NorthStar 958, Operator'S Manual, Page: 191 / 232

958 Operator’s Manual, Rev. A Page 183
Appendix A - Understanding GPS, WAAS, and
Radar
Introducing GPS and WAAS
What is GPS?
The nation’s Global Positioning System (GPS) was developed
by the United States Department of Defense, and comprises 21
active satellites (with three additional spares) that encircle the
earth with navigation information. The major advantage of
GPS over all other commercially available navigation systems
(including loran) is its all-weather, worldwide availability and
absolute accuracy.
Absolute accuracy is the ability of a
navigation device to determine your actual latitude and
longitude coordinates on the surface of the earth.
Repeatable
accuracy is the ability to return to a position whose
coordinates you’ve previously recorded with the same
equipment.
GPS has an absolute and repeatable accuracy of 20 to 30
meters (better than 100 feet) 95 percent of the time, according
to the U.S. government. This means that you can expect to be
within 100 feet of your intended position 95 percent of the
time, and beyond that area approximately five percent of the
time.
When they developed GPS, the Department of Defense
included a system of built-in errors, called Selective
Availability (SA), which they introduced as a national defense
strategy to intentionally degrade the GPS solution. The DOD
wanted to avoid the possibility that its precision could be used
by hostile forces as a source of accurate targeting data against
U.S. interests.
When it was enabled, SA reduced GPS’s absolute and
repeatable accuracy from a maximum of 30 meters (better
than 100 feet) 95 percent of the time, to 100 meters (better
than 330 feet) 95 percent of the time. However, this accuracy
didn’t meet the U.S. Coast Guard’s requirement of 8-20 meters
for navigating in harbors, so the Coast Guard then developed a
system of differential corrections—differential GPS or DGPS—to