An Introduction to Global Positioning System
The Global Positioning System (GPS) is a space age navigational
system that can pinpoint your position anywhere on the globe,
usually within a few yards or meters. This amazing technology is
available to everyone, everywhere, day and night, and best of
all, at no cost for use of the navigational data. GPS uses a
constellation of 24 satellites in precise orbits approximately
11,000 miles above the earth. The satellites transmit data via
high frequency radio waves back to Earth and, by locking onto
these signals, a GPS receiver can process this data to
triangulate its precise location on the globe.
GPS operates 24 hours a day, in all weather conditions, and can
be used worldwide for precise navigation on land, on water and
even in the air. Some of its many current applications include:
boating, fishing, hunting, scouting on land or from the air,
hiking, camping, biking, rafting, pack trips by horseback, hot
air ballooning, general aviation, snowmobiling and skiing, search
and rescue, emergency vehicle tracking, 4 wheeling, highway
driving and a host of other outdoor activities where accurate
positioning is required.
How GPS Determines Your Position
satellite ranging to triangulate your position. In other words,
the GPS unit simply measures the travel time of the signals
transmitted from the satellites, then multiplies them by the
speed of light to determine exactly how far the unit is from
every satellite it's sampling.
By locking onto the signals from a minimum of three different
satellites, a GPS receiver can calculate a 2D (two-dimensional)
positional fix, consisting of your latitude and longitude. By
locking onto a fourth satellite, the GPS can compute a 3D
(three-dimensional) fix, calculating your altitude as well as
your latitude/longitude position.
In order to do this Eagle uses a 12 parallel-channel
receiver in all of its current products. Three of the channels
lock on to satellites for triangulation. Another channel locks on
to a fourth satellite for 3D navigation, which lets the unit
calculate altitude in addition to latitude and longitude. These
four channels continuously and simultaneously track the four
satellites in the best geometrical positions relative to you. The
additional eight channels track all other visible satellites,
then add this data to the data from the original four satellites.
The unit then over-resolves a solution, creating an
accuracy-enhanced reading. The additional channels also ensure
reliable, continuous and uninterrupted navigation, even in
adverse conditions such as valleys or dense woods.
Accuracy of GPS
GPS was conceived in the 1970s, and is controlled by the United
States Department of Defense. Although GPS was initially
envisioned for military use, the Government realized early on
that there would be numerous civilian applications as well.
Subsequently, the Department of Defense (DOD) created two
transmission codes; the P code (Precision code) for military use,
and the C/A code (Civilian Access code) for civilian use.
The highest accuracy levels were to be reserved for the military
so as to prevent hostile enemy attacks against the U.S. using our
own navigational system. However, once in operation, the civilian
GPS receivers using the C/A code proved to be more accurate than
the DOD had intended. Consequently, the military developed a
system for randomly degrading the accuracy of the signals being
transmitted to civilian GPS receivers. This intentional
degradation in accuracy is called Selective Availability or S/A.
This reduced the civilian GPS accuracy levels to being within 100
meters or less, 95% of the time. However, typical accuracy for
most users averaged between 20 and 50 meters the majority of the
time. You could easily see the effects of S/A on a GPS receiver
when you were not moving. Typically, there would be random
movements in speed, altitude and position readings, along with
slow position "wandering" on the plotter trail. This was easily
seen when you were on a .1 or .2 mile zoom range and not moving.
For example, while parked at the dock in your boat, you would see
unexplainable changes in your digital speed readings up to a few
miles per hour, even though you were not moving.
Plot of position accuracy
using standard Eagle GPS receiver (stationary). Note the
differences in scale.
|5.5 hour period immediately
prior to shutoff of selective availability
|8 hour period immediately
after shutoff of selective availability
Effective May 2, 2000 selective availability (S/A) has been
eliminated. The United States Department of Defense now has the
technology to localize the control system to deny GPS signals to
selected areas. It is not often that your electronics products
increase in value after you've purchased them. Now boaters,
aviators, drivers, hikers, hunters, and outdoor enthusiasts of
all types can locate their position up to ten times more
precisely (within 10 to 20 meters) and navigate their way through
unfamiliar terrain. Anglers can now return to their favorite spot
on a lake or river instead of just their favorite area. A
Lowrance GPS receiver in combination with advanced technology of
today's GPS management will take you anywhere you want to go.
The decision to allow civilians so much accuracy in location
information was finally made because GPS is continually playing a
more important role in the lives of people around the world -
it's becoming a national utility. GPS is the global standard in
navigation because it is completely free of charge to the
Differential GPS (DGPS)
Differential GPS, or DGPS, has been developed to improve GPS
accuracy to within a few meters. DGPS was originally initiated by
the U.S. Coast Guard to counter the accuracy degradation caused
by Selective Availability. Even with S/A now eliminated, DGPS
continues to be a key tool for highly precise navigation on land
and sea. DGPS technology adds a land-based reference receiver
– located at an accurately surveyed site – to the
other GPS components. This non-moving DGPS reference station
knows where the satellites are located in space at any given
moment, as well as its own exact location. This allows the
station to compute theoretical distance and signal travel times
between itself and each satellite. When those theoretical
measurements are compared to actual satellite transmissions, any
differences represent the error in the satellite's signal. All
the DGPS reference station has to do is transmit the error
factors to your DGPS receiver, which gives the information to the
GPS receiver so it can use the data to correct its own
measurements and calculations.
The two most common sources of corrective DGPS signals
currently are: (1) Coast Guard, land-based beacon transmitters,
broadcasting the data at no charge to the public, covering all
coastal areas and much of the inland USA as well; and (2) FM
radio sub carrier transmissions available both in coastal and
inland areas, but limited to paid subscribers. In order to
receive DGPS correction data from Coast Guard beacon
transmitters, a mobile GPS unit requires a separate beacon
receiver. And to receive FM sub carrier DGPS signals from local
subscriber radio stations, the GPS unit requires a separate FM
receiver, normally the size of a pager. Naturally, your GPS unit
must have the capability to both receive and process DGPS
Wide Area Augmentation System (WAAS)
GPS is plenty accurate for route navigation, but the U.S. Federal
Aviation Administration has special need for aircraft traffic
control that go beyond basic GPS. The FAA has a plan under way to
boost GPS performance even further with its Wide Area
Augmentation System, or WAAS. This GPS add-on will include a time
control element that will help airliners fly closer together
while avoiding collisions. In addition to carefully spacing
airplanes along travel corridors, WAAS will eventually make
instrument landings and takeoffs more accurate as it replaces
existing aviation navigation systems.
Non aviators can use WAAS signals to make their GPS navigation
even more accurate. However, WAAS has some limits you should know
First, the U.S. government has not completed construction of
the WAAS system, so it is not yet fully operational. The ground
stations are in place, but only a few of the needed WAAS
satellites have been launched.
WAAS can boost the accuracy of land GPS navigation, but the
system is designed for aircraft. The satellites are in a fixed
orbit around the Equator, so they appear very low in the sky to
someone on the ground in North America. Aircraft and vessels on
open water can get consistently good WAAS reception, but terrain,
foliage or even large man-made structures frequently block the
WAAS signal from ground receivers.
You'll find that using your GPS receiver without WAAS is both
easy and amazingly accurate. It's easily the most accurate method
of electronic navigation available to the general public today.
Remember, however, that this receiver is only a tool. Always have
another method of navigation available, such as a map or chart
and a compass.