Novatel OM-20000077 user manual Message Formats, RTCM-Format Messages

Chapter 5

Message Formats

The chapter discusses the industry-standard message formats that can be used with your SUPERSTAR II receiver, including RTCM and NMEA. Refer to the L1 GPS Firmware Reference Manual for more information on using these message formats.

5.1RTCM-Format Messages

The Radio Technical Commission for Maritime Services (RTCM) was established to facilitate the establishment of various radio navigation standards, which includes recommended GPS differential standard formats.

The standards recommended by the Radio Technical Commission for Maritime Services Special Committee 104, Differential GPS Service (RTCM SC-104,Washington, D.C.), have been adopted by NovAtel for implementation into the receiver. Because the receiver is capable of utilizing RTCM formats, it can easily be integrated into positioning systems around the globe.

As it is beyond the scope of this manual to provide in-depth descriptions of the RTCM data formats, it is recommended that anyone requiring explicit descriptions of such, should obtain a copy of the published RTCM specifications. See Appendix D, Standards/References on Page 62 for reference information.

Message ID# 83, DGPS Configuration contains one or part of a RTCM message. The message type selected in the Set DGPS Configuration message (Message ID# 83, bytes 9 to 16) is retransmitted through this message. Message length is variable and a message can be transmitted up to once every 100 ms. A RTCM message always starts as the first byte of a message and always ends as the last byte of a message. Thus, a RTCM message can be output in one or many messages but a message block cannot contain more than one RTCM message. The control byte is used to determine the start and the end of a RTCM message. The sequence number of the control byte can be used to detect the loss of a message block on the transmitter side. It starts at 0 and increments by one for each consecutive message block (0,1,2,3,0,1,2,3,0,1,...). Refer also to the L1 GPS Firmware Reference Manual for details on this message.

All receiver messages adhere to the structure recommended by RTCM SC-104. Thus, all RTCM messages are composed of 30 bit words. Each word contains 24 data bits and 6 parity bits. All RTCM messages contain a 2- word header followed by 0 to 31 data words for a maximum of 33 words (990 bits) per message.

Message Frame Header		Data	Bits
Word 1	–	Message frame preamble for synchronization	8
	–	Frame/message type ID	6
	–	Base station ID	10
	–	Parity	6
Word 2	–	Modified z-count (time tag)	13
	–	Sequence number	3
	–	Length of message frame	5
	–	Base health	3
	–	Parity	6

The remainder of this section will provide further information concerning receiver RTCM data formats.

5.1.1RTCM1 Differential GPS Corrections (Fixed)

This is the primary RTCM message used for pseudorange differential corrections. This message follows the RTCM Standard Format for a Type 1 message. It contains the pseudorange differential correction data

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