Global Positioning System (GPS)
Radionavigation systems today rely on the Global Positioning
System (GPS) as the basis for navigation information. This all weather system is deployed by the US Department of
Defense and jointly managed by the Department’s of Defense and
Transportation. Unfortunately, the radionavigation data available from GPS is not sufficient to meet the needs of
highway users. Radionavigation systems are characterized by three primary parameters:
Accuracy, Availability, and Integrity.
The Federal Radionavigation Plan uses the following definitions for
these parameters:
Accuracy - The degree of conformance between the
estimated or measured position and/or velocity of a platform at a given time
and its true position or velocity. Radionavigation system accuracy is usually
presented as a statistical measure of system error and is specified as:
Predictable
- The accuracy of a radionavigation system’s position solution with respect to
the charted solution. Both the position solution and the chart must be based
upon the same geodetic datum. (Note: Chapter 4 in the FRS discusses chart
reference systems and the risks inherent in using charts in conjunction with
radionavigation systems.)
Repeatable - The accuracy with which a user can
return to a position whose coordinates have been measured at a previous time
with the same navigation system.
Relative - The accuracy with which a user can
measure position relative to that of another user of the same navigation system
at the same time.
Availability - The availability of a navigation system
is the percentage of time that the services of the system are usable.
Availability is an indication of the ability of the system to provide usable
service within the specified coverage area. Signal availability is the
percentage of time that navigation signals transmitted from external sources
are available for use. Availability is a function of both the physical
characteristics of the environment and the technical capabilities of the
transmitter facilities.
Integrity - Integrity is the ability of a system to
provide timely warnings to users when the system should not be used for
navigation.
GPS has a predictable accuracy of between 6 and 40 meters,
is available almost everywhere, and does not offer timely warnings of when it
should not be used. This level of
accuracy and integrity are not sufficient for many land transportation
applications. The availability of GPS
has helped to make it one of the most useful navigation systems ever
developed. It is sometimes referred to
as the next utility.
More on How GPS Works
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