A GPS Primer
The Global Positioning System (GPS) is a
satellite-based navigation system made up of a network of 24 satellites placed
into orbit by the U.S. Department of Defense. GPS works in any weather
conditions, anywhere in the world, 24 hours a day. GPS satellites circle the
earth twice a day in a very precise orbit and transmit signal information to
earth. GPS receivers take this information and use triangulation to calculate
the user's exact location.
A GPS receiver must be
locked on to the signal of at least three satellites to calculate a 2D
position (latitude and longitude) and track movement. With four or more
satellites in view, the receiver can determine the user's 3D position
(latitude, longitude and altitude). Once the user's position has been
determined, the GPS unit can calculate other information, such as speed,
bearing, track, trip distance, distance to destination, sunrise and sunset
time and more. This makes them ideal for navigating, even in dense fog or
whiteouts when no point of reference is visible.
Today's GPS receivers are
extremely accurate, providing position accuracy of less than 15 meters. 12
parallel channel receivers are quick to lock onto satellites when first turned
on and they maintain strong locks, even in moderately dense tree cover or
urban settings.
Some newer receivers with
WAAS (Wide Area Augmentation System) capability can improve accuracy to less
than three meters on average. WAAS consists of approximately 25 ground
reference stations positioned across the United States that monitor GPS
satellite data. Two master stations, located on either coast, collect data
from the reference stations and create a GPS correction message. Currently,
WAAS satellite coverage is only available in North America. For some users in
the U.S., the position of the satellites over the equator makes it difficult
to receive the signals when trees or mountains obstruct the view of the
horizon.
Users can also get better
accuracy with Differential GPS (DGPS), which corrects GPS signals to within an
average of three to five meters. This system consists of a network of towers
that receive GPS signals and transmit a corrected signal by beacon
transmitters. In order to get the corrected signal, users must have a
differential beacon receiver and beacon antenna in addition to their GPS.
Although many GPS receivers can interface with these devices, it is uncommon
for handheld users to make use of them.
One of the most useful
aspects of GPS receivers is their ability to record and store information
about specific locations. This storage feature allows you to record the
position of your destinations before you ever leave home. It also means you
can record "waypoints" so it's easier to navigate from point to point and stay
on track. Most GPS units allow you to save waypoints in ordered sets or
"routes." This means you can input information about entire routes from start
to finish, then follow them by asking your GPS receiver to point from one
waypoint to the next as you travel.
Don't confuse tracking
ability with routing ability. Nearly every GPS can "track" your movements,
leaving a "bread crumb" trail so you can see where you have been. Cheaper
units force you to backtrack manually, while more capable devices have a
"track back" feature that will point you in the right direction as you follow
the original route in reverse.
In general, the more
storage space (memory) a GPS receiver has for waypoints, routes and tracks,
the better. With a simple interface cable you can transfer waypoints, routes
and tracks between your GPS and a PC. Excellent digital map products are
available that allow you to draw routes and waypoints over detailed maps on
your computer and then transfer them to your GPS for easy navigation in the
field.
Some GPS receivers also
have built-in electronic maps that make navigation even easier. In general
their detail is quite limited. However, most of these will also allow you to
download more detailed maps into the unit from your PC. You can look at a map
of your general location right on your GPS screen, then use it to orient
yourself to your surroundings. Be sure to check out the level of detail of any
internal or downloadable maps before you depend on them. Many road and street
products can be quite detailed and up to date. However, for backcountry
navigation you shouldn't expect to find the level of detail that will allow
you to leave your paper maps at home.
GPS receivers can also
include additional functions. Although a standard GPS will provide a bearing
to a waypoint, it will only display your direction of travel as you move. GPS
units that include a digital compass can point you in a direction even when
standing still. Barometric altimeters are also quite useful, providing
altitude accuracies up to +/- 3 feet and often recording pressure changes
useful in forecasting weather.
Consider the options when
choosing your GPS unit. How will you be using it, purely for backcountry
navigation, or do you really need street level mapping for use while driving?
Mapping units are a great luxury in the backcountry, but are probably no
substitute for a unit that lasts longer on batteries, is lightweight and
rugged, and includes features like a digital compass or barometric altimeter.
Memory capacity, screen size and resolution may be key if you're planning on
downloading lots of maps or route information.
Accessories are also a
consideration. Do you need cigarette lighter adapters, mounts or PC interface
cables? Will the unit you're interested in work with the digital maps products
you prefer. Work through these questions and you'll quickly narrow down the
selection to the best choice for your needs.