Nationwide Differential GPS and Federal Radio Plan 2017

Articles about GPS, GLONASS, GALILEO, WAAS and other satellite navigation systems
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Nationwide Differential GPS and Federal Radio Plan 2017

Postby jimh » Mon Dec 04, 2017 12:24 pm

For decades the Coast Guard service of the United States of American (USCG) operated the LORAN radio position finding service. Several years ago, the USCG decided that it was no longer making use of their own LORAN position finding capabilities; no doubt they had transitioned to use the Air Force's NAVSTAR Global Positioning System (GPS).

There is a curious and often disregarded similarity between LORAN and GPS: they are both radio positioning services and work in exactly the same way: receivers measure the time delay between the transmission of a radio signal and its arrival at the receiver, and then deduce the distance away from the transmitter the receiver must be located. While the method is similar, the implementations are very different.

At some point in the planning for budget allocation, the USCG decided that it should let Congress know that it was not using LORAN anymore. In the early days of the Obama presidency, someone in some place in some government agency decided that if the USCG was not using LORAN, then the approximately $10-million per year it cost to operate LORAN should be saved, and the method to save the money was to completely dismantled, tear down, and scrap out the entire LORAN system.

It soon became apparent that going around and destroying perfectly good LORAN installations and their huge towers was actually costing more than just keeping them running or moth-balling them. I believe something like $55-million was spent blowing up towers and demolishing installations. Someone with some forward thinking finally decided that perhaps this was not such a great idea, and the LORAN might be upgraded to c.2015 technology eLORAN and provide a completely useful alternative to GPS for position, navigation, and timing (PNT) services, with the added benefits of much more robust signals and immunity to jamming. That process seems to be slowly gathering some momentum, but I leave that story for another time.

The USCG also, many years ago, embarked on a very elaborate service as an adjunct to GPS to provide enhanced precision. The USCG project was known as the Nationwide Differential Global Position System or NDGPS. It consisted of a large number of installations across the USA with towers and radio transmitters that sent out correction signals for GPS receiver on medium-frequencies. Each transmitter covered several hundred miles radius, and the signal could be received over rugged terrain or indoors. This project was begun long before the intentional dither of GPS was removed and perhaps about the same time or even before the Federal Aviation Administration (FAA) began their own project to enhance GPS precision for users in commercial airplanes, called the Wide Area Augmentation System (WAAS).

Once the FAA's WAAS augmentation came into use in c.2002, use of the DGPS system began to decline, particularly among boaters. The WAAS signal was transmitted on the same frequencies as the GPS signals, and it was not particularly difficult to add reception of WAAS to the same receiver and antenna that was receiving GPS. The NDGPS system required using a completely separate receiver operating at a much different frequency, requiring its own dedicated antenna. There was essentially no added cost to enhance GPS with WAAS at the user level, while there was significant added cost and bother for a user to add NDGPS to their GPS receiver. Also, in open water WAAS, designed to be received by aircraft, could be easily received on a boat, so the advantage of NDGPS's better radio signals was not influential. Today, recreational boaters use WAAS all the time and NDGPS just about never.

In what seemed like a rather strange repetition of the process used to decommission the perfectly good LORAN system, in August of 2015 the Department of Transportation, in coordination with the Coast Guard and the U.S. Army Corps of Engineers, announced they were planning to decommission 62 (of a total of 83) perfectly good and operational Nationwide Differential Global Positioning System Service (NDGPS) sites.

The rationale presented as leading to the proposed site reduction were:

--the U.S. Coast Guard change in policy to allow aids to navigation (ATON) to be positioned with a GPS receiver using Receiver Autonomous Integrity Monitoring (RAIM), and to allow USCG navigation in all waters using the WAAS receiver;

--limited availability of consumer grade NDGPS receivers;

--no USCG DGPS carriage requirement on any vessel within U.S. territorial waters;

--the Presidential Directive turning off GPS SA;

--continuing GPS modernization; and

--the Federal Railroad Administration's determination that neither NDGPS, nor High Accuracy NDGPS, are requirements for the successful implementation of Positive Train Control.

