Real-time correction messages: Why this is important and where it’s going
Most professional users of GNSS equipment are familiar with the term RTCM. You likely understand that RTCM is a language that is used for delivering real-time corrections to rovers. But that’s usually where the understanding of RTCM stops. This article outlines what RTCM is, why RTCM is important to the industry, a bit about its past, and a bit more about its future.
The subtitle of this article suggests that RTCM stands for Real-time Correction Messages. While this is true in practical terms, it is not true in fact. RTCM actually stands for Radio Technical Commission for Maritime Services. RTCM was established as a U.S. government federal advisory committee in 1947. It is now an independent membership organization with scientific, professional, governmental, and educational affiliations and is no longer restricted to maritime communication matters.
Organization
Within the parent organization are seven active special committees with subject-matter experts working on the development of standards across a variety of disciplines. These topics include emergency beacons, electronic charting technology, and the enhanced LORAN system.
Special Committee 104 (SC104) is the group that creates real-time correction messages that are used by professional-grade GNSS receivers. While SC104 deals primarily with the delivery of real-time corrections, its scope of responsibilities has grown in recent years.
SC104 meets three times per year and works online continuously through its working groups. Working groups confabulate on the development of industry standards for applications such as network RTK, BeiDou, coordinate transformations, and at least 11 other topics. The working group members are from network service providers, GNSS manufacturers, military representatives, and others. The primary work is to develop methods for delivering base station information to rovers through efficient and well-documented standards.
RTCM is the English language of RTK. Each manufacturer has its own proprietary RTK language. Trimble has a variety of message options such as CMR, CMR+ and CMRx. Spectra Precision uses a language called ATOM for their messages. Leica, Topcon, and others have their own proprietary presentations of these data. But RTCM is common to all the manufacturers. In this way, brands can work with other brands. Networks of reference stations can work with all rovers regardless of their manufacturer. RTCM means “interoperable” when speaking about RTK and other DGNSS services.
SC104
SC104 began publishing differential standards with the release of version 1 in 1983. Version 2 replaced version 1 in 1990. Version 2.1 added RTK support, and version 2.2 added Glonass support. Versions 2.2 and 2.3 are still being used in marine and various mapping applications.
During these years, the use of RTCM 2.2 and 2.3 became widely adopted, and therefore their shortcomings became obvious. One of the issues that surfaced was the fact that resolution of the position was down to only a centimeter. In the beginning, it was believed that this amount of granularity was adequate. But as technology increased the precision of the receivers, the demand for more decimals also increased.
Another weakness of RTCM 2 was its lack of efficiency. In those days, byte boundaries had to be respected. The skies were GPS only; there was plenty of time to get all of the data over the air at 4800 baud. Then the GLONASS system began its restoration phase, and there were rumblings about new systems being deployed such as Galileo by the Europeans, Compass by the Chinese, and QZSS by the Japanese. A new version of the standard was deemed to be necessary to provide a framework to manage these developments more efficiently and precisely.
Version 3 is approximately two and a half times more efficient than version 2, meaning that the base station has less data bits to broadcast and the rover has less to decode. This bandwidth savings means that more satellite data can be compressed into smaller packets, saving computational resources, cell phone bills, battery life, etc. Version 3 has also been structured to provide millimeter solutions to meet the most demanding needs.
RTCM version 3 was released in 2004. Today’s version 3.2 represents the best efforts of the industry’s top scientists to provide all the necessary data in a presentation as compact as possible. This allows rovers the ability to position themselves using all constellations in any combination across very long baselines with or without fee-based services.
Standards
The tri-annual meetings of SC104 tend to be held in conjunction with other industry events. The sessions have been hosted by diverse groups such as the European Space Agency, ION, Trimble, Caterpillar, Trinity House Lighthouse Authority, Berlin Senate Department for Urban Development, Wuhan Navigation, Swedish Land Survey, University of Bern, Ashtech, and others. Their recent May meeting was held in conjunction with the China Satellite Navigation Conference in Xi’an.
SC104 strives to meet the needs of an international audience with a wide variety of needs in a timely manner. Impossible? No, but it is difficult. Questions and compromises surrounding interoperability testing inevitably become tough subjects to agree upon. What is the best approach versus the most efficient approach versus the wrong approach? All of these issues must be vetted and then agreed upon by a plurality of the group. Sometimes these issues take years to solve.
The recent passage of the so-called Multiple Signal Messages (MSM) took more than four years to gain consensus to the point of adoption. In the end, competitors, participants, users, innovators, and scientists all agreed, and the standard was passed. MSM allows a service provider to create any combination of observables (frequency and constellation) and deliver this data to any rover that can then understand and decode the contents.
When passing a standard, gaining 100% agreement is ideal. It’s also impractical. Passage of a standard does not always mean that the industry has provided the best answer, but at least it represents an acceptable compromise. Typical SC104 plenary meetings have attendees numbering between 25 to more than 40. The worldwide audience of participants in the various working groups is much higher. Every company that participates has an agenda that may be at odds with another’s. Discussions are typically academic with a well-structured agenda, but occasionally they can become quite passionate and spirited.
Why Are Standards Important?
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They provide a basis for comparison.
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They reconcile regional differences in a global system.
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Their compliance enhances market acceptance.
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They represent industry-wide consensus.
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They facilitate technological advances.
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Their enforcement saves lives.
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They’re used to define minimal competencies.
How to Handle Today’s Signals
Today, if you add up all of the radio-navigation satellite signals coming from space, there are about 125 signals above the horizon that can be used for positioning on the ground. These signals come in a variety of constellations, frequencies, signal shapes, and physical locations.
Should a reference station network provide a rover with access to all signals? If so, what are the bandwidth requirements to pass this data through the cellular network? Can the network infrastructure support this massive delivery burden? Can a rover utilize all of this data? Does it need all the data? What combination works best in what conditions?
We don’t have clear answers to these questions today. A project hosted by Natural Resources Canada called MGEX is currently being conducted to try to answer these questions. The MSM standard mentioned above will then allow the network to deliver the best concatenation of signals and frequency to optimize rover performance.
In recent years, several initiatives have been standardized or are in the process of being standardized. Coordinate transformations can be delivered by the network instead of forcing each rover to calibrate. This standard was passed many years ago, but adoption has been limited to a handful of networks mostly in Europe.
NTRIP is a standard of SC104 that solved a need for service providers to augment their data (VRS or MAC) and get fee-based subscriptions protected by username and password. RINEX, a standard that is managed mostly by the International GNSS Society has been a collaborator with SC104 because many people would like to be able to convert data back and forth between RTCM 3 and RINEX 3 without any loss of fidelity to the data.
NMEA is another recent partner with SC104 as the industry is requesting more GNSS and receiver data. What began as a small organization working on the development of Coast Guard Beacon messages has grown into the standard for all differential and raw GNSS messaging.
The Future
Networks of GNSS reference stations have become infrastructure. They can be compared to electricity, plumbing, cable, etc. They provide high-accuracy positioning capabilities virtually anywhere in the world. The user base ranges the gamut from scientists to surveyors, contractors, machines, and more. The needs of these users extend beyond a simple ability to position themselves precisely.
Two-way communications, multiple/simultaneous solutions, emails and SMS, cameras, scanning, and many other needs will be clarified and standardized in the coming years. RTCM SC104 will participate in these developments. The future needs range from the obvious to the proposed to those not yet imagined. RTCM experts will be there to give advice and recommendations.
For more information about RTCM and its special committees, visit rtcm.org/
