule, uses the standard rtcm format focused on one geographic region (that is, a country or state) and on existing markets with low volume and high accuracy (such as surveying, agriculture, machine control). OSRs ensure centimeter level accuracy provided that the rover is within approximately 30 kilometers of the nearest reference station (base), they require two-way communication with high bandwidth, making it difficult to use them on a large scale to serve mass market applications ^[6]

Application: Single Base RTK, Network RTK ^[7]

Prior to version 3.1, the Observation Space Presentation Format (OSR) was used; after the release of version 3.1, the Status Space Presentation (SSR) format is used; earlier versions are considered obsolete.

State Space Representation (SSR) “State Space Representation” is the latest generation of GNSS correction services provided by new entrants and larger providers of legacy services.

The SSR method is based on the concentration and use of a network of reference stations to simulate key errors over large geographical regions and transfer them to Rover. The rover uses the incoming data to create a local GNSS error model, which it applies directly to GNSS observations. The performance of SSR services depends on what errors are reported to the rover. In addition, the broadcast nature of the SSR corrections allows accurate positioning services to more easily distribute corrections over IP or L-band satellite links. SSR services are capable of providing centimeter-level positioning accuracy in large geographic areas and are well-suited for mass market applications. ^[8]

Application: PPP, PPP-RTK ^[9]

Multiple Signal Messages (MSM) translated from English. - “Multiple Signals Messages” - The message format allows the receiver to use all satellite systems. Messages include compact and complete messages for pseudorange, phase measurements, carrier (signal) to noise ratio (standard and high resolution), frequency of phase measurements. Version 3 originally consisted of messages for GPS and GLONASS in their own format. A compatible (harmonized) universal format using the “new” signals from the upgraded GPS and GLONASS satellites, as well as BeiDou, Galileo and QZSS systems. ^[10]

Application: Multisystem-RTK

First introduced in version 3.2.

The RTCM SC-104 format uses 30-bit words; Of these, 24 bits are informational, followed by 6 bits - control. Each message has a two-word heading, followed by the words of the transmitted data that are specific to each particular type of message. Corrections and non-operational supporting information are transmitted as a continuous stream of messages consisting of separate information frames. One message includes (N + 2) words. Messages with GLONASS data are grouped into a block of 7 types ^[11]