Technological improvements in marine navigation continue at an incredible rate. Not long ago we all relied on loran C, the first reliable and accurate navigation system available to the general boating public at a reasonable price. But loran's numerous shortcomings, which included limited range, signal degradation in bad weather, and poor signal-crossing angles, resulting in low accuracy at some times and in some parts of the country, inevitably caused researchers to look elsewhere for a better system.
Satellites became the delivery mode of choice for navigational information. After early experimentation with the doomed Sat-Nav system (remember having to wait hours for the next satellite to appear overhead?), the Global Positioning System, or GPS was created. GPS would offer highly accurate position fixes in longitude and latitude, almost anywhere in the world.
Because all of these systems were military hand-me-downs to the general public, most came with a catch. In the case of GPS, it was Selective Availability (SA), an intentional degradation of the accuracy of GPS signals by the military. SA distorted GPS enough so that it was at first substantially inferior to loran in terms of repeatability, the ability to return to a specific spot again and again once you had been there and initially recorded the coordinates.
SAY No More!
SAY No More!
The U.S. Department of Transportation came to the rescue with differential corrections to SA. Differential works much like loran, with land-based radio transmitters broadcasting corrections to special differential receivers on boats. These receivers compensated for SA and greatly improved GPS accuracy.
But as of this past May 1, SA has been turned off. President Clinton ordered that Selective Availability be set to zero, after the various branches of the military concluded that doing so posed no threat to national security. (You can read more about navigation technology on the Coast Guard's navigation page at www.navcen.uscg.mil.)
Ordinary, non-differential GPS, therefore, just got a whole lot more accurate. So does this mean that the differential-equipped GPS you recently purchased is now worthless? No, it doesn't, since DGPS is still more accurate than ordinary GPS with SA-off. But to take the issue of accuracy one step further, a new system has appeared on the scene, called the Wide Area Augmentation System, or WAAS.
The Next Generation
The Next Generation
Developed by Raytheon under contract to the Federal Aviation Administration (FAA), WAAS uses satellites instead of land-based towers to deliver differential correction signals, and is expected to become the "next generation" navigation and landing system for aircraft when it's certified in 2002. WAAS, which has been in continuous operation since December 1999 provides a much greater area of coverage and inherently better accuracy than any other non-military positioning system.
Here's how it works: Land-based reference stations collect data that is then transmitted to a master station for analysis. The master station calculates a "correction value" for each of the GPS satellites in view, and a correction message is then up-linked to geostationary Inmarsat satellites, one of which is positioned over the Atlantic, the other over the Pacific. The Inmarsat satellite then broadcasts the correction down to the WAAS-equipped differential receiver on your boat, and the correction is combined with the GPS signal to give you a highly accurate position readout.
Basically, the WAAS system corrects GPS variations for which GPS alone, or differential GPS, cannot correct. These include such things as "clock drift," "orbit drift," and "ionosphere bend," highly technical problems that are too complicated to describe here. Obviously, we as fishermen are most concerned with what WAAS means to us in terms of accuracy of position and repeatability, two important aspects of finding fish.
"WAAS is the most accurate system for boaters," says Raytheon's National Sales Manager, Lyle St. Romain. "It's accurate to within five meters 95 percent of the time. In fact, in tests and trials we routinely see it down to 2.5 meters."
Bob Jackson, who heads up Raytheon's FAA Programs, provided the following data to compare the nominal accuracy of the three different systems.
- GPS with SA set to zero (turned off) - 60 feet
- GPS with conventional differential correction (DGPS) - 30 feet
- GPS with WAAS correction -15 feet
Because Raytheon has been so closely involved with the development of WAAS, its marine division will offer some of the first WAAS/GPS units for boaters this summer. The Raynav 300 and 301 will feature WAAS corrections on a five-inch monochrome screen, with a new and very fast 12-channel GPS receiver. The Raynav 300 will offer built-in WAAS correction and will list for about $900, while the Raynav 301 will contain both WAAS and conventional differential receivers and list for about $1,210. Both will feature NMEA and Seatalk interfacing, a track plotter, and a large readout of navigational data. Raytheon will also offer a stand-alone receiver, the RS 120, that will upgrade any late-model Raytheon GPS (as well as other brands) to WAAS. This is a small antenna about the size of an inverted teacup saucer that can be installed on a conventional antenna mount, or even flush-mounted into the deck, and is tough enough to walk on. Retail list price for the RS 120 will be about $635. For more information on these units, as well as when and where you can buy them, contact Raytheon at (800) 539-5539, ext. 2155.
These are surprisingly low prices for what you get - the very highest accuracy available in any type of marine navigation receiver. And you can now get that accuracy far from land and out of range of conventional GPS differential receivers. Bottom fishermen, for example, are certain to convert over to WAAS in droves, since accuracy is everything when searching for one small wreck or rock in a vast ocean.
You can learn more about WAAS by checking out Raytheon's web site at wwws.raytheontands.com/waas. No doubt about it, this is the navigation system of the future.