Understanding GNSS Correction Formats: RTCM, CMR+, MSM and what they mean
When setting up an RTK GNSS receiver or configuring a base station, you might come across various correction formats like RTCM 3.x, CMR+, or MSM4. If you’re unsure what they mean, you’re not alone — and choosing the right format matters for accuracy and compatibility.
In this article, we’ll break down the most common correction data formats and help you understand which one is right for your GNSS system.
🔍 What Are GNSS Correction Formats?
Correction formats are standardized ways of transmitting positioning error data from a GNSS base to a rover. They allow the rover to adjust its raw satellite signals and achieve centimeter-level accuracy in real time.
The correction format must be supported by both the base and the rover, or your device may not be able to apply the corrections (leading to float status or errors).
📘 Common GNSS Correction Formats
1. RTCM (Radio Technical Commission for Maritime Services)
- RTCM 2.x – Older format; still used in some basic DGPS setups
- RTCM 3.x – Modern and widely adopted; standard for RTK corrections
- RTCM 3.2 with MSM – Includes MSM (Multiple Signal Messages) for multi-constellation, multi-frequency support
✅ Best for: Most modern receivers (Trimble, Leica, South, Emlid, etc.)
🌍 Supports: GPS, GLONASS, Galileo, BeiDou, QZSS (if MSM-enabled)
2. CMR and CMR+
Proprietary formats developed by Trimble. CMR+ offers improved compression and satellite tracking.
- CMR – Basic GPS-only corrections
- CMR+ – GPS + GLONASS with better compatibility and smaller message size
✅ Best for: Trimble-to-Trimble systems
3. MSM (Multiple Signal Messages)
Part of RTCM 3.2, MSM is designed to handle all GNSS constellations and signal types with a unified format.
Examples:
- MSM4 – Standard with pseudo-range and carrier phase
- MSM5/7 – Advanced options with Doppler and signal-to-noise ratios
✅ Best for: Multi-constellation RTK with diverse hardware brands
📊 Comparison Table
| Format | Constellation Support | Proprietary? | Recommended Use |
|---|---|---|---|
| RTCM 2.x | GPS only | No | Legacy DGPS systems |
| RTCM 3.2 | GPS, GLONASS | No | Standard RTK correction |
| RTCM 3.2 + MSM4 | All (GPS, GLO, GAL, BDS) | No | Multi-constellation RTK |
| CMR | GPS only | Yes (Trimble) | Basic Trimble setups |
| CMR+ | GPS + GLONASS | Yes (Trimble) | Trimble RTK systems |
📌 Which Format Should You Use?
It depends on your equipment and workflow:
- Using mixed-brand GNSS gear? Choose RTCM 3.2 with MSM4 for compatibility
- Using only Trimble receivers? Use CMR+ if available
- Setting up your own base? Enable RTCM 3.2 and test with your rover
Always check your rover’s supported input formats in the specs or app settings.
🛒 Need Help Choosing Compatible Equipment?
We sell high-quality, pre-owned GNSS receivers tested for RTCM, MSM, and CMR+ compatibility. Choose from trusted brands like Leica, South, Trimble, and Emlid.
🔗 Browse our RTK-ready equipment here
Next up: Troubleshooting RTK Float Issues — 7 Common Causes & How to Fix Them