Multiple Signal Messages (MSM) represent a modern, standardized format within the RTCM 3.x protocol for transmitting GNSS observation data from reference stations to rovers. Developed to address the increasing complexity of multi-constellation, multi-frequency GNSS systems, MSM provides a unified, scalable framework for encoding raw measurements that accommodates current and future satellite signals without requiring constant protocol revisions.
The MSM format organizes observations by satellite system, with dedicated message types for each constellation: MSM1-MSM7 messages exist for GPS, GLONASS, Galileo, BeiDou, QZSS, SBAS, and other systems. Within each constellation family, the message number indicates the level of detail: MSM1 and MSM2 provide compact messages with limited precision for bandwidth-constrained applications; MSM4 provides full pseudorange and carrier phase measurements suitable for most RTK applications; MSM5 adds Doppler and signal-to-noise data; MSM6 and MSM7 provide extended resolution for the highest-precision applications.
The key advantage of MSM over legacy RTCM observation messages is its signal-agnostic design. Earlier RTCM formats required specific message types for each signal, a new GPS signal required a new message type definition, and different manufacturers might implement non-standard extensions. MSM uses a generalized structure where any signal from any satellite can be encoded using the same message framework, with signal types identified by standardized codes. This approach provides forward compatibility, receivers can process MSM messages containing new signals without firmware updates.
For organizations deploying GNSS correction infrastructure, MSM has become the preferred format for observation data transmission. Reference station networks broadcasting via NTRIP increasingly use MSM formats to ensure compatibility with diverse rover equipment. The standardization enables interoperability between correction services and receivers from different manufacturers, simplifying system integration and reducing vendor lock-in. As GNSS constellations continue evolving with new signals and satellites, MSM’s flexible architecture ensures that correction infrastructure can accommodate future developments without fundamental protocol changes.
