Compact Data Transmission Standard for High-Precision GPS

The performance of a Real-Time Kinematic (RTK) system is highly dependent on the datalink component between reference (base) and rovers. Real-Time Kinematic positioning is widely used for many applications. The reference station broadcasts carrier phase and code phase measurements to one or more rover stations. The rover stations combine locally collected measurements with those from the reference station to obtain position results at a centimetre-level. Frequently the datalink component of a Real-Time Kinematic system takes the form of digital radio/modems, however cellular phones or FM subcarrier signals on local radio stations may be used. In all datalink mediums, the available data bandwidth is at a premium. The design of a data transmission standard must attempt to minimise bandwidth. Real-Time Kinematic system manufacturers have adopted their own proprietary data transmission standards. Without a published standard, users cannot freely mix reference and rover equipment from different manufacturers. The Radio Technical Commission for Maritime services, Special Committee 104, (RTCM-SC104), addressed this issue by publishing message types 18-21 in their Version 2.1 standard, which support high-precision Real-Time Kinematic users. However, the RTCM messages have a large framing overhead and do not include significant data compression algorithms to make them practical for many datalink options. As community reference station infrastructure is implemented, a compact transmission standard will help Real-Time Kinematic techniques gain favour with users who only wish to purchase a rover unit. A widely used Real-Time Kinematic data transmission standard is publicly disclosed for the first time so that it can be used by all manufacturers across the industry. The Compact Measurement Record (CMR) format contains packet framing and message types for raw L1 and L2 carrier phase and pseudorange data, plus reference station location and description messages. Details of the compact data transmission standard are documented. Issues relating to mixing different receiver types are raised; of particular concern are carrier phase and pseudorange biases and antenna phase center variations. With nine satellites in view, the new standard allows for dual-frequency carrier phase and pseudorange data to be transmitted once per second in less than 2000 bits. The Compact Measurement Record format requires half the bandwidth of the equivalent RTCM-SC104 messages.