RTCM releases new differential standard

GPS World, June, 2004

The Radio Technical Commission for Maritime Services (RTCM) Special Committee 104 has completed a new standard for Differential Global Navigation Satellite System (DGNSS) services. Like its predecessors, the standard supports very high accuracy navigation and positioning through broadcasts from reference stations to mobile receivers.

Developed by RTCM's Special Committee 104 (SC 104), the Differential GNSS standard Version 3.0 broadcast contains data that augment the information obtained from GNSS satellites--including GPS, Russia's Glonass, and Europe's Galileo system now under development.

Using the RTCM messages broadcast from reference stations at precisely known locations, suitably equipped receivers can improve real-time horizontal positioning accuracy to a few meters or even as little as a few centimeters. For conventional differential operation, the data include pseudorange (approximate receiver/satellite distance) corrections that mitigate atmospheric effects and inaccuracies in the satellite ephemeris (orbital position and onboard clock) data.

For higher accuracy real-time kinematic (RTK) operations, the data elements are code and carrier-phase observables that support double-differencing algorithms in the mobile units.

Compared to Version 2.x series, the Version 3.x series standard is more efficient, incorporates a higher integrity scheme, and is easier for the receiver designer to use, according to the RTCM. The initial release of the new standard, Version 3.0, consists primarily of messages designed to support RTK operations, which requires the transmission of a considerable amount of information and thus benefits the most from an efficient data format. A typical Version 3.x RTK application uses less than half the data required by the previous version.

Version 3.0 provides messages that support GPS and GLONASS RTK operations, including code and carrier phase observables, antenna location and descriptors, and ancillary system parameters. The architecture of the new standard is designed to make it easy to add new capabilities.

In addition to the messages defined in the new standard, RTCM SC 104 is also developing a number of new messages. These messages address networked RTK systems, conventional differential systems, compact RTK, and legacy information from Version 2.x. As new messages and capabilities are demonstrated through validity and interoperability testing, they will be incorporated into future releases of the Version 3.x standard.

DGNSS standard Version 3.0 is available from RTCM at its online publication store <http://www.rtcm.org>.