62 OEM4 Family Installation and Operation User Manual Rev 12Chapter 5 Message Formats5.3 CMR Format MessagingThe Compact Measurement Record (CMR) message format was developed by Trimble NavigationLtd. as a proprietary data transmission standard for use in RTK applications. In 1996 Trimble publiclydisclosed this standard and allowed its use by all manufacturers in the GPS industry 1 .The NovAtel implementation allows a NovAtel rover receiver to operate in either RT-2 or RT-20mode while receiving pseudorange and carrier phase data via CMR messages (version 3.0) from anon-NovAtel base-station receiver. The NovAtel receiver can also transmit CMR messages (version3.0). The station ID must be ≤ 31 when transmitting CMR corrections, refer to Volume 2, Chapter 2,Commands.The message lengths of the three CMR messages are as follows:CMROBS = 6 (frame) + 6 (header) + (8*L1 channels) + (7*L2 channels) : (192 bytes maximum)CMRREF = 6 (frame) + 6 (header) + 19: (31 bytes)CMRDESC = 6 (frame) + 6 (header) + (variable:26 to 75): (38 bytes minimum; 87 bytes maximum)No guarantee is made that the OEM4 will meet its performance specifications if non-NovAtelequipment is used.5.3.1 Using RT-2 or RT-20 with CMR Format MessagesTo enable receiving CMR messages, follow these steps:1. Issue the COM command to the rover receiver to set its serial port parameters to the proper bitrate, parity, etc. This command is described in detail in Volume 2 of this manual.2. Issue the “INTERFACEMODE COMn CMR” command to the rover receiver, where “COMn”refers to the communication port that is connected to the data link. This command is described indetail in the OEM4 Family User Manual - Volume 2.Assuming that the base station is transmitting valid data, your rover receiver will now begin to operatein RT-2 or RT-20 mode. To send CMR messages, do the following:Periodically transmit three CMR messages at the base station:• A CMROBS message contains base station satellite observation information, and should besent once every 1 or 2 seconds.• A CMRREF message contains base station position information, and should be sent onceevery 10 seconds. Also, the rover receiver will automatically set an approximate positionfrom this message if it does not already have a position. Therefore this message can be usedin conjunction with an approximate time to improve TTFF, see Appendix E, TTFF andSatellite Acquisition on Page 161.• A CMRDESC message contains base station description information and should be sent onceevery 10 seconds, however, it should interlinked with the CMRREF message.1.Talbot, N.C. (1996), “Compact Data Transmission Standard for High-Precision GPS”. Proceeding of theION GPS-96 Conference, Kansas City, MO, September 1996, Vol. I, pp. 861-871
