News»View all»Differential GPS Shutdown Proposed in US
Posted on 26 August 2015

An Aug. 18 Federal Register notice proposes shutting down the Nationwide Differential Global Positioning System (NDGPS) in January 2016 because of a decline in its use, except for sites in coastal areas.

The notice, issued by the U.S. Coast Guard (USCG), Transportation Department (DOT) and Corps of Engineers (USACE), reads:

The Nationwide Differential Global Positioning System (NDGPS) service augments GPS by providing increased accuracy and integrity using land-based reference stations to transmit correction messages over radio beacon frequencies. The service was implemented through agreements between multiple federal agencies including the USCG, DOT, and Army Corps of Engineers, as well as several states and scientific organizations, all cooperating to provide the combined national DGPS utility.

However, a number of factors have contributed to declining use of NDGPS and, based on an assessment by the Department of Homeland Security, DOT and USACE. DHS, DOT and USACE are proposing to shut down and decommission 62 DGPS sites, which will leave 22 operational sites available to users in coastal areas.

Contributing factors cited in the decision are:

  • USCG changes in policy to allow aids to navigation (ATON) to be positioned with a GPS receiver using Receiver Autonomous Integrity Monitoring (RAIM), which assesses the integrity of a GPS signal within the receiver;
  • increased use of Wide Area Augmentation System (WAAS) in commercial maritime applications, which uses ground-based reference stations and satellite communications to improve accuracy;
  • limited availability of consumer-grade NDGPS receivers;
  • no NDGPS mandatory carriage requirement on any vessel within U.S. territorial waters;
  • the May 1, 2000 Presidential Directive discontinuing GPS Selective Availability
  • continuing GPS modernization; and
  • the DOT Federal Railroad Administration’s determination that NDGPS is not a requirement for the successful implementation of Positive Train Control (PTC), which provides the railway system the capability to positively enforce movement authorities along railroad systems.

In April 2013, announced that DHS and DOT were in the process of analyzing the need for NDGPS. “The response to the 2013 notice was limited, but the responses received were well informed on the NDGPS system, its use, and current and potential applications,” the notice reads. “While a limited number of responders found the broadcast of corrections to be beneficial, no respondents reported the discontinuance of DGPS broadcast to be detrimental or harmful. Ship pilots in particular noted that DGPS can be critical in confined waterways for precise ship-handling maneuvers.”

Public comments on the proposed shutdown and decommissioning of 62 DGPS sites are being accepted until Nov. 16. Termination of the NDGPS broadcast at these sites is planned to occur on Jan. 15, 2016.