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Marine Electronics Journal Blog

Upgrading GNSS accuracy

 Development of a global navigation satellite system—GNSS for short—is well on its way to fruition. Once completed the network of satellites will provide exceptional accuracy, especially with the addition of “differential” techniques to the equation. Differential refers to information-containing signals from land-based reference stations that are used in conjunction with satellite signals. Differential supplies the “D” in DGPS and now DGNSS.

Two experts from the U.S.-based organization RTCM---Radio Technical Commission for Maritime Services---recently reported on DGNSS for Marine Electronics Journal. The information below is excerpted from that article.


By Robert Markle and Kendall Ferguson


Satellite positioning has been a reality for civilian use for over 25 years.  GPS has become a word in the English language to the extent that some people who know what it is could not tell you that it stands for Global Positioning System.  Now we must learn a new acronym, “GNSS” for Global Navigation Satellite Systems since the US Air Force’s GPS system has been joined by a fully functional Russian GLONASS system, the European Galileo system, the Chinese BDS (BeiDou) system, as well as regional systems operated by Japan and India. Manufacturers now build receivers that can integrate signals from several of these systems to calculate even more accurate positions with increased reliability and availability. An example is Si-Tex's combination chart plotter/fishfinder that is compatible with Galileo, GLONASS and the Asian QZSS in addition to GPS.

One of the most critical phases of maritime navigation is navigation in channels and harbors where a few feet can make the difference between a great day on the water and going aground.  Although GPS had the capability to provide the accuracy needed for most maritime navigation situations, the US government years ago degraded the civilian signal in a process called Selective Availability. The intended goal was to limit the usefulness of GPS for military services not allied with the US and not having access to the encrypted GPS military signal.

GNSS receivers (including GPS) work by using messages broadcast by satellites that contain information about the exact location of each satellite at precise times.  The receiver calculates its distance from the satellite by comparing the time a message was sent to the time it arrives at the receiver.  The math requires adjustments for frequency shift caused by the speed of the orbiting satellite relative to the receiver, as well as corrections for the timing effects of both General and Special Relativity (Einstein was right).  By using similar information from three or more satellites, the receiver can compute its position on the surface of the earth.  With more satellites in view, accuracy improves, and altitude can be determined, which is, of course, important in air navigation.  Selective Availability degraded the accuracy of the position to the extent that it might be as much as 100 meters in error—certainly not good enough for navigation in close quarters.


Enter Differential GPS

The US Coast Guard wanted to improve the safety of channel and harbor navigation, and so developed the Differential GPS (DGPS) technique.  Differential GPS works by positioning reference stations at precisely surveyed locations near the waterways where improved navigation accuracy and safety are desired.  The reference station monitors the satellite signals, creates signal corrective information, and performs satellite signal integrity checks.  The station then broadcasts local correction and integrity information to navigation receivers that reduces shipboard position error to 8-10 meters 95% of the time, and often to within 5 meters.

RTCM partnered with the Coast Guard and RTCM’s vendor members to develop two DGPS standards.  One covered the operation of the reference stations and their associated integrity monitors.  The other standard defined the messages broadcast by the reference stations.  These standards have continued to be revised and updated by RTCM Special Committee 104.  GPS Selective Availability was discontinued in 2000, which brought uncorrected GPS accuracy to generally 10 meters or less, which DGPS could improve to 1 to 3 meters.  That’s because without the artificial errors introduced by Selective Availability, other errors result from atmospheric conditions, which differential corrections can address.  As other GNSS systems were introduced, RTCM’s standards (now DGNSS) were and still are being revised to include them and to include still-evolving DGNSS techniques.


