WAAS, DGPS and the Mariner’s Toolkit

CDR Curtis Dubay
LT Terry Johns
Office of Aids to Navigation
US Coast Guard

On August 24, 2000, the Federal Aviation Administration (FAA) announced that their space-based, L-band Wide Area Augmentation System (WAAS) became available for use by "some aviation and all non-aviation" users. The FAA announcement has prompted numerous inquiries to the Coast Guard regarding the maritime use of WAAS and the status of the Coast Guard DGPS system. The following Questions and Answers are directed at helping to clarify the status of these two systems for the mariner.

1.      Why did the Government design and build two different GPS augmentation systems?

The 1994 National Telecommunications and Information Administration (NTIA) Technical Report to DOT on a National Approach to Augmented GPS Services studied the necessity of expanded government efforts in providing DGPS services. Its goal was to recommend the optimum integrated system to meet aviation and terrestrial navigation needs. A variety of systems were being proposed at the time. The study concluded that a combination of two systems, the FAA’s Wide/Local Area Augmentation Systems (WAAS/LAAS) and the USCG’s DGPS system, was the optimum mix. This integrated system, consisting of the L-band line-of-sight WAAS for aviation users, and the terrain-following medium frequency DGPS for maritime and terrestrial users, meets the vast majority of the nation’s precise navigation and positioning needs.

2.      Is WAAS currently certified for maritime navigation?

No. WAAS is not yet fully operational and is currently in a testing status, undergoing further development. It is not certified for use as a safety of life navigation system in the maritime navigation environment. WAAS may be used, with caution, in the maritime environment to improve overall situational awareness, but it should not be relied upon for safety-critical maritime navigation. The Maritime DGPS Service, on the other hand, is fully operational and meets all the standards for the harbor entrance and approach phases of navigation

3.      After WAAS reaches initial operating capability (IOC) in a few years, will it be suitable for maritime navigation?

WAAS is not optimized for surface (maritime and terrestrial) use, rather, it was designed primarily for aviation use. It is intended to eventually support aeronautical enroute through precision approach air navigation. The current WAAS test signals are transmitted by two geo-stationary satellites on a line-of-sight, L-band radio frequency. This means that if anything obstructs the view of the portion of the sky where the satellite is, the WAAS signal will be blocked. Since geo-stationary satellites are positioned over the equator, the farther north users are, the lower the geo-stationary satellites are in the sky - increasing the likelihood of an obstruction. In contrast, the medium frequency (285-325 kHz) radiobeacon-based Maritime DGPS Service is optimized for surface (maritime and terrestrial) applications because it's ground wave signals "hug the earth" and wrap around objects. This means that the Coast Guard DGPS system is well suited for the marine environment (down in the "ground clutter") where a geo-stationary satellite can be blocked by terrain, harbor equipment and other man-made and natural objects.

4.      Can the Coast Guard’s DGPS system be used by aviation?

That’s up to the FAA. However, the Coast Guard’s system was designed with the surface (maritime and terrestrial) user in mind. It was neither designed nor intended to meet aviation requirements. Although aviation users could potentially get some modest benefit from the Coast Guard’s DGPS for applications such as surface traffic management at airports or General Aviation, it could not attain the type and level of aeronautical service for which WAAS and LAAS are designed, without significant re-engineering.

5.      Is the Coast Guard DGPS system a "transient technology" that is here today but will be gone tomorrow?

No. DGPS has already been adopted globally as an international maritime standard established by the 1994 International Telecommunications Union document ITU-R-M.823. It meets IMO Resolution A 815(19) standards for navigation in harbor entrances and approaches. Over 40 nations have fully embraced this robust technology and are implementing DGPS services identical to our own.

6.      Which system is more accurate, WAAS or DGPS?

On the average, WAAS and DGPS accuracy are virtually the same, although DGPS accuracy is better when the user is near a DGPS transmitting site. The WAAS architecture is designed to provide uniform 7m accuracy (95%) regardless of the location of the receiver – within the WAAS service area. DGPS is designed to provide better than 10m navigation service (95%), but typically provides better than 1 meter horizontal positioning accuracy (95%) when the user is less than 100 nautical miles from the DGPS transmitting site. Accuracy then degrades at a rate of approximately 1 meter per hundred nautical miles as the user moves away from the transmitting site. A total of 56 maritime DGPS sites provide coastal coverage of the continental United States, the Great Lakes, Puerto Rico, portions of Alaska and Hawaii, and portions of the Mississippi River Basin..

Conclusions: Once WAAS becomes fully operational, the combination of Coast Guard and FAA systems is expected to provide a robust, complementary service to all modes of transportation. We look forward to the day that industry provides the public with a fully integrated receiver, one that uses all available radionavigation systems to provide unprecedented accuracy, integrity, and availability.

Despite the differences between DGPS and WAAS, it should always be kept in mind that both services ultimately rely upon a single navigation system – GPS – which is vulnerable to interruption at any time. This lends additional credence to the recommended practice of using all available means of navigation and not relying upon any single system. Remember, prudent mariners will always keep looking out the window!