Tech Chat - What is RTCM format in RTK correction data transmission?

With the progress of science and technology, the global positioning system is becoming more and more complete, and a variety of positioning and navigation products have gradually entered our lives.  However, in order to have a high accuracy and good positioning effect, it is necessary not only to capture more satellites, but also to accept the correction data transmitted by the base station at all times.  When we set up the base station or save the raw data, we often need to select the differential data format, including RTcm2.X, RTCM3.X format, so what is the RTCM format?

In order to deal with GNSS real-time data service, the Radio Technical Commission for Maritime Services (RTCM) proposed a general GNSS data encoding format for network communication, similar to RINEX file format commonly used in post-processing.  RTCM can be said to be the RINEX file in real-time GNSS service. RTCM is mostly used in real-time PPP and RTK positioning calculation. In actual use, RTCM is broadcast as an array of binary sequences.

RTCM Types

RTCM2.X: It mainly defines more than 30 message types, only supports part of the system, a single function of the expansion.

RTCM3.0: a transitional version from Rtcm2.x to rtcm3.x, which is not in use much at present.

RTCM3.2: used more, for the standard format.

RTCM3.3: the format is basically the same as RTCM3.2. On the basis of RTCM3.2, it adds the Big Dike calendar, GA1-7 ephemeris, SBAS system, etc.  

The following are the five most commonly used RTCM packets:

RTCM Data Content  

The RTCM data format includes packet header, data, and parity frame tail.

The first 24 bits are packet header, which consists of three parts:

• Preamble: begins with D3 00; 8bit
• Reserved bits: reserved bits complement 0, there are six binary bits; 6bit
• Data length: the valid data length of the message; 10bit
• Data: the 24-bit parity bit

The following are the types of data commonly used by RTCM:  

• Observation messages: a single satellite system is a packet of messages, so there will be multiple messages in a single epoch of the multi-system.  
• Ephemeris: the information of a single satellite is 1 packet of messages, which are broadcast when there is satellite information update. Ephemeris of different satellite systems are updated in different periods.  
• Base station message: including base station coordinates, base station antenna information, auxiliary information, etc.  
• SSR message: including precision orbit, precision clock difference.

It is because the receiver receives this information that the positioning effect with centimeter accuracy can be achieved. Now high-precision products have gradually entered our life, in surveying and mapping, agriculture, auto driving, personnel positioning and other widely used, greatly promoted the development and progress of society.

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