In today’s GNSS landscape, with the wide range of constellations that GNSS sensors like the Leica GS18, GS16, GS05, and others can track, it’s common for a GS sensor to track 25 or more satellites at any given moment.   This in turn creates a better RTK environment for survey users of GNSS sensors such as: availability, reliability, geometry, performance, faster fix times and much more.  A common method of RTK is through the usage of UHF (Ultra High Frequency) radios, as this article specifically looks at the transmission of correction data via UHF radios.

RTK UHF radios generally have much narrower bandwidths than internet-based communication, which can limit their ability to transmit all RTK correction data from the base station to the rover.  For instance, in some regions, local regulations restrict radio channel bandwidth to just 12.5 kHz. When using the RTCM v3 format and tracking 25 or more satellites, a data rate of 9600 bps is insufficient to transmit the complete correction message within one second. To handle the increased data volume without violating regulatory limits, higher bandwidth efficiency is often required.

As such, when configuring a base station with a GS sensor, it’s important to consider the message type and the data being transmitted to the rover.  It is recommended to use RTCM V3 (MSM) or RTCM V3 as the message type during base setup, as it is continually evolving RTCM standard significantly increases the number of supported message types. RTCM V3 is specifically designed to easily accommodate emerging systems, such as network RTK, new satellite constellations like BDS and Galileo, and updates to existing systems, including support for new signals like L2C and L5.

The RTCM Multiple Signal Messages (MSM) are part of the RTCM standard used for GNSS (Global Navigation Satellite System) data transmission.  For more information on RTCM Standards, please visit the DIFFERENTIAL GLOBAL NAVIGATION SATELLITE (DGNSS) STANDARDS – Radio Technical Commission for Maritime Services page:

Once RTCM V3 is selected, in the Data rates tab, users can choose between Extended and Compact Message type:

Here are the differences Between Compact and Extended Message Type:

Here are the key differences between MSM3 and MSM5:

MSM3: This message type includes both code and phase measurements for GNSS signals. It provides essential data for positioning but does not include CNR (Carrier to Noise Ratio) info or Doppler measurements.

MSM5: This message type is more comprehensive, including code, phase, CNR and Doppler measurements. The inclusion of Doppler measurements allows for more precise velocity calculations.

In summary, MSM5 provides more detailed information than MSM3, making it better suited for applications that require higher precision and velocity data. The Compact format is sufficient for standard RTK GNSS measurements, while the Extended format is better suited for highly dynamic environments. Compact messages are also recommended for radio setups, as their smaller size enables lower over-the-air (OTA) transmission rates.

Finally, when configuring a GNSS base station to transmit via UHF radio, several key factors must be considered:

1. The exact UHF radio chosen for the task. (Satel, Pacific Crest, and others)

2. Transport Method - the specific radio protocol used to transmit the correction data from Base to Rover. (The UHF radio protocols define how the correction messages are structured and transmitted over these radio links.)  They can be but are not limited to: RTCM, Leica4G, CMR/CMRx, TPS, HPS, etc.

3. Frequency to use to transport correction messages (For more information please see the articles  (Best practices for UHF radio link communication; How to Configure a UHF Radio Channel Frequency in Leica Captivate)

4. Physical location of the setup for the Base sensor vs Rover sensor (Line-of-sight ranges (5–10 km typically) with limitations such as susceptibility to signal blockage and interference)

5. Selecting Compact and Extended Messages

Once again, it is recommended to use RTCM V3 (MSM) or RTCM V3 as the message type during GNSS Base setup