The DOT solicited comments. Apparently they got some replies, and they must have listened to them. The government recently announced in its Federal Radio Plan for 2017 as follows:

    "The shutdown will be limited to 37 NDGPS sites, leaving 46 operational sites available to users in the maritime and coastal regions. Maritime sites to be disestablished are:

    -- Brunswick, ME
    -- Cold Bay, AK
    -- Eglin, FL
    -- Isabela, PR
    -- Lompac, CA
    -- Pickford, MI
    -- Saginaw Bay, MI
    -- Sturgeon Bay, WI
    -- Key West, FL

    "Inland sites to be disestablished are:

    -- Albuquerque, NM
    -- Austin, NV
    -- Bakersfield, CA
    -- Billings, MT
    -- Chico, CA
    -- Clark, SD
    -- Dandridge, TN
    -- Essex, CA
    -- Flagstaff, AZ
    -- Greensboro, NC
    -- Hackleburg, AL
    -- Hagerstown, MD
    -- Hartsville, TN
    -- Hawk Run, PA
    -- Klamath Falls, OR
    -- Macon, GA
    -- Medora, ND
    -- Myton, UT
    -- Pine River, MN
    -- Polson, MT
    -- Pueblo, CO
    -- Savannah, GA
    -- Seneca, OR
    -- Spokane, WA
    -- St. Marys, WV
    -- Summerfield, TX
    -- Topeka, KS
    -- Whitney, NE"

As a boater you might have never noticed a NDGPS site, but, as it happens, I have seen three of them: Detroit, Cheboygan, and Upper Keewanaw. Each site consists of a radio transmitting antenna formed by a triangular guyed tower of about 100-feet height. The Detroit site is on the west bank of the Detroit River, near the Ambassador Bridge. The Cheybogan site is on the east bank of the Cheboygan River at its mouth on Lake Huron. The Upper Keewenaw site is on the north bank of the Keewanaw Waterway at its western end near Lake Superior.

Availability of moderate cost recreational grade NDGPS receivers remains the stumbling block for recreational boaters to make use of the system. There are none available. I also suspect that modern recreational grade chart plotters may not support a connection of an NDGPS correction signal, which would be made via NMEA-0183. The correction data would be sent in a format called RTCM RECOMMENDED STANDARDS FOR DIFFERENTIAL NAVSTAR GPS SERVICE, also commonly referred to as RTCM SC 104-STD. I have not studied this carefully, but I am skeptical that in today's market you could rely on every random recreational grade chart plotter being able to accept and use NDGPS data; many modern chart plotters don't even have NMEA-0183 inputs any more.

Radio antenna tower at Upper Keweenaw site
NDGPS_TransmitterSiteUpperKeweenaw.jpg (45.87 KiB) Viewed 11709 times

NDGPS Coverage Map for Eastern USA
NDGPSCoverageEastern815x725.png (81.28 KiB) Viewed 11710 times

NDGPS Coverage Map for Western USA
NDGPSCoverageWestern413x612.png (33.01 KiB) Viewed 11690 times

For more about the RTCM SC-104 protocol, see this UNIX man page.

Posts: 11781
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula

End of USCG Differential GPS Service

Postby jimh » Tue Apr 03, 2018 7:28 am

A notice in the FEDERAL REGISTER on March 21, 2018 announces the United States Coast Guard will discontinue its differential GPS correction broadcast services known as the Nationwide Differential Global Positioning System (NDGPS) service, beginning in September 2018 and concluding in September 2020. The formal announcement notes:

The Maritime Differential GPS system was established in the late 1980s to augment the existing GPS signal with accuracy corrections and integrity monitoring. This augmentation signal was broadcast over Medium Frequency from terrestrial broadcast sites. At the time, the publicly available GPS signal was intentionally degraded through Selective Availability (SA), and thus augmentation was necessary to meet minimum requirements for maritime positioning and navigation. Selective Availability was permanently discontinued in 2000, and as system technology has improved, observed positional accuracy for un-augmented GPS consistently meets requirements for harbor/harbor-approach navigation on modern GPS receivers...

The USCG has continued assessments and outreach affirming that the positional accuracy provided by un-augmented GPS and GPS augmented by the U.S. Wide Area Augmentation System (WAAS) is sufficient to meet its mission requirements and navigational safety requirements for harbor approaches. Because there is no regulatory requirement for the carriage of Differential Global Positioning System (DGPS) equipment, and other GPS augmentation systems such as WAAS are already in prevalent use by marine navigation equipment, the USCG cannot justify further investment to upgrade and maintain the NDGPS system. Additionally, the Coast Guard no longer has a mission requirement for DGPS to position Maritime Aids to Navigation because current Coast Guard policy allows the placement of aids to navigation with un-augmented GPS or GPS augmented by WAAS. Finally, other government and commercial augmentation systems (e.g. WAAS) are readily available to provide GPS accuracy corrections. For these reasons, the NDGPS system was reduced from the 2015 constellation of 84 sites, to the current constellation of 38 maritime sites. Pursuant to this announcement, the USCG's remaining 38 maritime sites will be discontinued in stages, beginning in September of 2018 and ending in September of 2020.

For more current information, visit: