It's Been a Good Year for GPS

Lt/C Ron Mosley, AP

Information for this article was taken from an on-line article edited by William Sisson. Early in the year, the Clinton administration announced it would halt the intentional distortion (for military purposes) of GPS signals known as selective availability or SA. Overnight, the accuracy improved from a nominal 328 feet to about 65 feet, the upgrade was free. Shortly after SA went by the boards, Raytheon Marine announced a further enhancement of the GPS system that it has been developing under a contract with the Federal Aviation Administration (FAA). This new system is called WAAS, which stands for wide area augmentation system. WAAS is essentially an improved differential system that broadcasts correction signals from a pair of Inmarsat satellites.

Its main goal is better position accuracy for airplanes, but boaters will be among the first beneficiaries. This is because the FAA has not yet approved it for wide spread use in the airline industry. The most impressive thing about WAAS is that it provides even greater accuracy than the differential system operated by the Coast Guard and for less cost. In addition, you don't need a "black box" to receive the satellite corrections. Any GPS receiver with embedded WAAS circuitry will be able to decode the corrected signals.

According to Craig Cushman, Raytheon's global marketing communications manager, "the accuracy is amazing." Raytheon conducted 21- day stability test for the FAA earlier this summer, during which time the system provided accuracies of 1 meter horizontally and 3 meters vertically (the contract requirement was for 7.6 meters). Following this test, the FAA declared the system available to some aviation and all non-aviation users.

In addition to greater position accuracy, WAAS provides more accurate boat speeds, better repeatability, and faster updating and is less susceptible to atmospheric interference, according to Cushman. There are no coverage blackout areas in North America.

If you have differential already, the difference between that and WAAS (15 to 30 feet as opposed to 3 to 10 feet) is probably not enough to make you shelve your DGPS. But WAAS is certainly something to consider next time you're upgrading your electronics.

The WAAS network consists to about 25 ground reference stations across the country that receive GPS signals. The positions of the stations have been very accurately surveyed, which enables them to determine any errors in the GPS signals. The stations send the information to a master station, which computes the correction information and relays it to leased Inmarsat satellites, which in turn forward the corrections to any WAAS-capable GPS receiver. The communication satellites also act as navigational satellites, providing additional signals for determining positions, according to the FAA. Raytheon is the first to build WAAS-capable receivers, but the technology is not proprietary to the company. Raytheon has two WAAS ready GPS units currently available, the RayNav 300 receiver, with a manufacturer's list price of $900, and the Raychart 320 plotter, which lists for $1,210. Also available is the Raystar 120, a GPS satellite sensor that will upgrade existing electronic equipment to receive WAAS. We can probably look for hand-held WAAS receivers (not from Raytheon) by early next year.