GNSS Correction Standards
GNSS correction standards promote the use of a unique set of conventions, models and methodologies for the computation and application of GNSS correction messages. WAAS, Coast Guard DGPS, RTK, Network RTK, and PPP are examples of augmentation services that provide GNSS correction streams to different user communities. Compliance to standards gives users confidence that service providers control and monitor the accuracy, reliability, and integrity of the information they distribute. As standards for ‘navigation safety’ and ‘precise positioning’ can differ significantly, they are discussed separately in the following.
Navigation Safety Standards
International standards bodies, such as RTCM, RTCA and the National Marine Electronics Association (NMEA) have been leaders in documenting the format of broadcast GNSS correction messages for marine and air navigation.
Air navigation standards were developed by WAAS for ground and satellite based augmentation services (GBAS and SBAS). While SBAS provide continental coverage for en-route air navigation via satcom, GBAS are dedicated to operations at local airports. The Radio Technical Commission for Aeronautics (RTCA) correction messages are designed for wide-area coverage and contain integrity information updated within 10 seconds to notify users of a system fault. Manufacturers of WAAS receivers also implement Receiver Autonomous Integrity Monitor (RAIM) algorithms in their user sets to comply with civil aviation requirements.
Marine navigation is supported by Coast Guard DGPS beacons broadcasting high-frequency radio signals along the North-American coastline, St. Lawrence Seaway, and Great Lakes. Reference sites with co-located master and monitor GNSS stations provide built-in integrity and reliable autonomous operations. The RTCM-104 DGNSS standards define the protocol, message structure and content of the correction messages. Detailed information about RTCM standards can be downloaded from here.
Besides internationally accepted RTCM standards, some receiver manufacturers have also developed proprietary formats to support the transmission of GNSS data between master and rover RTK units. For instance, in 1996, Trimble introduced a Compact Measurement Record (CMR) format which has been widely used.
RTCA: RTCA Special Committee -159 maintains a suite of minimum operational performance standards (MOPS) for aviation equipment using aircraft-based, ground-based, and satellite-based augmentation systems (ABAS, GBAS, and SBAS, respectively). These standards are in widespread use today but apply mainly to the aviation community who relies exclusively on the GPS L1 coarse/acquisition (C/A) code.
RTCM: Radio Technical Commission for Maritime Services (RTCM) standards for differential GNSS are defined by RTCM Special Committee 104. RTCM-104 specifies the communication protocols between GPS base stations and rovers and the content of correction messages. The messages can support real-time dual frequency differential positioning using the RTK method. Detailed information about RTCM standards can be downloaded here (behind a paywall for non-members).
NTRIP: The Network Transport of RTCM via Internet Protocol (NTRIP) standard supports the distribution of GNSS differential corrections over the internet. NTRIP documentation and a list of casters providing access to real-time GNSS data streams can be found here.
NMEA: National Marine Electronics Association (NMEA) provides standard protocols and messages to communicate GNSS positions between electronic devices. Information on the NMEA standard can be found at their website.
Precise Positioning Standards
To improve the productivity of positioning applications that are not safety critical, a number of real-time or post-mission correction services are offered. In general, the correction messages and communication protocols they use are customized to deliver accuracy and fast ambiguity resolution. Services include regional single-base and network real-time kinematic (RTK, NRTK), global real-time Precise Point Positioning (PPP), as well as services providing a combination of NRTK and PPP (such as Trimble’s Real-Time eXtended (RTX) positioning).
While single-station RTK corrections may be relayed using the maritime RTCM-104 standard, most modern network-based precise positioning services now offer corrections in a state space representation (SSR). SSR messages were designed to relay optimal estimates of satellite and atmospheric corrections from regional or wide-area GNSS networks. RTCM has recently adopted the SSR messaging standard.
Private companies offering regional NRTK services use different methodologies to estimate SSR corrections within their networks. The Virtual Reference Station (VRS), Master Auxiliary Concept (MAC) and Flächen Korrektur Parameter (FKP)) are the most common regional modeling approaches currently used. These solutions are often delivered in a closed network environment using proprietary formats. In Canada, network RTK services are provided mainly by the private sector. Their networks cover most of the populated regions along our southern border. By subscribing to these services, users in these regions gain access to streams of real-time GNSS corrections from their cellular phones and can achieve centimetre-level precision with high-end GNSS receivers.
Global PPP and combined PPP/NRTK solutions are based on near real-time predictions of GNSS satellite orbits and clocks. While PPP software solutions may vary, most conventions, models, and algorithms have been documented and made available by the IGS (“A guide to using International GNSS Service (IGS) products”). The IGS also provides access to internet streams of global real-time GNSS corrections computed by several of its analysis centers (see RTS website). Commercial real-time extended services, based on a similar approach, deliver the global corrections to subscribers over the air using different combinations of satellite and cellular communications.
In collaboration with CGRSC member agencies, NRCan has published “Guidelines for RTK/RTN GNSS Surveying in Canada (2015)” and “Best Practices for GNSS Reference Station Installation and Operation” to support GNSS users seeking to achieve the highest accuracy in the national standard. In order to promote interoperability and ensure the compatibility of coordinate solutions obtained using correction streams from different providers, CGRSC also developed a compliance agreement to validate the coordinates assigned to all RTK references operating in Canada. This agreement has resulted in the continuous update and public posting of coordinate time series for all compliant RTK networks. A description of the agreement and a map of reference stations operated by compliant NRTK service providers can be found at the NRCan NRTK monitoring website.