II Morrow Inc. Apollo Precedus, User Manual

Page: 10 / 105

unit can calculate latitude and longitude (usually abbreviated
Lat/Lon); with four or more satellites, GPS altitude can also
be calculated.
GPS
System
Accuracy
The GPS allows a high degree of position fix accuracy. The
system can produce a position fix accurate to within less
than one meter. Due to concern for national security, the
U.S. Department of Defense introduces constant errors to the
transmitted satellite data to degrade the accuracy of the
system. Called Selective Availability, this practice limits
GPS position fix accuracy to about 100 meters, although the
relative position of the satellites to one another, their
elevation above the horizon, and other factors can also affect
accuracy of the position fix. Under optimal conditions,
accuracy can improve to within 10 meters. GPS position fix
accuracy is not affected by atmospheric conditions.
GPS altitude is based on a mathematical model of the sphere
of the earth. Including intentional degradation, GPS altitude
may differ from barometric altitude by several hundred feet.
Summary Unlike navigation aids providing a position fix with data
from land-based sources, the
Precedus
™ can provide an
accurate position fix over land or sea anywhere in the world.
The unit includes an extensive database of useful waypoint
information and allows you to create up to 1,000
“customized” waypoints of your own. With the power of this
navigation device in the cockpit, you can easily navigate
with unsurpassed accuracy.
2
Introduction to GPS Navigation