NorthStar 941X Reference Manual page 14

selective
availability
GPS and DGPS
Page 6
from a chart or other reference. Repeatable accuracy refers to the ability
to return to a position whose coordinates you have previously recorded
with the same equipment.)
Developed by the United States Department of Defense (DOD) the GPS
system of 21 active satellites (with 3 additional spares) literally encircles
the earth with navigation information. GPS is offered in two forms: for
military users and for civil users.
With GPS available to virtually anyone around the world, 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. To
guard against such actions, they introduced intentional errors into the
GPS signals that only U.S. forces could bypass. Called Selective Avai
ability (or SA), it involves a complex formula of data encryption that can
ability
ability ability
be turned on or off without the end-user ever knowing for sure whether
it is in effect or not. DOD's stated position is that SA will remain in effect
indefinitely.
Selective Availability effectively reduces GPS's absolute and repeatable
accuracy from 30 meters (roughly 100 feet) 95% of the time, to 100 me-
ters (330 feet) 95% of the time. This means that, with SA, you can expect
to be within 300 feet of your intended position 95% of the time, and be-
yond that area approximately 5% of the time. At first, this was accepted
by most end-users as an inevitable part of GPS, a tradeoff against its
global availability. But it didn't meet the Coast Guard's requirement of 8-
20 meter accuracy for navigating harbors. It was then that a system of
differential corrections was developed by the Coast Guard that enabled
them to effectively circumvent SA's built-in errors.
These differential corrections are generated by fixed "reference stations"
and broadcast by marine radiobeacons over a relatively limited area—
usually a maximum broadcast radius of 250 miles—to Differential-
capable GPS (DGPS) receivers. The precise geographic location of the
reference station is used to calculate corrections based on the GPS data
received from the satellites, and the corrections are then broadcast to
DGPS receivers. The receivers then convert this data into extremely
precise position, heading and speed information. (Referred to as
1 — Introduction 1 — Introduction 1 — Introduction 1 — Introduction
Selective Avail l l l - - - -
Selective Avai Selective Avai
Rev. D