A quote from Lowrance:
Effective May 2, 2000 selective availability (S/A) has been eliminated. The United States Department of Defense now has the technology to localize the control system to deny GPS signals to selected areas. It is not often that your electronics products increase in value after you've purchased them. Now boaters, aviators, drivers, hikers, hunters, and outdoor enthusiasts of all types can locate their position up to ten times more precisely (within 10 to 20 meters) and navigate their way through unfamiliar terrain. Anglers can now return to their favorite spot on a lake or river instead of just their favorite area. A Lowrance GPS receiver in combination with advanced technology of today's GPS management will take you anywhere you want to go.
The decision to allow civilians so much accuracy in location information was finally made because GPS is continually playing a more important role in the lives of people around the world - it's becoming a national utility. GPS is the global standard in navigation because it is completely free of charge to the public.
Differential GPS (DGPS)
Differential GPS, or DGPS, has been developed to improve GPS accuracy to within a few meters. DGPS was originally initiated by the U.S. Coast Guard to counter the accuracy degradation caused by Selective Availability. Even with S/A now eliminated, DGPS continues to be a key tool for highly precise navigation on land and sea. DGPS technology adds a land-based reference receiver – located at an accurately surveyed site – to the other GPS components. This non-moving DGPS reference station knows where the satellites are located in space at any given moment, as well as its own exact location. This allows the station to compute theoretical distance and signal travel times between itself and each satellite. When those theoretical measurements are compared to actual satellite transmissions, any differences represent the error in the satellite's signal. All the DGPS reference station has to do is transmit the error factors to your DGPS receiver, which gives the information to the GPS receiver so it can use the data to correct its own measurements and calculations.
The two most common sources of corrective DGPS signals currently are: (1) Coast Guard, land-based beacon transmitters, broadcasting the data at no charge to the public, covering all coastal areas and much of the inland USA as well; and (2) FM radio sub carrier transmissions available both in coastal and inland areas, but limited to paid subscribers. In order to receive DGPS correction data from Coast Guard beacon transmitters, a mobile GPS unit requires a separate beacon receiver. And to receive FM sub carrier DGPS signals from local subscriber radio stations, the GPS unit requires a separate FM receiver, normally the size of a pager. Naturally, your GPS unit must have the capability to both receive and process DGPS data. Most Lowrance GPS receivers are DGPS ready.
Wide Area Augmentation System (WAAS)
GPS is plenty accurate for route navigation, but the U.S. Federal Aviation Administration has special need for aircraft traffic control that go beyond basic GPS. The FAA has a plan under way to boost GPS performance even further with its Wide Area Augmentation System, or WAAS. This GPS add-on will include a time control element that will help airliners fly closer together while avoiding collisions. In addition to carefully spacing airplanes along travel corridors, WAAS will eventually make instrument landings and takeoffs more accurate as it replaces existing aviation navigation systems.
Non aviators can use WAAS signals to make their GPS navigation even more accurate. However, WAAS has some limits you should know about.
First, the U.S. government has not completed construction of the WAAS system, so it is not yet fully operational. The ground stations are in place, but only a few of the needed WAAS satellites have been launched.
WAAS can boost the accuracy of land GPS navigation, but the system is designed for aircraft. The satellites are in a fixed orbit around the Equator, so they appear very low in the sky to someone on the ground in North America. Aircraft and vessels on open water can get consistently good WAAS reception, but terrain, foliage or even large man-made structures frequently block the WAAS signal from ground receivers.
You'll find that using your GPS receiver without WAAS is both easy and amazingly accurate. It's easily the most accurate method of electronic navigation available to the general public today. Remember, however, that this receiver is only a tool. Always have another method of navigation available, such as a map or chart and a compass.
smiley as you did with your "Hence.. 2-3 GPS units in your vehicle 
