- What is GPS?
Global Positioning System (GPS) consists of 24 navigation satellites in low
Earth orbit. It began as the Navigation Satellite Timing and Ranging (NAVSTAR)
system by the Department of Defense in 1973. Selective Availability was
discontinued on May 2, 2000.
- How does GPS work?
The satellites continually beam their location to receivers on the ground.
A GPS receiver does triangulation from several satellite signals to compute
its own location. The accuracy depends on how many satellites it sees and the
strength of their signals. I’ve gotten as close as 12 feet.
- What does a GPS receiver do?
All GPS receivers tune in to the satellite signals, calculate and display
their latitude and longitude, record tracks and waypoints (see below), and
connect with a computer. Some models also contain digitized maps, driving
- What are the limitations?
A GPS receiver needs to have a clear view of at least four satellites.
It usually doesn’t work indoors or underground, and may have a problem in
a forest or in a city with tall buildings. Fortunately for hikers, your
best reception is in the open desert or on a mountain top.
- What are NAD27, WGS84, etc.?
The Earth is not a perfect sphere. It isn’t a perfect ellipsoid
either, but for any region on the surface of the Earth, there is an
ellipsoid that comes close. These ellipsoids, which approximate the
shape of the Earth within a limited geographic region, have names such
as North American 1927 Datum (NAD27), World Geodetic Survey 1984
(WGS 84), etc. You have to tell the GPS receiver which ellipsoid to
use in its calculations. Most of Arizona is under NAD27.
- What is a waypoint?
A particular location, recorded as latitude, longitude, and elevation.
- What is a track?
A line drawn through a series of points recorded automatically as you walk.
- Why should hikers carry a GPS?
A GPS will provide continuous readings of the distance and direction from
where you are to a chosen waypoint (such as the trailhead you started from,
or a destination you programmed in). It will record the latitude and
longitude of the gold mine you found, or where to send emergency help.