RTK and high-accuracy positioning

It used to be that high precision could only be accomplished with post-processing after data had been collected.  In 2004, RTCM published a new standard with a more compact message structure designed to work efficiently—the Real Time Kinematic (RTK) method, which can use several reference stations as a network. In 2006, Special Committee 104 added another standard to the family.  This one is called NTRIP (Networked Transmission of RTCM via Internet Protocol).  NTRIP is designed to disseminate differential correction data or other kinds of GNSS data to stationary or mobile users over the Internet, allowing simultaneous PC, Laptop, PDA, or receiver connections to a broadcasting host.  Users can access data through their computer’s Internet connection, or via the cellular telephone network, thereby skipping the necessity of having a radio infrastructure. 

You may have heard that the US Coast Guard is in the process of shutting down some of its DGPS reference stations.  Like the unfortunate decision to shut down Loran-C stations, this is a result of continuing budget pressures on a service that is expected to take on additional tasks with limited funding.  There is also a sense that GPS and other GNSS systems provide adequate accuracy for most maritime navigation purposes, and that DGPS use is declining because of that.  Still, the Coast Guard will keep nine coastal stations open for now in the hope that these will provide adequate maritime coverage. At upper right is the station at Pt. Loma, CA. At left is the station at Albuquerque, NM.

But a growing interest in autonomous ships suggests that there will be a need in the future for more precise positioning with even greater safety in the maritime domain, since there would not be a human navigator on board to make up for the few meters of uncertainty that present navigation systems provide.  This suggests that the more precise positioning methods with greater integrity provided by RTCM standards may be needed for maritime navigation in the future.



About the authors

Bob Markle is an independent marine safety consultant, who retired as RTCM President in 2016 after 14 years.  He is the convener of IMO’s Correspondence Group on Modernization of the Global Maritime Distress and Safety System and current acting RTCM President.

Kendall Ferguson, longtime Chair of RTCM Special Committee 104, is recognized as one of the foremost experts in this area. Ferguson is employed by Sapcorda Services GmbH, which was created in 2017 to bring high-precision GNSS positioning services to mass market applications.




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Comments | Leave a Comment
Page 1 of 3 ( 11 comments)

Hard-Over with Brushed APilot Pump:(12/18/2017 5:37:05 PM) "Jim.
What do you mean by ...."Garmin GHP 20 with SmartPump...Because it is a brushless system, it is fail-safe and won’t execute a hard-over turn the way a brushed pump can."

Thanks for the note. Since the description came from Garmin I contacted the company for an explanation. Here's what one of their engineers told me:

On brushed DC actuators, a single-point failure in the drive circuit (shorted wire or blown component inside the controller) could cause the motor to run full speed in one direction and take the rudder all the way to one rail. A brushless actuator relies on timing-controlled commutation, so a short or component fail would cause the actuator to stop moving rather than moving at full speed.

Hope this helps,

trawlerdeejay:(10/13/2017 3:46:51 PM) "Excellent article. I had no idea what the differences were between o183 and 2000, Thank you so much."
Darryl:(3/27/2017 10:17:15 PM) "Putting the MSRP with each unit reviewed would have been helpful. If each unit was actually tested, the reports on each unit would have been helpful too.

Thanks Darryl---we generally don't mention prices due to confusion over so many variations---MSRP (mfg. suggested retail price), MAP (min. advertised price), MRP (min. resale price) and then there are internet prices on some websites that go their own way. But your point is well taken--buyers need to know if something is in their price range. We'll work on it.
There is independent testing of some of these products on sites like but the information we receive from manufacturers rarely cites the results of any shootouts they may conduct against the competition's products. "
Laurie Seibert:(2/16/2017 2:00:20 AM) "Thanks EV Collier for sharing this informative blog. It is important to know the causes of EMI filters. We use these parts in our daily life in the electronic products so we should know that what are the causes are cures of EMI Filters.

Great job and keep updating!

Laurie Seibert"
Yes:(2/10/2017 7:22:40 AM) "EMI/RFI filter causes and cure. There are very few people who share such information with everyone. I was looking to read such informative blog!

Great job!

Lisa Wilson
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